XFEL.EU THESIS-2021-002

Ultrafast Dynamics
in Light-Harvesting
Molecules with
Optical and X-ray
Spectroscopies
Hamburg, July 2021

Tae Kyu Choi
FXE, European XFEL

European X-Ray Free-Electron Laser Facility GmbH
Holzkoppel 422869 Schenefeld
GermanyUltrafast Dynamics inLight-Harvesting Molecules with
Optical and X-ray Spectroscopies

DISSERTATION
zur Erlangung des Doktorgrades
an der Fakultät für Mathematik, Informatik und
Naturwissenschaften
Fachbereich Chemie der Universität Hamburg

vorgelegt von
Tae Kyu Choi
aus Chuncheon, Südkorea

Hamburg
2021

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Gutachter der Dissertation:

Prof. Dr. Wojciech Gawelda
Prof. Dr. Gabriel Bester

Gutachter der Disputation:

Prof. Dr. Wojciech Gawelda
Prof. Dr. Gabriel Bester
Prof. Dr. Nils Huse
Prof. Dr. Simone MascottoProf. Dr. Robin Santra

Datum der Disputation:

24.09.2021

Vorsitzender des Prüfungsausschusses:

Prof. Dr. Gabriel Bester

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Contents
List of publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2

Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1

2.2

2.3

3

Coordination chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.1

Cyclometalation and carbene coordination . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.2

Solvation and magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Optical light-matter interaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1

Absorption bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.2

Relaxation dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.3

Remark: Dark chemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

X-ray light-matter interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.1

X-ray absorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3.2

X-ray fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.3.3

Remark: Resonant inelastic X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . 42

Experimental methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.1

Time-resolved experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.2

Femtosecond optical laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.1

3.3

Synchrotron light source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1

3.4

4

Experimental setup at University of Bern . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

11-ID-D Endstation at APS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

X-ray free-electron laser (XFEL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4.1

BL3EH2 Endstation at SACLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.4.2

FXE Endstation at European XFEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5d6 Ir(III)-Co(III) photocatalyst. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.2

Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.3

TOAS on Ir(III) photosensitizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

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5

6

7

8

4.4

TOAS on Ir(III)-Co(III) triads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.5

Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4d6 Ru(II)-Mo(VI) photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.2

Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.3

TOAS on Ru(II) photosensitizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.4

TOAS on Ru(II)-Mo(VI) dyad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5.5

Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4d6 Ru(II) photosensitizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

6.2

Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

6.3

Time-resolved EXAFS on Ru(II) photosensitizers . . . . . . . . . . . . . . . . . . . . . . . 97

6.4

Lifetime studies in acetonitrile and water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6.5

Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

3d10 Cu(I) photosensitizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

7.2

Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

7.3

Femtosecond-resolved Kα and Kβ1,3 XES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7.4

Femtosecond-resolved Kβ2,5 XES and XSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

7.5

Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

3d6 Fe(II)-Co(III) photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
8.1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

8.2

Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

8.3

Femtosecond-resolved XES on Fe(II)-Co(III) dyad . . . . . . . . . . . . . . . . . . . . . . 123

8.4

Femtosecond-resolved XES on Fe(II) photosensitizer . . . . . . . . . . . . . . . . . . 128

8.5

Conclusion and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

9

Outlook and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

A

PETRA III P01 data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

B

SACLA XES data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

CPython data analysis codes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

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D

Supplementary information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
D.1

Chapter 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

D.2

Chapter 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

D.3

Chapter 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

D.4

Chapter 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

D.5

Chapter 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

E

List of hazardous substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

F

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

Eidesstattliche Versicherung. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

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List of publications
1

Galler, A., Gawelda, W., Biednov, M., Bomer, C., Britz, A., Brockhauser, S.,
Choi, T.-K. et al. Scientific instrument Femtosecond X-ray Experiments (FXE):
instrumentation and baseline experimental capabilities. J. Synchrotron Rad. 26,
1432-1447 (2019).

2

Khakhulin, D., Otte, F., Biednov, M., Bömer, C., Choi, T.-K. et al.
Ultrafast X-ray Photochemistry at European XFEL: Capabilities of the
Femtosecond X-ray Experiments (FXE) Instrument. Appl. Sci. 10, 995 (2020).

3

Choi, T.-K. et al. Femtosecond Molecular Flattening in [Cu(dmp)2 ]+ Probed
by X-ray Emission Spectroscopy and Solution Scattering. International
Conference on Ultrafast Phenomena 2020, 10.1364/UP.2020.Th1A.5 (2020).

4

Nazari Haghighi Pashaki, M., Choi, T.-K. et al. Unveiling the originof photo-induced enhancement of oxidation catalysis at Mo(VI)centres of Ru(II)-Mo(VI) dyads. Chem. Commun. 57, 4142-4145 (2021).

5

Huber-Gedert, M., Nowakowski, M., Kertmen, A., Burkhardt, L., Lindner,N., Schoch, R., Herbst-Irmer, R., Neuba, A., Schmitz, L., Choi, T.-K. et al.
Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal
Iron(II)-Cobalt(III) Dyad. Chem. Eur. J. 27, 9905-9918 (2021).

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Zusammenfassung
Lichtsammelnde Materialien, wie der Farbstoff N719, wurden als eine derEnergiequellen der nächsten Generation eingeführt. Photophysikalische undelektrochemische Studien an solchen Systemen wurden durchgeführt, um die
Lichtsammelphänomene zu verstehen, wobei ein besonderes Interesse anedelmetallfreien Photosensibilisatoren und Photokatalysatoren besteht. Die
Realisierung von ultrakurzen optischen und Röntgenpulsen, zusammen mit ihren
spektroskopischen Anwendungen, ermöglichte es, die entsprechenden ultraschnellen
Veränderungen in den Photosensibilisatoren und Photokatalysatoren fein zu verfolgen,sowohl global als auch lokal.
Diese Studie untersucht die ultraschnelle Dynamik der Energieumwandlung inModellkomplexen mittels ultraschneller optischer und röntgentechnischer Methoden,
um weitere Einblicke in deren stationäre Leistungen zu erhalten. Unterschiede
im optischen Absorptionsspektrum in Bezug auf die Ankunftszeitänderungenzwischen zwei Pulsen zeigten eine globale Wechselwirkung in einem
bimetallischen Photokatalysator wie Ir(III)-Co(III)-Triaden, Ru(II)-Mo(VI)-Dyaden
und Fe(II)-Co(III)-Dyaden. Komplementäre, zeitaufgelöste Röntgenabsorptionsund -emissionsspektroskopie visualisierte die elektronischen und geometrischen
Strukturänderungen des Elements, die inhärent die relevanten chemischen Ereignissein photoaktiven Molekülen wie Ru(II), Cu(I) und Fe(II)-Photosensibilisatoren
auslösen. Diese Studie zeigt, dass die 3d-Chelate nach der Ladungstrennung einen
Femtosekunden schnellen Symmetriebruch durchlaufen, der oft nicht-strahlend
rekombiniert, während die 4d- und 5d-Chelate mit ihrem chemischen Potential
im Zustand der Ladungstrennung die nahegelegenen Elektronendichten sostören können, dass entweder eine chemische Energie oder ein Elektron auf derPikosekunden-Zeitskala übertragen wird.

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Abstract
Light-harvesting materials, such as the N719 dye, have been introduced as one of
the next-generation energy sources. Photophysical and electrochemical studies onsuch systems have been carried out to understand the light-harvesting phenomena
with a particular interest in noble-metal free photosensitizers and photocatalysts. The
realization of ultrashort optical and X-ray pulses together with their spectroscopic
applications enabled to finely trace the corresponding ultrafast changes in thephotosensitizers and photocatalysts, both globally and locally.
This study investigates the ultrafast dynamics of energy conversion in model
complexes by means of ultrafast optical and X-ray methods to obtain more insights
into their steady state performances. Differences in the optical absorption spectrumwith respect to the arrival time changes between two pulses revealed a global
interaction in a bimetallic photocatalyst such as Ir(III)-Co(III) triads, Ru(II)-Mo(VI)dyads, and Fe(II)-Co(III) dyads. Complementary to this, time-resolved X-ray
absorption and emission spectroscopy visualized the electronic and geometricstructure changes of the target element that inherently trigger the relevant chemical
events in photoactive molecules like Ru(II), Cu(I), and Fe(II) photosensitizers. This
study finds that the 3d-chelates undergo a femtosecond symmetry breaking after
the charge separation that often recombines non-radiatively, whereas the 4d- and5d-chelates with their chemical potential in the charge separation state can perturb
the nearby electron densities such that either a chemical energy or an electron gets
transferred on the picosecond timescale.

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Acknowledgements
This was quite an amount of knowledge and experience for such a novice like me
within ∼4 years of the Ph.D. course, and it was only possible with uncountable helps
from my lovely people in all around the world.
First of all, I would love to thank both Prof. Dr. Christian Bressler and Prof. Dr.
Wojciech Gawelda who offered me a Ph.D. position in FXE group at European
XFEL, and taught me a lot of great lessons for the last four years. All the chancesof networking, experiments, conferences, and publications were only possible with
their helps. Particularly, I will never forget our funny memories at the XAFS 2018conference in Kraków. :)I would also like to thank other FXE (ex-)members who helped and encouraged
me a lot during my stay at FXE, Dr. Chris Milne, Dr. Andreas Galler, Dr. Dmitry
Khakhulin, Dr. Peter Zalden, Dr. Frederico Alves Lima, Dr. Katharina Kubicek, Dr.
Angel Rodriguez-Fernandez, Dr. Sophie Canton, Dr. Alexander Britz, Dr. Mykola
Biednov, Dr. Yifeng Jiang, Dr. Vandana Tiwari, Dr. Fernando Ardana-Lamas, Dr.
Christina Bömer, Michael Diez, Florian Otte, Martin Knoll, Paul Frankenberger, andTimo Korsch. In addition, I appreciate all the helps from the European XFEL staff
members and Ph.D. students in many different occasions, special thanks to Dr. Vasilii
Bazhenov, Prof. Dr. Adrian Mancuso, Halimah Shie, Julia Galler, Dr. Chan Kim, Dr.
Yoonhee Kim, Dr. Huijong Han, Dr. Tokushi Sato, Benoit Rio, René Wagner, Dr. Jia
Liu, Marieke Sander, Dr. Frank Poppe, Karina Kazarian, Torsten Lange, Gabriella
Mulá-Mathews, and Maik Neumann.
The whole Ph.D. course was under the IMPRS-UFAST graduate school at DESY, andthere I would love to thank Dr. Julia Quante, Dr. Neda Lotfiomran, Dr. Sang-Kil Son,
Prof. Dr. Nils Huse, Dr. Aleksandr Kalinko, Dr. Chiwon Lee, Hyein Hwang, Kyung-Min
Lee, Jihyun Jeong, Dr. Young Yong Kim, Dr. Sihyun Sung, Manuel Harder, Daniel
Haynes, and Ivette Bermúdez Macias for their numerous supports (special thanks
to Prof. Dr. Gabriel Bester for his second supervision!). Also, from the University of
Hamburg many people helped me, and I would like to thank Dr. Young-Joo Lee for her
valuable advices.
After all, I learned significant amount of my current knowledge and hands-on
experiences at SACLA during my internship stay for about two years, and I sincerely
thank Dr. Makina Yabashi, Dr. Tetsuo Katayama, and Yasuko Matsumoto for theirinvaluable supports. In line with this, I would also like to thank all our collaborators
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in various research projects so far, Prof. Dr. Andrea Cannizzo, Dr. Maryam Nazari
Haghighi Pashaki, Prof. Dr. Anne-Kathrin Duhme-Klair, Dr. Xiaoyi Zhang, Dr.
Kristoffer Haldrup, Dr. György Vankó, Dr. Jakub Szlachetko, Dr. Tom Penfold, Prof. Dr.
Matthias Bauer, Dr. Michał Nowakowski, and Marina Huber-Gedert for their time to
discuss with me.
Apart from the scientific activities, many precious memories with colleagues andfriends helped me a lot to ease the arrival and settlement in Hamburg. A big handgoes to the FXE geeks, especially Dr. Christina Bömer, Michael Diez, and Florian
Otte at the same office, always sharing lots of humors and laughs, hanging out
together to the cool places and bars in Hamburg, such an everyday chit-chat reallyrelaxed my Ph.D. studies. At the same time, I would love to thank Dr. Katharina
Kubicek and René Wagner for sharing their time with me while I was writing the thesis,
our regular dinner was one of my most important escape at that time. Countlessbarbeques, parties, picnics, Kohltours, Feierabendbierchen also with the koreansociety in Hamburg should be equally recognized, I am literally so much grateful foreverything that I could receive during my Ph.D. course. One thing I must mention isthe business trips that I made to the various occasions, I thank for all the supports onthat way and really enjoyed the travels.
Last but not least, I would like to thank all my old friends and my beloved family,
relatives who still cheer me up from Korea, and a special thanks goes to the family
Heinrich and my love Vanessa Heinrich, who support me a lot in every single sense.
Finally, I would like to thank you for visiting and taking your time to read my thesis.

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1 Introduction

Microscopic to macroscopic world, and vice versa.

The motivation to study physical chemistry became clearer during my doctoral
course. That is, what has been described in modern physics with computational
methods for microscopic world can be effectively correlated to macroscopic world
via physical chemistry, such that one can "see" and understand the system fromboth points of view. In this regard the thesis will try to visualize both microscopic
and macroscopic perspectives on light-harvesting materials, primarily consisting of
inorganic chelates in solution phase. That means, the inorganic chelates in solution
phase for light-harvesting applications will be investigated not only by applying
quantized pictures to the system but also by translating them into chemistry termsof thermodynamics and kinetics. Furthermore, this thesis will also try to resolve thephenomena in time especially when it comes to the very first step of light-harvesting,unveiling the so-called ultrafast dynamics on picosecond or femtosecond timescalethat determines how the intermediate and steady states of the system can beaccomplished to chemically react further.
Inorganic chelates in solution (condensed) phase for light-harvesting applications
have recently attracted a number of physicists, chemists, and other scientists allover the world not only because they in fact appear often in nature but also because
their photophysical and electrochemical properties can be easily tuned compared tothe organic counterparts, due predominantly to their rather simpler symmetry andthus clearer valence orbital energetics. In addition, the inorganic chelates are morerobust with higher quantum yields of photoabsorption and luminescence owing to thestrong ligand field in their ligand coordination. Hence, these inorganic chelates are
frequently applicable to converting the solar energy to a transient molecular dipole, i.e.
a transient electric potential (<1 µs) inside the system, that can trigger consequentchemical reactions like photosynthesis. Although many of them have been quite
thoroughly characterized in their ground and excited states, the underlying ultrafast
electron and energy transfer processes in higher time resolution are still yet to be fully
explored, in particular when the molecular orbitals, spin states, and vibrations in the
excited state are strongly coupled to each other.

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This thesis will therefore exploit the time-resolved experimental methodologies withultrashort optical and X-ray pulses, to elucidate the ultrafast dynamics in some of therepresentative 5d-, 4d-, and 3d-chelates. Global interactions in a supramolecular
photocatalyst for example can be observed with the optical probe window, whereas
a local electronic or geometric structure change requires rather the X-ray probe
window. The ultrafast phenomena interpreted with quantized energy and momentum
diagrams will be then correlated to the infinite timescale properties, namely the steadystate characteristics, further gaining insights into the corresponding light-harvesting
reactions. At the end of the thesis we will realize how unique the light-harvestingprocesses are in terms of energy transformation, since the photon energy absorption
at selective wavelengths by the inorganic chelates can lead to different reaction
or relaxation pathways accordingly, which is not common in electrochemical andthermodynamical methods.
Chapter 2 starts with reviewing how the inorganic chelates in solution phase areunderstood in coordination chemistry. The ligand coordination to metal ion hybrid
orbitals and the following π-donation backbonds configure a certain molecularsymmetry and valence orbital energetics by the ligand field. This accompanies
some characteristic optical absorption bands, at which the photoexcitation resultsin the localization of the excited electron either on the ligand or in the metal ion,
eventually decaying back to the ground state radiatively or non-radiatively. The
X-ray photoabsorption in contrast leads to a core-hole intermediate state that will
be instantaneously refilled by an outer-shell electron, emitting the respective X-ray
fluorescence. The photoelectron interacting with the coordination shell structure, andthe final state of the X-ray fluorescence interacting with the spin density in the system,
reveal the local electronic and geometric structures.
In Chapter 3, the time-resolved experiments with ultrashort optical and X-ray pulses
will be reviewed. A brief history of the femtosecond optical laser system is followedby an actual setup at University of Bern that was employed for the transient opticalabsorption spectroscopy experiments in Chapter 4 and 5. Another brief history of thesynchrotron light source will explain how the extremely bright and collimated X-ray
pulses are generated that can be used for the optical pump X-ray probe absorption
experiments (Chapter 6). Moreover, the recent realization of the X-ray free-electron
lasers further enables to simultaneously monitor the femtosecond electronic andnuclear dynamics in a photosensitizer or photocatalyst, disentangling its strongly
coupled degrees of freedom (Chapter 7 and 8). The relevant experimental setupsin 11-ID-D endstation at Advanced Photon Source and in BL3EH2 endstation at
SPring-8 Angstrom Compact free-electron LAser will be described.
In Chapter 4, the Ir(III)-Co(III) bimetallic photocatalysts with a newly synthesized

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that facilitates the photocatalytic proton reduction. An effective quenching of theexcited electron density with a ∼100 ps time constant was observed, although its
arrival in the Co(III) moiety was not detected.
In Chapter 5, a biomimetic Mo(VI) catalyst covalently linked to a Ru(II) photosensitizer
was studied with the transient optical absorption spectroscopy method to elucidate
the light-induced activation mechanism in the bimetallic photocatalyst. By comparing
with the spectroelectrochemical oxidation spectrum of the catalytic Mo(VI) complex, aphoto-induced intramolecular electron transfer from the vicinity of the Mo(VI) moiety
towards the Ru(II) moiety was unveiled to occur on the picosecond timescale.
In Chapter 6, the geometric structures of the light-switching and photo-oxidizing Ru(II)photosensitizers in their excited states were obtained by exploiting the time-resolved
X-ray absorption spectroscopy. The homo- and heteroleptic Ru(II) complexes withthe localized excited electron on different ligands hardly showed any difference, eventhough the solvent molecule coordination effect and the thermal population of themetal-centered excited state were consistently observed.
In Chapter 7, the femtosecond-resolved X-ray emission spectroscopy andsolution scattering experiments were simultaneously conducted on a model Cu(I)photosensitizer, to unravel its strongly coupled electronic and nuclear dynamics for the
first picosecond after the optical photoabsorption. A molecular symmetry breaking
with a 400-700 fs time constant was clearly identified together with the coherent
vibrational wavepacket, but the spin-flip of the excited electron was not able to be
distinguished within the probe time window.
In Chapter 8, a novel Fe(II)-Co(III) photocatalyst was subjected to the
femtosecond-resolved X-ray emission spectroscopy experiments, because theintramolecular excited electron transfer from the Fe(II) to Co(III) moiety had not been
detected by the transient optical absorption spectroscopy. A chemical energy transfer
instead of the electron transfer was proven to occur on the femtosecond timescale,
inducing the second excited electron in the Co(III) moiety.

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2 Theoretical background

Science without numbers and equations is just a novel.

2.1 Coordination chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.1

Cyclometalation and carbene coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.2

Solvation and magnetism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2 Optical light-matter interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1

Absorption bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2.2

Relaxation dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.3

Remark: Dark chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.3 X-ray light-matter interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.1

X-ray absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3.2

X-ray fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.3.3

Remark: Resonant inelastic X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Before we start our journey to the time-resolved spectroscopy science, we will review
some theoretical backgrounds for the inorganic chelates in solution phase as well as
the optical and X-ray light-matter interaction. This thesis does not include all details
behind the theories presented because either these are presumed to be known orthese are beyond the scope of this thesis and can be found elsewhere. An additional
purpose of this chapter is to make sure that we are sharing the same backgroundknowledge and terminologies for later discussions. This chapter therefore consists ofan overview on coordination chemistry, optical light-matter interactions for ultrafast
optical photon energy transformation, and X-ray light-matter interactions for mapping
the electronic and geometric structure of the inorganic chelates in solution phase.

2.1 Coordination chemistry
Coordination chemistry is one of the most important chemistry branches covering
the science of transition metal complexes, which involves numerous applications
in real world. The formation of transition metal complexes, namely one metal ion
and more than one ligand molecule binding to each other as the simplest example,
is thermodynamically favourable and can be also described as Lewis-acid andLewis-base reaction. That means, two nonbonding electrons from the ligand molecule

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Figure 2.1: A number of cobalt chelates in solution phase with different colors, i.e. different
valence electronic structures absorbing selective wavelengths of incident visible light.

are donated to a vacant spn dm -hybrid orbital from the metal ion, forming a covalentbond. The covalency of the bonds between the metal ion and the ligands depends onthe effective partial charge of the ligands, and this affects the orbital energetics at thevalence-level as well as the core-level of the metal ion [118; 42; 98].

Figure 2.2: Schematic illustration of σ-coordination bond and π-donation backbond in transition
metal complexes. The π-donation backbond follows the σ-coordination bond to balance theexcessive electron density on the metal ion [98].

During the formation of a σ-coordination bond, a π-donation backbond follows withan overlap between an occupied π-symmetry d-orbital of the metal ion and an
antibonding π* orbital of the ligand, to balance the excessive electron density on themetal ion [98]. These valence orbital overlaps, i.e. bindings between the metal ion
and the ligands, therefore configure a certain molecular symmetry to achieve thelowest potential energy of the system, exhibiting its structured energetics in opticalabsorption range (Fig. 2.1). In other words, the so-called ligand field (∆) via σ- andπ-character overlaps splits the valence molecular orbitals into a few energy levels

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according to the symmetry, making the photoabsorption at certain wavelengths be
more probable (Fig. 2.3).
The π-donation backbonds, which connect the valence occupied d-orbitals of themetal ion to the lowest unoccupied ligand π* orbitals, play a very important role inso-called Metal-to-Ligand Charge Transfer (MLCT) transitions, leading to the transient
molecular dipole which can trigger further chemical reactions for the light-harvesting.
These MLCT transitions as well as core-to-valence and valence-to-core transitions,whose probabilities are highly dependent on the valence orbital overlaps between themetal ion and the ligands, will be discussed in later sections.

Figure 2.3: A simplified molecular orbital diagram of the nd6 transition metal complexes inoctahedral symmetry (6 coordinations) represented with four probable photoexcitation pathways(MC = Metal-Centered, LC = Ligand-Centered, LMCT = Ligand-to-Metal Charge Transfer).
Left-hand side represents the atomic orbitals of the metal ion, and right-hand side represents
the molecular orbitals of the ligands. The color and thickness of each arrow indicate the relative
photon energy needed for the corresponding dipole transition and its relative probability
respectively. Here we assume that the ligands contain π-conjugation, which is in fact the
primary case in this thesis. The ligand field splitting in nd-orbitals by ∆0 stems from the σ(dx 2 −y 2 , dz 2 ) and π-symmetry (dxy , dxz , dyz ) overlaps with the ligand valence orbitals.

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2.1.1 Cyclometalation and carbene coordination
The ligand field, which determines the valence orbital energetics and thus the excitedelectron dynamics, varies on how the coordination bonds are formed. An electronlone-pair from nitrogen 2sp2 -orbital is the most frequent source of the coordinationbond, but that from carbon 2sp2 -orbital is also commonly considered if necessary.
Two types of the carbon coordination will be introduced and discussed in this thesis; acyclometalation reaction and a persistent carbene coordination. The cyclometalation
mostly occurs on pyrolysis of the reaction mixture consisting of metal salt (M-X) andligand molecule with good leaving group (H-C-...-N; [20]). According to the chargebalance in this reaction (Fig. 2.4), the carbon atom after the reaction possesses one
excessive electron in its nonbonding 2sp2 -hybrid orbital which is coordinated to themetal ion [132; 48]. That means a negative formal charge on the carbon atom is
exposed to the metal ion, influencing the valence electronic structure of the chelate
via its ligand field [60]. In result the MLCT excited state on this cyclometalated ligand(π* orbital) becomes energetically higher, letting the metal-to-ligand charge transfer bemore directional to other ligands [15].

Figure 2.4: Cyclometalation. N = donor nitrogen, M = metal ion, H = leaving group (Figure from[20]).

On the other hand, a persistent carbene molecule such as N-heterocyclic carbene(NHC) contains a neutral carbon atom with two unshared valence electrons that isready to coordinate. The stabilized carbon atom via the σ-coordination bond therefore
does not have any π-character bond, i.e. any π-donation backbond with the metal ion.
The NHC ligand is thus also called as a π-deficient ligand, and cannot balance theexcessive electron density on the metal ion properly. Hence the MC excited stateof the chelates with the NHC ligand, where an excited electron occupies one of eg *orbitals (Fig. 2.3), becomes energetically higher and less probable to be populated,
whereas the MLCT excited state on the NHC ligand (π* orbital) becomes relatively
lower and more probable to be populated.

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2.1.2 Solvation and magnetism
Once the orbital energetics in the inorganic chelates are settled by effective nuclear
charges, valence hybridized orbitals of the metal ion coordinated with σ-donations
from the ligand molecules, and accompanying π-back donations from the metal
ion highest occupied molecular orbitals (HOMOs) to the ligand lowest unoccupied
molecular orbitals (LUMOs), which collectively cause a certain symmetry in themolecular structure and thus the "ligand field", now the inorganic chelates are ready to
be dissolved in solvent. Contrary to gas phase or lattice structure, in solution phasewhich is still a condensed phase, the inorganic chelates are so-called solvated by thesurrounding solvent molecules. The solvation process in general equilibrates thesystem by forming its own "field" as well, in accordance with the local effective dipole
moments of the solute and solvent molecules [153; 157]. The solvation shells, i.e. thesolvent cages, consequently affect the valence orbital energetics of the solute.
Besides, when the ligand field becomes weaker by certain external stimuli such astemperature, pressure, or photoexcitation, some inorganic chelates especially fromperiod 4 rearrange its electronic structure to reach the new lowest potential energyof the system [59]. This often returns unpaired parallel valence electrons resulting
from coulomb interaction terms and spin-orbit coupling, forming a magnetic field inthe system. This magnetic field from higher multiplicity in the inorganic chelates inturn affects the electric dipole transition processes like photon energy absorption andemission. Detailed discussion on the solvation effect as well as the magnetism will be
however directly referred to at each relevant case in following sections and chapters.

2.2 Optical light-matter interaction
Since there is a sun, the optical light-matter interaction is always, everywhere on
Earth. One should therefore notice that this means not only optical scattering but also
optical photoabsorption and consequent photoemission or photodegradation may
happen anytime if available.
The optical light-matter interaction, predominantly between visible light and valence
electrons, covers from classical Fresnel equation to modern nonlinear optics.
Optical photon energy ranges from ∼1.5 eV to ∼3.5 eV, and is in the low-energy
2limit of Compton scattering (ν << mehc ), thus when the optical photon interacts withthe valence electrons Rayleigh scattering primarily happens rather than inelastic
scattering, although Raman scattering can also happen. That means, the boundelectrons in the valence orbitals with their oscillation frequency comparable to that ofincident light accelerate by the electric field and resonate in phase with its polarization,
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emitting the same photon energy in the same polarization (dipole radiation).
c
The refractive index n(λ) of the medium, namely the phase velocity n(λ)
ofelectromagnetic wave in the medium, and the symmetry inside the medium are also
very important in optical light-matter interaction especially when it comes to the opticallaser, where the fluence of light can become so high that nonlinear effects occur. In
this thesis however we will mostly consider particle-like properties of visible light, i.e.
Einstein coefficients, and accompanying energy transformation between the opticalphoton and the bound valence electron. Here we assume that only one electroninteracts with one photon at a time.

2.2.1 Absorption bands
Optical photoabsorption such as MLCT or LC transition (Fig. 2.3) is a dipole transitionof a valence electron from an energetically lower state to a higher state or evencontinuum upon visible light absorption, or an imaginary part of the refractive
index n(λ) of the medium. A well-known diagram and equation for the opticalphotoabsorption, i.e. optical attenuation, is the Beer-Lambert Law;
I(λ) = I0 (λ) ∗ e−(λ)cl(2.1)

Figure 2.5: A representative illustration of Beer-Lambert law. Apart from scatterings andnonlinear effects, photoabsorption has the largest cross section particularly at low photon
energy.

where I0 (λ) is the incident photon flux and I(λ) is the transmitted photon flux, (λ) is
the molar attenuation coefficient of the absorbent in case of solution phase, c[M] is
the molar concentration of the absorbent and l[cm] is the optical path length. The
molar attenuation coefficient (λ) is directly related to the attenuation cross section
σ(λ) of the absorbent like,σ(λ)[cm2 ] ∼ 3.82353216 ∗ 10−21 ∗ (λ)(2.2)

Therefore by measuring −ln II(λ)
at certain absorbent concentration and optical path
0 (λ)
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length, we obtain (λ) and σ(λ). This wavelength-dependent attenuation coefficient or
cross section indicates how probable the dipole transition upon corresponding photon
energy absorption in the target system is, i.e. Einstein B12 coefficient.
The dipole transition from the ground state (GS) to the excited state (ES) and viceversa is instantaneous that all relevant coordinates or degrees of freedom in the
inorganic chelates can be assumed to be stationary during the transition process(Franck-Condon principle). Thus, the vibrational overlap as well as the electronic
space and spin overlap between the GS and the ES are maximized at higher
vibrational level of the ES than that of the GS, resulting in so-called vibrationally "hot"excited state [186]. This region in a potential energy surface (PES) diagram with
relevant coordinates to the inorganic chelates, where the dipole transition proceeds
under the Franck-Condon principle, is therefore called as Franck-Condon region. This
principle is one of the most important rules in spectroscopy and quantum chemistry,
because the excessive vibrational energy not only triggers coherent nuclear motions
at certain frequencies but also allows an additional access to other excited states
while it dissipates into different nuclear and electronic degrees of freedom in therelaxation process.

Figure 2.6: One example of the Franck-Condon principle represented in the PES diagram of aCu(I) model complex (Figure from [94]). The sky blue arrow and the dashed arrow indicate two
Franck-Condon regions where the relevant dipole transition occurs to a higher vibrational levelof the final state.

Figure 2.7 shows optical photoabsorption spectra of several Fe(II) chelates with(pseudo-)octahedral symmetry in different solvents as an example [112]. Let’s

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first have a look on [Fe(bpy)3 ]2+ (bpy = 2,2’-bipyridine) in water. If we imagine avery similar molecular orbital diagram for this chelate with the one in Fig. 2.3, werecognize that the dipole transitions between 300 nm and 700 nm are not delta
functions, i.e. not occuring in a very narrow photon energy range precisely from A
state to B state, but rather broadened and overlapped with each other. This reflects
not only the complexity of the valence orbital energetics in the inorganic chelates,
but also the fundamental limit of probing the exact electronic structure inside thematerial. That is, the valence molecular orbitals consist of coupled electron clouds
that are vulnerable to thermodynamic parameters like temperature or pressure,
as well as nearby circumstances such as solvent molecules or dissolved oxygen
surrounding the inorganic chelates, rearranging its electronic structure accordingly
into a new equilibrated state and consequently allowing some additional dipole
or even quadrupole transitions. Room temperature, namely ∼25 meV, for example
may perturb the dipole transitions between different vibrational levels, and different
solvent environments may vary the dipole transitions significantly. The fundamental
limit in optical spectroscopy predominantly originates from instrumentation, in otherwords the incident electromagnetic wave is not ideally monochromatic, leading to thebroadened dipole transitions in Gaussian profile. All these factors collectively result inthe so-called absorption "bands".

Figure 2.7: Optical photoabsorption spectra of several Fe(II) chelates with (pseudo-)octahedral
symmetry in different solvents (terpy = 2,2’;6’,2”-terpyridine; Figure from [112]).

In case of [Fe(bpy)3 ]2+ in water, the absorption bands below ∼320 nm correspond to
LC transitions (from ligand π to ligand π*), meaning that these absorption bands arecharacteristics of bpy-ligand. At least two absorption bands lie between ∼320 nm and∼400 nm with the tails from the LC absorption bands overlapped, which correspond to
MLCT transitions (from metal ion t2g to ligand π*). Additional two absorption bands
between ∼460 nm and ∼560 nm also correspond to MLCT transitions, but from the
GS to the lowest singlet MLCT excited state (molecular spin multiplicity = 1), i.e. thelowest Franck-Condon state.

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The analogous interpretation of the absorption bands can be applied to other Fe(II)chelates. [Fe(terpy)2 ]2+ in water however shows an additional MLCT absorption band
above ∼600 nm, which corresponds to the spin-forbidden dipole transition from thesinglet GS directly to the triplet MLCT excited state (molecular spin multiplicity = 3).
Such a transition is formally forbidden by selection rule, but can be partially allowed
especially when the molecular symmetry is not perfect and the nuclear vibrations of
the chelates are coupled to its valence molecular orbitals, so that the spin-forbidden
dipole transition gains some probability via overlapped spin-allowed transitions. Sameapplies to [Fe(terpy)(CN)3 ]- in water where one terpy-ligand is replaced by three
cyanides, thus the molecular symmetry is even further broken. The absorption bandsof [Fe(terpy)(CN)3 ]- change significantly however in DMF, demanding an additional
perspective for its interpretation (solvatochromic effect).
In comparison with another similar case [112], where two bpy-ligands of [Fe(bpy)3 ]2+are replaced by four cyanides, the same solvatochromic effect is observed particularlyin the solvents with -OH hydroxyl group. The effect has been ascribed to theinteraction of strong Lewis acidity solvents such as water or MeOH with the nitrogenlone-pair of the cyanide ligand, destabilizing the LUMOs on bpy-ligand. Hence thelowest MLCT absorption band corresponds to the lowest singlet MLCT excitedstate on the bpy- or terpy-ligand, and the second lowest MLCT absorption bandcorresponds to energetically higher MLCT excited state on the bpy- or terpy-ligand.
Back to the [Fe(terpy)(CN)3 ]- in DMF, an absorption band above ∼640 nm is thenattributed to the triplet MLCT excited state, and another between ∼330 nm and ∼360
nm is attributed to energetically much higher MLCT excited state on the bpy- or
terpy-ligand.
Optical photoabsorption is a unique way of energy transformation in which theincident photon energy is harvested into the potential energy of the absorbent. Unlike
electrochemical or thermal excitation, with the photon energy one can selectively
approach to a specific excited state, being able to tune the behaviour of an excitedelectron, i.e. a quasi-particle, in a desired way.

2.2.2 Relaxation dynamics
Once a valence electron is promoted to the excited state manifold with an excessive
vibrational energy, this intermediate state relaxes non-adiabatically within so-called
ultrafast timescale, generally shorter than a µs, releasing its potential energy into
different nuclear and electronic degrees of freedom and eventually getting back to
the lowest equilibrium state, i.e. the GS. The intermediate state here is defined
as an excited state where a vacancy in the HOMO orbital appears and in turn an

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excited electron is present in one of the LUMO orbitals, thus forming a transient dipole
moment inside the chelate under adjusted effective nuclear charges and shielding
effects. In this state the ligand field predominantly becomes weaker due to a newly
occupied antibonding molecular orbital and affects the electronic structure accordingly,
which often stabilizes itself with higher multiplicity particularly in Fe(II) or Co(III) 3d6
chelates.
Throughout the thesis we will primarily deal with the MLCT excited state, because ingeneral compared to other excited states one can utilize the incident photon energy
most efficiently [141]. According to the selection rule, the singlet MLCT state (1 MLCT
state) is the most probable intermediate state right after the optical photoabsorption
in the inorganic chelates. In this 1 MLCT state, where the chelate still retains its
symmetry, i.e. the Franck-Condon state, the chelate can fluoresce the same photon
energy as the incidence by directly relaxing back to the GS [25; 69; 24; 94], or anothercompeting relaxation pathway is a so-called intersystem crossing (ISC), flipping
the spin state of an excited electron in 1 MLCT manifold to the triplet (3 MLCT state;molecular spin multiplicity = 3). This spin-flip is very favourable since the 1 MLCT and
3

MLCT states are energetically and spatially so close to each other, proceeding

typically within 100 fs which is way faster than the fluorescence (∼10 ns), dominating
the first relaxation pathway.

Figure 2.8: Schematic illustration of the relaxation dynamics in [Fe(bpy)3 ]2+ upon 530 nm
photoexcitation, represented in Fe-N distance coordinate (LS = Low-Spin, HS = High-Spin;
Figure from [123]).

At the same time the excessive vibrational energy in the 1,3 MLCT manifold starts
dissipating into a few nuclear coherent motions at certain matching frequencies (Fig.
2.8). While following relevant relaxation dynamics proceed, the nuclear wavepacket
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hundred of femtosecond [123].
The weakened ligand field also plays a very important role in the beginning of therelaxation as we shortly discussed, not only by rearranging the electronic structure
but also by distorting the molecular symmetry accordingly to lower the overall energyof the chelate (Jahn-Teller effect). The Jahn-Teller effect as well as the coherent
nuclear vibration is usually stronger where the electron density associated withthe coordination bonds is more concentrated, i.e. in 3d-chelates than in 4d- or
5d-chelates. Hence the 3d-chelates, for example [Fe(bpy)3 ]2+ (Fig. 2.8), tend toundergo the spin-allowed transition named as spin crossover (SCO) from the 3 MLCT
state to a so-called HS state (molecular spin multiplicity > 3) via the 3 MC state (eachof two parallel unpaired electrons in eg * and t2g orbital respectively), and the distortionof its geometric structure from the Franck-Condon state to further minimize the overall
energy of the intermediate state. Such a non-radiative decay from the MLCT state to
the MC state also competes with another spin-forbidden radiative decay getting backto the GS, which thus phosphoresces a lower photon energy than the incidence [152].
The chelates with relatively weak ligand field (small ∆) therefore exhibit the high spin
multiplicity as well as the distorted molecular structure without any phosphorescenceupon photoexcitation, whereas the chelates with strong ligand field (large ∆) show thephosphorescence for a comparatively long lifetime, i.e. >1 ns, and keep its molecularsymmetry [59].
The transient dipole moment in the 3 MLCT state is not only the driving force to trigger
the sequential reactions via energy transfer, but also the vulnerable instant where anelectron acceptor can formally oxidize the chelate [141]. Thus residing in the 3 MLCT
state long enough for the following reactions to take place is the key of designing thelight-harvesting inorganic chelates. In case of the 4d- and 5d-chelates, the ligand field
is strong enough to have the 3 MLCT state as the lowest excited state with a bit ofexceptions in Ru(II) 4d6 chelates [60]. In the 3d-chelates however, especially the Fe(II)or Co(III) 3d6 chelates, the ligand field is often so weak that the LS to HS transition(SCO) dominates the second relaxation pathway, typically within 1 ps.
In this regard one way of tuning the ligand field in 3d6 chelates is the NHC ligand
coordination, replacing the nitrogen coordinations with the carbon ones. As webriefly reviewed in Section 2.1.1, the unbalanced electron density on the metal ion
from the NHC ligand coordination consequently makes the MC excited states beenergetically higher (Fig. 2.9), such that either the 3 MLCT state or the 3 MC state
becomes the lowest excited state of the 3d6 chelate, eventually extending the lifetimeof the transient dipole moment in the 3 MLCT state.

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Figure 2.9: Comparison of the relaxation dynamics in [Fe(bpy)3 ]2+ (left) and one of the
Fe(II) NHC chelates (right), where 4 nitrogen coordinations are replaced with carbon ones,
represented in a simplified PES diagram of each system (Figure from [130]).

The third relaxation pathway from the MC excited states back to the GS is formallyspin- and symmetry-forbidden and non-radiative, thus taking usually longer than∼100 ps [24]. This relaxation is however still favourable because in the MC states themolecular symmetry is broken by the weakened ligand field, namely an occupied
σ*(eg *) molecular orbital, making the actual transitions be coupled with other
dipole-allowed relaxation processes.
From the flashback to Fig. 2.3, we recognize that the excited electron, i.e. thequasi-particle, can be present in either a π* molecular orbital or a σ* molecular orbital,
each of which corresponds to the MLCT state or the MC state respectively. Hence
there are not many different options for the relaxation pathways in the inorganic
chelates, but the complexity of it in fact comes from numerous couplings between
vibrations, spin states, and orbitals at the valence-level. After each dipole transition
between two quantum states, the vibrational level in the final state is higher than thatin the initial state (Franck-Condon principle), and a so-called vibrational cooling to thelowest vibrational level in the final state takes usually a few to tens of picoseconds[34; 175]. The solvent cages also respond to the relaxation dynamics in the inorganic
chelates upon photoexcitation, and the consequent rearrangement of the solvation
shells proceeds on different timescales depending on the solvent viscosity and thenumber of nuclei involved, ranging from few tens of femtoseconds to hundreds ofpicoseconds [126; 180; 26].

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2.2.3 Remark: Dark chemistry
What causes a chemical reaction? A spontaneous chemical reaction in a closed
system occurs in the direction of lowering its free energy, i.e. the new equilibrium.
A collision between reactants is a prerequisite for the spontaneous chemical
reaction in a mobile phase, with exceptions of self-decomposition or isomerization.
Rate-determining steps in the reaction could be therefore the collision of the reactants
in a proper orientation, or the activation energy of the reaction to be overcome(Maxwell-Boltzmann distribution). Hence the chemical reaction is often limited bygeneral diffusion processes, but still proceeds as long as the free energy of thesystem can be lowered.
As we briefly remarked before, the optical photoabsorption is a unique way of energy
transformation since the resulting excited state, in other words the activated complex,
can possess an enough potential energy to overcome the reaction barrier and thus
open up the following reactions which are otherwise not at all spontaneous. At thesame time however, many important chemical reactions in nature such as catalysis do
not require visible light (dark chemistry). Thus what we have discussed in this section
is not much relevant to the dark chemistry, but is yet essential for the light-driven
chemistry like photosynthesis.

2.3 X-ray light-matter interaction
Unlike visible light, X-ray is neither always nor everywhere on Earth, but it isnevertheless familiar to us thanks to its useful applications in practice. X-ray, ingerman word Röntgenstrahlung, is defined as photon whose energy is between∼0.1 keV and ∼100 keV, or electromagnetic wave whose wavelength is between∼12.4 nm and ∼0.124 Å, although both soft X-ray region up to ∼1 keV and hard X-ray
region above ∼10 keV can be also classified as extreme ultra-violet (EUV or XUV) andGamma-ray respectively. Since such a broad range of photon energy can be identified
as X-ray but the light-matter interaction quite differs depending on the photon energy,
commonly the X-ray science differentiates it into the soft X-ray (up to ∼3 keV) andthe hard X-ray (above ∼5 keV). The photon energy between ∼3 keV and ∼5 keV isoften named as tender X-ray, but this photon energy is less accessible either due tothe limited number of available photon sources or interesting scientific cases. This
thesis will predominantly handle the hard X-ray and its interaction with the core-level
electrons in the metal ion, because these are highly relevant to mapping the electronic
and geometric structure of the inorganic chelates rather than other X-ray regime.
The hard X-ray photon energy covers from K-shell absorption edge of Titanium to that
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Figure 2.10: The femtosecond, coherent hard X-ray beam from the SASE1 undulator at
European XFEL. The fluorescence emitted by irradiated nitrogen molecules in air was recorded(Figure from European XFEL).

of all base and noble metals, and the hard X-ray wavelength ranges from ∼2.48 Å to∼0.124 Å which is on the scale of all kinds of chemical bond lengths or even shorter.
Hence the hard X-ray is the best probe for the inorganic chelates in solution phase not
only to element-specifically investigate its electronic structure by K-shell core-hole
spectroscopy, but also to directly observe the distribution of chemical bond lengths
inside by X-ray solution scattering. In the end, strongly coupled ultrafast dynamics in
the inorganic chelates upon optical photoexcitation can be disentangled with thesesimultaneous and complementary hard X-ray techniques.

2.3.1 X-ray absorption
The hard X-ray light-matter interaction is by itself an established research field
concerning the refractive index n(λ) smaller than 1 in any material (thus the phasecvelocity n(λ)
of the X-ray in any material "exceeds" the speed of light in vacuum) andthe Bragg’s law (reflection geometry; the Laue equations in transmission geometry
won’t be discussed in this thesis). Moreover when it comes to the X-ray optics, one
should take account of both the real and imaginary part of the refractive index at
certain incident photon energy in certain material, since the X-ray photoabsorption
and accompanying thermal deformation (heat load) of the material can affect its

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refractive index accordingly. This is also important in terms of the loss of X-ray photon
flux, and in case of Bragg diffraction resulting changes in the lattice constant of thecrystal can broaden the diffracted beam in energy.
In this thesis the incident photon energy between ∼7 keV and ∼22 keV will be primarily
discussed, and as shown in Fig. 2.11, within this range the X-ray photoabsorption
dominates the light-matter interaction unless the incident angle is close to the criticalangle of the surface material or satisfies the Bragg’s condition, although for lightelements such as carbon or nitrogen the incoherent (e.g. Compton) scattering
processes start dominating above ∼20 keV. Therefore, in the following sections andchapters the K-shell core-hole spectroscopy upon X-ray photoabsorption will be thecentral topic.

Figure 2.11: X-ray mass attenuation coefficients for all the cross sections in carbon and iron.
The X-ray photoabsorption is the main process that causes X-ray attenuation in carbon and ironup to ∼20 keV (Figure from https://xraypy.github.io/XrayDB/).

The K-shell core-hole spectroscopy in general consists of X-ray absorption
spectroscopy (XAS) and X-ray emission spectroscopy (XES). Briefly speaking, theincident hard X-ray photon ejects one of the K-shell electrons in the metal ion, i.e.
one of the 1s2 electrons by photoelectric effect, either completely (photoelectron)or into one of the unoccupied (and also the unpaired) valence orbitals dependingon the photon energy (see also the Fermi’s golden rule from time-dependent
perturbation theory). In other words, as the incident photon energy gets higher acrossthe so-called K-edge (K-shell absorption edge) of the metal ion, the 1s electron
undergoes a dipole transition first to the unoccupied or unpaired valence orbital(pre-edge absorption), and afterwards to the continuum (post-edge absorption),
becoming a photoelectron whose energy is approximately the difference between theincident photon energy and the binding energy of itself.

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Right after this dipole transition, one core-hole in the K-shell thus appears which
will be immediately refilled by one of the outer electrons in the metal ion, releasing
the corresponding potential energy difference in hard X-ray fluorescence. Such a
dipole-allowed transition therefore only involves 2p- and 3p-orbitals in case of the
3d-chelates, and the X-ray fluorescence from each orbital is called as Kα and Kβ1,3emission lines respectively. Another very small cross section still exists at higher
fluorescence energy than the Kβ1,3 emission lines, namely the Kβ2,5 emission lines,
which involve not only the quadrupole transitions from 3d-orbitals but also theso-called cross-over transitions from ligand 2sp2 - and π-orbitals. The cross-overtransitions are possible thanks to the valence orbital overlaps between the metal
ion and the ligands, and will be discussed in the following section. The consequenthole state in the outer shell after the hard X-ray fluorescence will be then refilled
by either an electron from the valence orbitals or the photoelectron from the X-ray
photoabsorption (the Auger effect will be also discussed in the following section).

Figure 2.12: K-shell core-hole spectroscopy on a Cu(I) model complex in solution phase. The
simplified atomic orbital diagram represents the core-hole intermediate state right after the
X-ray absorption and before the X-ray emission. The corresponding XAS spectrum (Figure from[102]) and XES spectra are also shown.

Hence, the XAS is optimal to investigate the unoccupied valence electronic structure
as well as the first and second coordination shell structure, whereas the XES is
unique for studying the occupied core- and valence-level electronic structure, both inan element-specific manner. Figure 2.12 shows an example of the K-shell core-hole
spectroscopy.

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X-ray absorption fine structure (XAFS) refers to the complex absorption feature in
the vicinity of the K-edge of target element, including the pre-edge structure andthe so-called white line (intense absorption peaks near the K-edge). The region is
also named as X-ray absorption near edge structure (XANES; Fig. 2.13), and its
complexity originates not only from the coupled valence molecular orbitals but also
from the core-hole intermediate state. That means, in case of single atom XANES
in gas phase such as noble gases its spectrum is relatively featureless, and whenit comes to the specific chemical environment surrounding the target element its
spectrum shows quite consistent XAFS features (Fig. 2.15). It is trivial that thecore-hole intermediate state (Fig. 2.12) is extremely unstable, and thus perturbs theeffective nuclear charge as well as the overall electron clouds accordingly. In termsof the Fermi’s golden rule this state is the final state of a dipole transition upon the
X-ray absorption, and the probability of this transition, i.e. the photoabsorption crosssection, therefore reflects the strength of the coupling between the initial state (GS)and the final perturbed state. In other words, the XANES feature is often difficult to
be attributed to one particular transition, but rather to the mixture of many different
contributions in the complex system (e.g. the coupled valence molecular orbitals
getting further perturbed in the final state).

Figure 2.13: XANES spectra of three cobalt chelates (sep = sepulchrate) in solution phasemeasured with Kα1 high-energy-resolution fluorescence detection (HERFD) technique. All
three complexes have 6 nitrogen coordinations and (pseudo-)octahedral symmetry. The
pre-edge structure clearly identifies the spin density in each system, whereas the post-edge
structure does not show any straightforward information.

The pre-edge structure however is sensitive to the molecular symmetry andspin density (Fig. 2.13), mainly because the corresponding dipole transition
concerns the core-to-valence transition (1s to 3d for the 3d-chelates) which isformally symmetry-forbidden. This is yet another privilege of the K-shell core-hole
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spectroscopy compared to its soft X-ray counterpart (L3 -shell core-hole spectroscopy),
namely the dipole-forbidden core-to-valence and valence-to-core transitions become
partially allowed if the molecular symmetry gets distorted or the spin density getscoupled to other degrees of freedom, such that the pre-edge absorption gains highsensitivity to the valence electronic and geometric structure [74].
It is noteworthy that the binding energy of the 1s electron is not necessarily equal to
the K-edge energy, since the final state of the K-edge absorption, i.e. the core-holeintermediate state, affects the energy needed for the transition. Once the incident
photon energy exceeds the K-edge energy, the 1s electron becomes a photoelectronwith the extra kinetic energy being the difference between the incident photon energy(E i ) and the K-edge energy (E K ), and propagates to the neighboring ligand system as
an outgoing quantum wave with its wavenumber k .
√
2π
k=
=
λ

2me (E i − E K )
h̷

(2.3)

Figure 2.14: Pictorial view of the multiple scattering of an outgoing wave off neighboring atoms(Figure from [164]). The topmost atom is the original source of the wave, which diffracts first off
the atom at the lower left and finally off the atom at the lower right. Each successive outgoingspherical wave is weaker, which is reflected in the thickness of the spherical wavefronts.

where λ is the de Broglie wavelength of the photoelectron, me is the electron mass,and h̷ is the reduced Planck constant. The photoelectron wave will then reflect offof the nearest atom once encountered, and in turn reflects another quantum wave
back toward the origin, so will other nearby atoms reflect the photoelectron wave
back towards the original atom (Fig. 2.14; [164]). The amplitude of all the reflected
quantum waves at the X-ray absorbing atom add either constructively or destructively
to the outgoing photoelectron wave and hence modulate the transition matrix elementin the Fermi’s golden rule between the initial state (GS) and the final core-hole state,
which controls the strength of the corresponding X-ray photoabsorption. Because this
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interference pattern changes with the energy of the photoelectron, i.e. its de Broglie
wavelength λ, the matrix element and consequently the X-ray absorption will exhibit
similar oscillations (Fig. 2.15). These generally weak oscillatory wiggles beyond
about 30 eV above the K-edge are termed EXAFS (extended X-ray absorption finestructure).

Figure 2.15: XANES and EXAFS spectra of two ruthenium chelates in solution phase (phen =1,10-phenanthroline; TAP = 1,4,5,8-tetraazaphenanthrene) measured in total-fluorescence-yield(TFY) mode. Given the similar chemical environment near the Ru(II) ion, the XANES features
as well as the EXAFS features of [Ru(phen)3 ]2+ and [Ru(TAP)3 ]2+ are almost identical.

The EXAFS contains precise information about the local atomic structure around theatom that absorbed the X-ray, namely the first and second coordination shell structure
in case of the inorganic chelates. This is because the final core-hole state, i.e. thephotoelectron state, and its wavelike nature must interfere with the neighboring atomic
structure as noted above. The theory of EXAFS has been therefore quantitatively
parameterized like the following equation,
χ(k) = S02 ∑ NRR

2 2
∣feff (k )∣∗ sin(2kR + 2δ c + Φ)e−2R/λk e−2σ k
2
kR

(2.4)

where the structural parameters are the interatomic distances R, the coordination
number NR , and the temperature-dependent root-mean-square (rms) fluctuation inbond length σ, which should also include effects due to structural disorder. In addition,∣feff (k)∣ is the effective backscattering amplitude, δ c is central-atom partial-wave
phase shift of the final state [164], and λk is the energy-dependent mean-free-pathof the photoelectron predominantly limited by inelastic losses as the quantum wave
traverses the material. For a wave reflected straight back by a neighboring shell of
atoms in distance R, the phase difference between the outgoing photoelectron wave

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and the reflected wave is approximately 2kR. Thus the dependence of the oscillatory
structure on interatomic distance and energy is clearly reflected by the sin(2kR) term.
The decay of the wave due to the mean-free-path or finite lifetime (including core-hole
lifetime) of the photoelectron is captured by the exponential term e−2R/λk . The phaseshifts δ c and Φ account for the difference between the measured and geometrical
2 2interatomic distances. The Debye-Waller factor, i.e. the vibrational damping, e−2σ kis due partly to thermal effects, which cause all of the atoms to jiggle around theirequilibrium atomic positions. Finally, the overall amplitude factor S02 is a many-bodyeffect due to the relaxation of the system in response to the creation of the core-hole(the shielding effect becomes weaker and the electrons become more stable). The
normalized EXAFS spectrum χ(k) is obtained from,(2.5)

χ(k) = [µ(k) − µ0 (k )]/∆µ0
where the X-ray absorption coefficient µ(k) without the atomic-like backgroundabsorption µ0 (k ) is normalized by ∆µ0 [164], and conventionally defined with respect
to the photoelectron momentum index k (2.3).

Figure 2.16: (left) A geometric structure of [Ru(phen)3 ]2+ illustrated by Chimera (center
oceanblue: Ru(II), blue: nitrogen, beige: carbon, white: hydrogen), (right) the normalized
EXAFS spectrum of [Ru(phen)3 ]2+ from Fig. 2.15 weighted by k 2 (above) and its Fouriertransform ∣χ(R)∣ (below). The fitting result of the EXAFS equation (2.4) to the experimental
data is also shown. The interatomic distance to the first coordination shell is ∼0.5 Å shorter than
the geometrical value (R ∼2.06 Å), since the standard equation (2.4) does not include the 1s
core-hole lifetime-broadening effect especially for the element with Z > 40 [181].

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Figure 2.16 shows the χ(k) spectrum of [Ru(phen)3 ]2+ weighted by k 2 as an example,
along with the fitting result of the EXAFS equation (2.4) to the experimental data
(S02 ∼0.91, R ∼2.06 Å, NR = 6, σ 2 ∼0.0021; detailed data process and fitting procedurecan be found in Chapter 6). The Fourier transform of the χ(k) spectrum corresponds
to the effective radial distribution function ∣χ(R)∣, with peaks near the first few
neighboring shell distances [164]. Although the ∣χ(R)∣ reveals several peaks up
to ∼4 Å or longer (Fig. 2.16), for the inorganic chelates only the first and second
coordination shell structure could be clearly identified owing to the e−2R/λk term. Thatmeans, the effective mean-free-path of the photoelectron wave is limited by not only
the inelastic losses on the way of propagation but also the 1s core-hole lifetime, sothat the coordination shells further away cannot significantly interact with the quantum
wave [181].
The 1s core-hole lifetime refers to the decay time constant of the 1s core-hole state by
the X-ray fluorescence, and this restricts the spatial resolution in the ∣χ(R)∣ in terms of
the limited mean-free-path, which simultaneously indicates that the energy resolution
in the χ(k) spectrum is also limited. Through the Heisenberg uncertainty principle,
any state with a finite lifetime does not have a sharp, delta-function-like energy level,
but rather a lifetime-broadened energy level. This gives rise to an energy uncertainty
̷of the level inversely proportional to its lifetime, i.e. t ∼ h/∆E.
In case of the optical photoabsorption, the excited state lifetime generally ranges
from ∼1 ps to ∼1 µs, which returns the energy uncertainty of ∼1 meV or smallerthat is negligible compared to the half width at half maximum (HWHM) of a usual
femtosecond pulse in energy. On the other hand, the excited state lifetime upon the
X-ray photoabsorption, namely the 1s core-hole state lifetime in our case, is ∼1 fs oreven shorter for the transition metal elements and this leads to the energy uncertaintyof ∼1 eV or larger [116]. In other words, the final state of the X-ray absorption is
uncertain in energy, making the XAFS (XANES + EXAFS) spectrum be broadened.
As we discussed earlier on the energy-broadening in the optical absorption bands, i.e.
the fundamental limit of probing the exact electronic structure inside the material, forK-shell core-hole spectroscopy the fundamental limit usually lies on the 1s core-hole
lifetime-broadening, namely the natural level width of the corresponding K-shell,
which results in the broadened dipole transitions in Lorentzian profile [181]. The
lifetime-broadening effect is frequently found in X-ray spectroscopies, especially if thecore-shell is so deep in potential energy that its absorption edge is in the hard X-ray
region [116]. Hence this effect will be also one of the central topics in the following
sections.

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2.3.2 X-ray fluorescenceNow we are in the second phase of the X-ray photoelectric effect upon the X-ray
absorption (Fig. 2.12). As noted earlier, the 1s core-hole in the intermediate state will
be refilled after its lifetime in accordance with the selection and golden rule, i.e. thehighest probability lies on the 2p to 1s dipole transition (Kα emission lines) followed by
the 3p to 1s dipole transition (Kβ1,3 emission lines). Thus, for the X-ray fluorescence
the initial state is the 1s core-hole state and the final state is yet another core- or
valence-hole state, not the GS (Fig. 2.17). That means, the X-ray fluorescence rate is
now determined by the strength of the coupling between the lifetime-broadened 1s
core-hole state and the final lifetime-broadened core- or valence-hole state.

Figure 2.17: (a) The first- and (b) second-order processes in the X-ray photoelectric effect(Figure from [181]). In the first-order transition an atom in the GS (o) absorbs an X-ray photon
in mode λ, and the atom gets excited to a photoelectron state k with the refilled core-hole byan outer electron from a state α, which consequently emits the X-ray fluorescence σ. In thesecond-order diagram, now there is a core-hole lifetime which delays the refilling process.

In the previous section, the critical factors for the XAS on the inorganic chelates were
the incident photon energy and the corresponding final state of the X-ray absorption,even though this is quite complicated for the inorganic chelates. The resulting
core-hole state has a very short lifetime which leads to the lifetime-broadened X-ray
absorption in Lorentzian profile (the K-level width in HWHM, ΓK ; [116]). The 1s
core-hole will be then occupied by the 2p3/2 electron in case of the Kα1 emission, andthe 2p3/2 electron undergoes the 2p3/2 core-hole lifetime-broadened transition to the1s core-hole lifetime-broadened energy level (the L3 -level width in HWHM, ΓL3 ; [45]).
Therefore, the natural shape of an emission line in the frequency or energy scale is a
Lorentzian, according to the both classical and quantum theory [84]. This is observedin practice for a large number of Kα emission lines, and they are highly symmetricand can be represented by a Lorentzian profile. By contrast, it is also well-known thatthe experimentally observed Kα and Kβ1,3 emission lines of the 3d transition metals
are pronouncedly asymmetric. The first appearance of the asymmetric features
was clearly found at an incident photon energy coinciding with the threshold for the

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36 of 171creation of a 1s3d two-hole configuration (multi-electron excitation; [84]). In otherwords, the 3d electron as well as the 1s electron gets photoexcited once the incident
photons have sufficient energy, and the presence of a 3d "spectator"-hole during theemission process introduces additional splitting in the initial and final energy levels,
consequently increasing the number of distinct transition lines [45].
Accordingly, Hölzer et al. were able to show that the line shapes of the Cu Kα andKβ1,3 emission lines can be fully accounted for by contributions from 3d-spectatortransitions only, in addition to the diagram ones (pure one-hole, single-electrontransitions; [84]). Nevertheless, to fully interpret the origins of the emission lines
we should also consider the ligand field multiplet structure of each emission line, if
needed including the molecular orbital theory, and the electron-electron interactions
as well as the corresponding spin-orbit coupling [74].
In other words, the emission processes in the 3d transition metals are highly
dependent on the spin state of the electron taking part in the transition, namely if
the 1s electron is spin up for example (Fig. 2.12), the 2p or 3p electron refilling the1s core-hole should be spin up as well. Due to the large overlap between the 2p(3p) and the 3d wave functions [74], the exchange interactions between the 2p or
3p core-hole and the 3d electrons are considerable especially if the 3d electrons
are unpaired, affecting the Kα and Kβ1,3 emission processes magnetically. That is
how the emission lines get sensitive to the spin density in the 3d-chelates, and why
the emission lines become pronouncedly asymmetric (for high Z materials, such as
rhodium or uranium, where the 3p spin-orbit splitting is large enough, the Kβ1 and Kβ3emission line appear distinct and symmetric; [74]).
In addition to the Lorentzian nature of the line shapes, the Gaussian broadening
from the instrumental factors contributes inevitably, and this aspect will be further
discussed in Chapter 3 and 7.
With a deeper understanding of the X-ray fluorescence processes, we will
now discuss the Kα, Kβ1,3 , and Kβ2,5 emission lines of the 3d-chelates bytaking advantage of the [Cu(dmp)2 ]+ (dmp = 2,9-dimethyl-1,10-phenanthroline)
photosensitizer data as an example (Fig. 2.12). Even though no single theoretical
spectrum will be introduced in this thesis (the theoretical spectra can be found in [74;45] for instance), we will try to analyse and interpret the emission lines by fitting the
Voigt profile V (x; σ, γ) (a probability distribution given by a convolution of a Lorentzian
distribution L(x; γ) and a Gaussian distribution G(x; σ); see below).

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Figure 2.18: The simplified energy scheme and nomenclature for the X-ray fluorescenceprocesses in the 3d-chelates upon the K-edge absorption (Figure from [74]). The respective
electronic configuration denotes the corresponding hole state. Here, only the diagramtransitions are indicated.
∞

G(x ′ ; σ)L(x − x ′ ; γ) dx ′V (x; σ, γ) = ∫−∞1x2
G(x; σ) = √ exp(− 2 )
2σ
σ 2π
γ/π
L(x; γ) = 2
x + γ2The Voigt profile is chosen to cover both the natural Lorentzian line width and the
instrumental Gaussian line width [134; 1; 106]. The data presented in this section
were measured with the incident photon energy of ∼10 keV (∼1 keV above the Cu
K-edge). Detailed experimental conditions can be found in Chapter 7.
Figure 2.19 shows the Kα emission spectrum of the [Cu(dmp)2 ]+ photosensitizer.
The main two peaks of Kα1 and Kα2 stem from the 2p3/2 and 2p1/2 to 1s dipole
transition respectively. The fluorescence energy splitting between Kα1 and Kα2(∼20 eV) originates from strong spin-orbit interaction of the 2p core-hole in the final
state of the Kα fluorescence [45; 74]. The best fitting required two Voigt profile
components for the Kα1 but only one component for the Kα2 , mainly due to the very
weak intensity of a potential Kα22 emission line which was therefore difficult to be fit[45]. As denoted next to the Kα1 emission line, the Kα11 fluorescence corresponds
to the 2p3/2 to 1s diagram transition with only one core-hole involved, whereas the

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Figure 2.19: Kα emission lines from the [Cu(dmp)2 ]+ photosensitizer analysed by fitting the
Voigt profile components [45; 134]. The corresponding initial and final hole states to theemission lines are denoted. Detailed data process and fitting procedure can be found in
Appendix B and C.3.

Kα12 fluorescence represents the 2p3/2 to 1s dipole transition in the presence of anadditional 3d spectator-hole created by the so-called double-ionization (two-electron
excitation; [45]). The additional fluorescence energy splitting between Kα11 and Kα12(∼2 eV) is hence owing to the exchange interaction between the 2p core-hole and the
3d electrons [74].
Figure 2.20 shows the fitting result of Voigt profile components to the Kβ1,3 emission
spectrum of the [Cu(dmp)2 ]+ photosensitizer. The best fitting required 5 Voigt profile
components, each of which is attributed to Kβ1 , Kβ3 , Kβ1′ , Kβ ′ , and KMN emission
lines [45; 54]. The main two peaks of Kβ1 and Kβ3 stem from the 3p3/2 and 3p1/2 to
1s diagram transition respectively, but this time the fluorescence energy splitting
between them is ∼10 times smaller (∼2 eV) than that between the Kα1 and Kα2 ,owing to much weaker spin-orbit interaction of the 3p core-hole in the final stateof the Kβ1,3 fluorescence [45; 74]. However, in case of the Kβ1,3 fluorescence theelectron-electron interactions between the 3p and 3d wave functions are ∼3 times
stronger, which thus result in its distinct and asymmetric spectrum line shape with Kβ1′
and Kβ ′ spectator-hole satellite lines present [74].
The spectator-hole satellite lines correspond to the 3p3/2 and 3p1/2 to 1s dipole
transition under the (3p, 3d) exchange interaction after the double-ionization [45;74]. That means, an additional hole to the 1s core-hole appears simultaneously in
3d-(or even in 3p-, 3s-)orbital during the 10 keV photoabsorption, and the following
spin density from the unpaired electrons in M-shell as well as the 3d spectator-hole

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Figure 2.20: Kβ1,3 emission lines from the [Cu(dmp)2 ]+ photosensitizer analysed by fitting the
Voigt profile components [45; 134]. Detailed data process and fitting procedure can be found in
Appendix B and C.3.

affects the Kβ1,3 fluorescence. Therefore, the Kβ1′ and Kβ ′ spectator-hole satellite
lines appear at either lower or higher fluorescence energy than the Kβ1 and Kβ3
diagram transitions, and become much broader than the Kβ1 and Kβ3 emission lines
due primarily to the abovementioned interactions. In addition, a very weak and broad
KMN satellite line appears from the radiative Auger decay, i.e. the Kβ1,3 fluorescenceloses its partial energy to the M-shell to N-shell excitation (double-ionization during
the emission; [54]).
The Kβ2,5 emission spectrum of the [Cu(dmp)2 ]+ photosensitizer was sharply resolved(Fig. 2.21), although the signal-to-noise ratio (SNR) is much lower than that of the Kα
and Kβ1,3 emission spectra. The origins of the Kβ2,5 emission lines are predominantly
the cross-over transitions from the ligand 2sp2 - and π-orbitals [74]. The main two
peaks right below the pre-edge absorption energy of [Cu(dmp)2 ]+ (8979.5 eV;[102]), which represents the quadrupole-allowed transitions of the 1s electron to theunoccupied 3d-orbitals, correspond to the dipole-allowed transitions of an electronfrom the HOMOs to the metal ion 1s core-hole.
In fact, an additional Kβ ′′ satellite line from the ligand 2sp2 -orbital to the metal ion 1s
core-hole (this transition is not dipole-forbidden since the center of symmetry of two
orbitals are different; [74]) should appear ∼10 eV below the main Kβ2,5 fluorescence incase of the nitrogen coordination [138; 134; 159; 43; 196]. However, in our case theKβ ′′ satellite line was not clearly observed owing to an overlapping KNN satellite line(radiative Auger decay; the Kβ2,5 fluorescence loses its partial energy to the valenceshell to continuum excitation [54]) at the similar fluorescence energy. Instead, other

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Figure 2.21: Kβ2,5 emission lines from the [Cu(dmp)2 ]+ photosensitizer analysed by fitting the
Voigt profile components [54; 134]. The final hole states of the emission lines as well as the
KNN satellite line are denoted. Detailed data process and fitting procedure can be found in
Appendix B and C.3.

peaks above the Fermi level were clearly observed, and these satellite lines result
from the so-called shake (or multiple electronic) transitions due mainly to the 1s
core-hole [74]. As we shortly discussed earlier, within the 1s core-hole lifetime theweakened screening effect causes a readjustment of the overall electron clouds to theperturbed potential, letting some LUMOs like ligand π* or σ* orbital be temporarily
occupied and thus the cross-over transitions from the LUMOs to the metal ion 1s
core-hole be possible [74]. Furthermore, an additional spectator-hole from the
double-ionization and the following exchange interactions as well as the molecularorbital theory, will further increase the number of factors associated with the Kβ2,5emission lines. For instance, we expect the KLβ emission line appearing at ∼8990 eV[73], which corresponds to the Kβ1,3 fluorescence in presence of the 2p core-hole andthe 1s core-hole (Zn(Z+1) L3 -edge + Cu Kβ1,3 - Cu L3 -edge ∼8990 eV), because the
10 keV incoming photon energy was enough to create an additional 2p core-hole tothe 1s core-hole (Cu K-edge + Zn(Z+1) L3 -edge ∼10 keV).
Up to now we have discussed how the hard X-ray photons are absorbed and emitted
by the inorganic chelates. Indeed, these are not simple in the condensed phase
especially with the deep core-hole in presence, and this thesis has only shown a
few specific cases with very limited level of the theory, namely the fitting analysis
provided is only phenomenological. Nevertheless, we will be able to extract thechemical information of our interest from the X-ray spectroscopic data we measured,in accordance with our understanding from this chapter.

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41 of 171As a remark here, the resulting hole state in the outer shell in response to thereoccupied 1s core-hole in the third phase of the X-ray photoelectric effect upon
X-ray absorption, will be refilled by either an electron from the valence orbitals or thephotoelectron from the X-ray absorption. The Auger effect is a non-radiative decay
in the second phase of the X-ray photoelectric effect, where the dipole transition
from the outer shell to the 1s core-hole emits not the photon but the Auger electron,
whose kinetic energy becomes then approximately the difference between the Kα2
fluorescence energy and the L3 -edge energy in case of the KL2 L3 Auger transition.
While the Auger process is the main relaxation process after the soft X-ray absorption,
the radiative decay becomes dominant in the hard X-ray regime.

2.3.3 Remark: Resonant inelastic X-ray scattering
As pointed out earlier, the first appearance of the asymmetric features in the emission
lines was clearly found at an incident photon energy coinciding with the threshold forthe creation of a 1s3d two-hole configuration [84]. In other words, the 2p3/2 core-hole
lifetime-broadened transition to the 1s core-hole lifetime-broadened energy level (Kα1emission) is dependent on the incident photon energy. In the previous section, we
discussed the Kα, Kβ1,3 , and Kβ2,5 emission lines of the [Cu(dmp)2 ]+ upon 10 keV
photoabsorption (∼1 keV above the Cu K-edge, where the EXAFS feature is no longer
present). What will happen to the emission lines if the incident photon energy getscloser to the Cu K-edge, or even to the pre-edge structure?
The pre-edge absorption (core-to-valence transition; 1s to 3d for the 3d-chelates) is
the resonant excitation especially for the Kβ2,5 emission (valence-to-core transition; 3d
to 1s for the 3d-chelates although it is only quadrupole-allowed), and in terms of theone photon-in, one photon-out concept (Fig. 2.17), the other emission processes canbe treated as inelastic X-ray scattering processes. Hence, the resonant inelastic X-ray
scattering (RIXS) spectroscopy is to map the inelastically scattered emission lines
while scanning the resonant excitation energies across the pre-edge structure.
In this section, we will again refer to the same energy scheme (Fig. 2.22) as Fig. 2.18,
and accordingly indicate the total energy of the intermediate state, where the excited
1s electron is in one of the LUMOs, with the incident photon energy Ω, and the totalenergy of the final state, where the excited 1s electron is still in the same LUMO but
one of the 2p or 3p electrons undergoes the dipole transition to the 1s core-hole,with the energy transfer Ω − ω (not with the fluorescence energy ω; [74]). In this way,
the RIXS spectra can be viewed as a map of the final states in the vicinity of theintermediate states. That means, through the RIXS plane we are able to monitor
both the final states approachable from one specific intermediate state, and the

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intermediate states that can proceed to one specific final state. Moreover, one of themain transition channels (ω) between the initial and final states can be chosen toprobe the transition rates in the XANES region (Fig. 2.13).

Figure 2.22: The simplified energy scheme and nomenclature for the RIXS processes (Figure
from [74]). The relative transition intensities are given as an example. Here, only the diagramtransitions are indicated.

In the XANES region, particularly below the K-edge, the excited 1s electron is in oneof the unoccupied or unpaired valence orbitals, and the X-ray fluorescence processes
proceed in the presence of a spectator-"electron" [74]. Therefore, the final statesnear the pre-edge structure become more characteristic to the electronic structureof the corresponding molecular system. Figure 2.23 shows the 1s2p RIXS planeof the [Co(bpy)3 ]2+ in solution phase as an example (same data with Fig. 2.13; the1s2p RIXS planes of other cobalt chelates can be found in Appendix A). The final
states near the pre-edge structure clearly separate themselves from the strong
dipole-allowed K-edge absorption, and exhibit distinct features of the [Co(bpy)3 ]2+ ,even though they are very weak owing to the quadrupole transitions.
The two strong transition channels between the initial and final states are the Kα1 andKα2 , and the XAFS measurement technique along the specific channel is called as
HERFD (Fig. 2.13), since one particular final configuration is observing the X-ray
absorption rates across the XAFS range, excluding any other line width broadening
effects from other final configurations. The XAFS spectrum can be recorded by either
measuring the transmitted photon flux or the TFY (Fig. 2.15), but both techniques
measure the line width broadening effects from all final electronic configurations.
Thus, as long as the instrumental Gaussian line width does not overwhelm the natural
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Figure 2.23: 1s2p RIXS plane of [Co(bpy)3 ]2+ in solution phase. The instrumental Gaussianline width was ∼1 eV in full width at half maximum (FWHM). The horizontal axis also
corresponds to the total energy of the intermediate state, and the vertical axis to the totalenergy of the final state. The two strong transition channels represent the Kα1 and Kα2 , which
are asymmetric due to the multiplet interaction [74].

Lorentzian line width, the HERFD technique improves the energy resolution of the
XAFS spectrum [74].
The Kα1 and Kα2 emission lines along the y-axis at an incident photon energy close
to the Co K-edge are still asymmetric, due primarily to the multiplet interaction. The
characteristic pre-edge features can be also interpreted by analysing the spectra on
the x- and y-axis [74]. Hence, the very advantage of the RIXS spectroscopy is toeffectively separate out the lowest unoccupied molecular orbitals localized on themetal ion using the hard X-rays, to investigate the valence electronic structure in amuch detailed manner.

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3 Experimental methodology

Seeing is believing, and believing is seeing.

3.1 Time-resolved experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 Femtosecond optical laser system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.1

Experimental setup at University of Bern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 Synchrotron light source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1

11-ID-D Endstation at APS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

3.4 X-ray free-electron laser (XFEL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.4.1

BL3EH2 Endstation at SACLA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.4.2

FXE Endstation at European XFEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

The inorganic chelates consist of a metal ion with deeply localized core electrons, andthe ligands coordinated to the valence hybridized orbitals of the metal ion. Opticaland hard X-ray photons interact with the inorganic chelates in many different ways,
mostly depending on their photon energy. But when it comes to the photoabsorption,
the annihilation of a photon leads to an excited state which will decay through various
intermediate states and processes, on the ultrafast timescale (<1 µs).
Following the discussions in Chapter 2, this chapter will be dedicated to the
instrumentations for the study of ultrafast dynamics upon the optical or X-ray
photoabsorption. Firstly, the basic idea of the time-resolved experiments with
so-called ultrashort pulses will be discussed, and then the femtosecond optical laser
system as well as the endstations at synchrotron light source and X-ray free-electron
laser (XFEL) facilities employed for this thesis will be introduced. This thesis however
does not cover all the details behind the experimental setups presented, since theseare beyond the scope of the thesis and can be found elsewhere.

3.1 Time-resolved experiments
How do we resolve dynamics in time? We need first of all our "eye" to process
the transmitted signal from the dynamics. For instance, we perceive the opticalsignal from our daily dynamics via our eyes, and the timescale of the dynamics
typically ranges from a second to an hour. We can further resolve the dynamics into

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45 of 171a much shorter time domain, once the recording frequency of our eye gets higher.
Nowadays one can reach the recording frequency of ∼1 MHz or higher by exploitingan "electronic" eye (detector), thus achieve the time resolution of ∼1 µs or shorter.
But what if the dynamics of our interest occur on even shorter timescales than amicrosecond, e.g. the very first step of photosynthesis within a picosecond?
For such an ultrafast regime, we can think of employing the ultrashort pulses. Thatmeans, two ultrashort pulses with a certain arrival time delay between them interact
with the system of our interest. The first pulse (pump pulse) initiates the relevant
ultrafast dynamics in the system and the second pulse (probe pulse) then takes asnap shot of the dynamics after the time delay. The snap shot contains the integrated
information within the full width at half maximum (FWHM) of the pulse in time domain,which therefore determines the time resolution of the measurement. For example, if
the FWHM of the pulse in time is on the order of a few femtoseconds, the dynamics
taking place for longer than a picosecond can be resolved by applying the time delayof few tens of femtoseconds between the pump and probe pulses. The probe pulse
with the integrated information of the dynamics after the time delay within its FWHM
will be then recorded by a detector.
The ultrashort pulses can be in principle anything, e.g. light, electron bunches,
pulsed magnetic fields, electrochemical potentials, temperature and pressure.
Their Gaussian broadening in time domain generally confines the time resolutionof the measurement, although a shorter resolution can be achieved by so-called
self-referenced techniques [81; 46]. In this thesis, we will focus on deciphering theultrafast dynamics in the light-harvesting inorganic chelates, hence the pump pulse
will be an optical laser. The relaxation dynamics upon optical photoexcitation will bethus monitored by recording the probe pulse in the order of the varying time delay,i.e. the varying difference in propagated distances between the pump and probe
pulses (∼3 µm corresponds to ∼10 fs). For the global and localized investigation of therelevant dynamics, in this thesis we will exploit the optical and hard X-ray photons as
the probe pulses respectively, and conduct so-called pump-probe experiments.
Figure 3.1 shows an overview of the time-resolved pump-probe experimental
methodologies used for this thesis. The core concept here is again the ultrashortpump and probe pulses approaching the sample with a time delay of t (thegeneralized parameters for both pulses are summarized). Before the interaction withthe sample system, both pump and probe pulses are characterized first (I0 ). The
optical pump pulse promotes the system to the excited state of our interest, and thefollowing optical or hard X-ray probe pulse then once more promotes the excited statesystem after the time delay of t, capturing either the global electron density (I1 ) or the
localized electronic structure (XAS, XES; see also the Section 2.3) in the excited

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Figure 3.1: A simplified scheme of the time-resolved pump-probe experimental methodologies
used for this thesis. The generalized parameters for the pump and probe pulses are
summarized. The pump and probe pulses are diagnosed prior to the interaction with thesample system. The probed signals with and without the optical excitation can be read out bydifferent detectors in different geometries.

state respectively. In addition, the X-ray solution scattering (XSS) measurement canbe conducted simultaneously in optical pump hard X-ray probe experiment [113;117; 63; 109], to further disentangle the nuclear dynamics from the electronic ones.
That means, the probe pulse recording the ultrafast dynamics can deliver different
information at the same time, which will be read by either single-shot detectors (1D) or
charge-integrating detectors (0D, 2D) in different geometries. The pump pulse onlyinteracts with the sample system at the half repetition rate of the probe pulse, suchthat the probed signals with and without the optical excitation can be compared (herewe assume that either the target system completely recovers back to the GS before
the next probe pulse, or the system is being renewed faster than the repetition rate).
In the following sections, details on the light sources for the pump and probe
pulses as well as the spectroscopic methods in both optical and hard X-ray regime
will be discussed. Depending on the timescale of our interest, the pulse duration(FWHM) of the optical pump can be either shorter than ∼50 fs or longer than 1 ps.
The corresponding Fourier-transform-limited spectral bandwidth then restricts theminimum possible FWHM of the pump pulse spectrum (uncertainty principle), which isusually still broader than the lifetime-broadened energy level of the excited state (∼1
meV or smaller) as denoted earlier. In other words, there is the fundamental limit of
exploiting the ideally monochromatic photon energy for the pump pulse. These are
however not that critical to the probe pulse whose spectral bandwidth is much broader
or pulse duration is inherently longer than the Fourier-transform-limit. Moreover,

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47 of 171we will briefly cover the two-photon absorption process in which an energetically
twice higher resonant state results from the simultaneous absorption of two identicalphotons, once the pump laser fluence exceeds the linear one-photon absorption
regime (this is a nonlinear optical process and different with the double-ionization
process discussed in Section 2.3.2).

3.2 Femtosecond optical laser system
The light-harvesting phenomena to convert photon energy into useful chemical energy
start with a separation of charge that creates the chemical potential, i.e. the transient
dipole moment, inside the inorganic chelates that is needed to drive further chemical
reactions [141]. Although important equilibrium and global events in chemistry occur
on much longer timescale, these events are inherently triggered by changes at earlytimes and short distances such as bond vibrations and atomic fluctuations in thepicosecond to nanosecond time domain [36]. Accordingly, a complete picture of a
relevant chemical event implies following its temporal evolution from the shortest time
onward.
Not only because we are interested in the light-harvesting phenomena but also
because no electronics can measure the probed signal in the picosecond or shortertime domain, such a time resolution is achieved by exploiting the pump-probe schemewith the ultrashort pulses. These are limiting our stimulus pump pulse only to theoptical laser, but in many instances non-light-driven ultrafast dynamics in chemistry
can be mimicked by triggering them with the optical photons [36].
Since the first coherent light pulse was constructed in 1960 [133], there have been
tremendous improvements on the shortest possible pulse duration, the highest
possible laser power, the frequency tunability from the deep UV (DUV) to the mid-IR,
and the best SNR within a reasonable data accumulation time [10]. In their earlystages, ultrafast studies had to rely on the accidental coincidence of given laser lines(mostly from sub-ps dye lasers) with molecular absorptions, both in the spectrumof the GS and the excited state absorption (ESA) spectrum of the intermediates.
In prominent cases, this was already sufficient for a crucial breakthrough inunderstanding the underlying mechanism [203]. In other cases however, a fairamount of "chemical intuition" had to be added to resolve any ambiguities. Ideally, thephotoexcitation should not be chosen by instrumental constraints but in accordancewith the ultrafast dynamics of our interest [142].
The 1990s have witnessed a revolution in ultrashort laser technology, thanks to theadvent of solid state active materials like Ti:sapphire, and powerful mode-locking

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Figure 3.2: A representative illustration of the femtosecond-resolved pump-probe experiment(Figure from [36]). The focused pump and probe pulses on the sample position interact with thelight-harvesting inorganic chelates, and the detector reads out the resulting probe pulse.

techniques such as Kerr lens mode-locking (KLM; [31]). With these advances,
femtosecond optical lasers have gained remarkable reliability and user-friendliness,becoming "turn-key" devices available to a wide community of non-specialists.
Another landmark of femtosecond laser technology has been the chirped pulse
amplification (CPA) technique [31], which enabled increasing the laser intensity
by 2 to 3 orders of magnitude, from the mJ to the multi-J level. Such an increasein peak power makes it possible to access a whole new class of nonlinear opticalphenomena, triggering a renaissance in the field of nonlinear optics. Parallel tothese developments has been the discovery of novel nonlinear optical crystals, suchas β-barium borate (BBO) and lithium triborate (LBO), combining improved opticalcharacteristics (high nonlinear optical coefficients, low group velocity dispersion(GVD), and broad transparency ranges) with high damage thresholds [31].
The tunable frequency of the femtosecond optical laser is obtained by the opticalparametric amplification (OPA) of the KLM Ti:sapphire laser amplified by the CPA
technique. A collinear OPA together with difference frequency mixing gives access tothe spectral range from near-IR to mid-IR, whereas a noncollinear geometry (NOPA)
with the help of frequency doubling and sum frequency mixing allows for the shortest
pulses from the UV to the near-IR [142]. This development enabled a number ofchoices for the selective dipole transitions, thereby dramatically enlarging the range ofultrafast dynamics that can be studied with transient optical absorption spectroscopy(TOAS).
The available tunability can be also used to probe the intermediate states from the

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49 of 171selective photoexcitation. However, employing probe pulses that cover an ultrabroad
spectral range at once greatly simplifies the experiment. The wider the probe
spectrum, the more spectral signatures of the intermediate states involved in theultrafast dynamics can be monitored simultaneously, which significantly facilitates theassignment of the underlying transient species. Towards this goal, one approach is
to exploit the broadband NOPA pulses that have a Fourier-transform-limit of only a
few femtoseconds [142]. Combining such a probe pulse with a narrowband NOPA
excitation allows us to achieve a time resolution in the sub-100 fs regime, which is
tailored to the investigation of the fastest nuclear and electronic dynamics [94; 24].
The alternative approach is to use supercontinua. These are generated by focusing
an ultrashort laser pulse under proper conditions into optically nonlinear transparent
media like gases, liquids, photonic crystal fibers and solids [142]. The supercontinua
from bulk materials have been shown to be a very broad and smooth spectrum,
high temporal and spatial coherence and a very high pulse-to-pulse energy stability[142]. Hence in our TOAS experiments for Chapter 4 and 5, we employed thefrequency-doubled NOPA pump pulse centered at ∼425 nm (∆λ ∼3 nm; MLCT
transition), and the supercontinuum probe pulse ranging from ∼320 nm to ∼720 nm
generated by focusing a fraction of the Ti:sapphire laser source into a 5-mm-thick
CaF2 crystal [156].
The idea of the TOAS is rather simple; the excited state system after the interaction
with the pump pulse (∼425 nm) absorbs the following probe pulse (∼320 nm to ∼720nm). In addition to the ESA, which characterizes the corresponding transient species,
the stimulated emission (SE; Einstein coefficient B21 ) can also happen. The so-called
transient absorption (TA) spectrum is then plotted by subtracting the GS absorption
spectrum from the ES one (ES (λ) − GS (λ) = ∆(λ)), and thus shows in general three
types of the probed signals (Fig. 3.3; [36]). The ground state bleach (GSB) signal
indicates an absorption band in the GS disappearing in the ES, and accordingly a new
ESA band arises usually at a lower photon energy (unoccupied valence electronic
structure now seen by ES, not GS). The SE signal appears as a negative feature at
the lowest photon energy, since the probed signals are recorded by the detector intransmission mode (Fig. 3.1), and the corresponding transition proceeds under theFranck-Condon principle (Fig. 2.6).
The very advantage of the TOAS compared to other transient optical spectroscopic
methods is that the recovery of the GS and the relaxation dynamics of the ES canbe detected simultaneously. That means the relaxation pathways to the GS can be
distinguished from those to other excited state manifold. Detailed data reduction andfitting analysis as well as the experimental setup will be followed in the next section,
Chapter 4 and 5.

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Figure 3.3: (left) A simplified scheme of the three representative signals in an optical TA
spectrum and (right) the corresponding transitions described in a PES diagram (Figure from[36]). The dipole transitions occur under the Franck-Condon principle.

3.2.1 Experimental setup at University of Bern
The TOAS experiments for Chapter 4 and 5 were carried out at University of Bern(Prof. Dr. Andrea Cannizzo, Dr. Maryam Nazari) in May 2018 and December 2017
respectively. Herein the experimental setup (Fig. 3.4) will be briefly introduced
focusing primarily on the pump and probe pulse generation processes. Further
information on other components and aspects can be found in [156].
The oscillator is a KLM Ti:sapphire designed by Kapteyn-Murnane laboratories,
pumped by a continuous wave (CW) 5W pump at 532 nm (Finesse from Laser
Quantum). The oscillator output (80 MHz repetition rate, at 795 nm with a bandwidthof 12 nm) is used to seed a Ti:sapphire chirped pulse amplifier (Legend Elite by
Coherent), pumped with a DARWIN-527 from Quantronix. The output of the cavity
is a 5 kHz laser pulse, horizontally polarized, with max. pulse energy of ∼0.5 mJ at
800 nm and pulse duration of ∼100 fs (FWHM). The output of the amplifier is sent to acommercial NOPA, tunable between 500 nm and 750 nm, with a pulse duration of∼35 fs and a spectral bandwidth of ∼50 nm (TOPAS-White by Light conversion). The
NOPA is used to generate ultrashort tunable and broadband pulses over the visible
range of the spectrum.
Pump beam generation is divided into visible and DUV spectral regions. In order

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Figure 3.4: A layout of the femtosecond-resolved pump-probe experimental setup at Universityof Bern (Figure from [156]).

to generate visible pump pulses, the NOPA output is tuned to the desired pumpwavelength between 500 nm and 750 nm, with typically 30 nm bandwidth (FWHM).
Sometimes the pump bandwidth was reduced to have a more selective excitation ofelectronic transition by inserting a slit in a two prism based compressor on the path,where the pump pulse was dispersed (S1). For sake of completeness, the prism
compressor is built with two highly dispersive SF11 prisms (Pr1 and Pr2). For thepump pulses in the DUV region, the visible NOPA output is focused by a parabolic
mirror with f = 150 mm (PM1) on a BBO crystal with 250 µm thickness and thenfrequency doubled by choosing the right phase matching angle [31]. The max. energy
per generated pump pulse was ∼1 µJ with a bandwidth of ∼2 to 3 nm (FWHM). A 250µm thickness was chosen to have sufficient excitation intensity without an importantdegradation of the time resolution.
The supercontinuum generation for the broadband femtosecond probe pulse (∼320
nm to ∼720 nm) was achieved by focusing a small fraction of the Ti:sapphire laser
source onto a 5-mm-thick CaF2 . Several phenomena like self-phase modulationand (anti)-stokes Raman shift amplification occur and broaden the fundamental
beam [75]. To avoid any damage on the CaF2 plate due to accumulated heat load,

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the crystal was mounted on a motorized stage horizontally wobbling. Adjustment on
the fundamental beam size and pulse energy to generate white light was fulfilled
by a fine adjustable aperture and a variable neutral density filter, respectively. The
white light was then collimated by a UV-enhanced parabolic mirror (f = 100 mm), andthe fundamental beam was filtered out by a 2-mm-thick short-pass filter (BG40 from
Schott). The polarization of the white light was vertical and parallel to the fundamental
beam. Since CaF2 shows a nonlinear anisotropic birefringency, the crystal has been
mounted on a rotational stage to optimize the mutual orientation of the crystal axis at
the laser polarization by means of a polarizer. This optimal orientation corresponds toan optimal spectral bandwidth and stability. The polarization of the white light was
double-checked at the sample position to ensure that other optical components or
misalignments did not affect the polarization.
After the supercontinuum generation, the probe pulse was collimated by a f = 50 mm
parabolic mirror (PM-WL in Fig. 3.4), and then splitted into two by a 50%-50% UVbeamsplitter. One of the two pulses was focused by PM3 to spatially overlap with thepump pulse at the sample position. The temporal overlap and time delay between
the pump and probe pulses were controlled by a 30 cm motorized delay stage placedin the pump path. The delay stage can move down to ∼0.15 µm (∼0.5 fs) per stepand span up to 2 ns. The other probe pulse passes through another sample region
not overlapped with the pump pulse, and was exploited for the reference spectrum.
The probe beam size at the sample position was ∼30 µm in diameter (1/e2 ). Boththe reference and probe beam after the interaction with the sample system were
re-collimated by another f = 120 mm parabolic mirror (PM4). Their polarization wererotated to be horizontal by a 90° periscope, and then both beam were dispersed by aprism on two Hamamatsu CMOS sensors (S11105 series) with two chips (Fig. 3.4).
Each sensor consists of an array of 512 pixels with the pixel dimension of 12.5 by 250µm, and the achieved spectral resolution was ∼1 to 2 nm (FWHM). Two cylindricallenses (f = 20 mm) were placed in front of the sensors to focus the reference andprobe beam vertically onto each pixel, to improve the photon collection and thus the
SNR.
Using the frequency-doubled NOPA pump pulse centered at ∼425 nm (∆λ ∼3 nm;
MLCT transition) and the supercontinuum probe pulse covering from ∼320 nm to ∼720
nm, the TOAS experiments were conducted on two bimetallic photocatalysts, namely
Ir(III)-Co(III) triads and Ru(II)-Mo(VI) dyad, to figure out how the excited electron in thephotoactive moiety contributes to the enhanced catalytic performance on the othermoiety. The fitting analysis and interpretation of the reduced TOAS data will be further
discussed in Chapter 4 and 5.

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3.3 Synchrotron light source
The unoccupied valence electronic structure seen by the GS and ES in a
femtosecond-resolved manner reveals plenty of information on the relevant ultrafast
dynamics particularly dealing with the global electron density changes in the inorganic
chelates (e.g. an electron, energy transfer process or a conformational relaxation
upon optical photoabsorption; [7]). Nevertheless, the limited photon energy window
between ∼1.5 eV and ∼3.5 eV cannot fully unravel the strongly coupled vibration, spin,and nuclear dynamics because there are numerous resonant states simultaneously
detectable in such a complex system within the optical regime.
To further gain the selectivity and sensitivity to specific and localized dynamics in the
inorganic chelates, the hard X-ray probe pulse suits our needs well (see also theSection 2.3). Therefore, the endstations at the two contemporary X-ray facilities, i.e.
synchrotron light source and XFEL, have been employed for such scientific cases
where the geometric structure changes with a picometer-scale spatial resolution andspin density changes to shed light on a complete relaxation scheme can be studied.
In this and the next section, the instrumentations and corresponding hard X-ray
methods exploited for this thesis will be introduced along with the X-ray light sources.
Detailed data reduction and fitting analysis will be followed in Chapter 6, 7, and 8.
The whole history of Nobel Prizes in physics has started with the discovery of X-rays
in November 1895 at the University of Würzburg by W. C. Röntgen, and until today∼40 Nobel Prizes were awarded to the X-ray sciences. However, progress in both thetheoretical understanding of the X-ray light-matter interaction and the experimental
methodology was steady from the period covering their discovery through to the mid
1970s [3]. The central limitation in that period was the source, which had remained
essentially unchanged from about 1912 [84].
In a conventional X-ray laboratory, the X-ray beam was generated in a so-called X-ray
tube (or is still generated in airports and hospitals). The tube basically consists of twoparts; the filament and the water-cooled target material. A large voltage difference(20 kV to 50 kV) is produced between the filament and target, while the filament is
operated by a small voltage of approximately 6 to 20 V. This is sufficient to generate
electrons from the filament by thermionic emission. The huge voltage difference
between the filament and target material accelerates the generated electrons in the
direction of the target material. When some of the accelerated electrons hit the target
material, they are stopped abruptly and emit in turn an electromagnetic wave. The
higher the voltage of the filament is, the more intense X-rays are generated.

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The spectrum of the X-rays generated from the electrons impinging on a metal anodehas two distinct components. There is a continuous part due to the electrons being
decelerated, and eventually stopped in the metal (not all electrons are stopped atonce but some have multiple collisions with the target, and this leads to a generationof further electromagnetic waves of increasing wavelengths). This is consequently
known as bremsstrahlung, and has its maximum photon energy that corresponds tothe high voltage applied to the tube regardless of the target material. Superimposedon this broad spectrum is a sharp and characteristic line spectrum from the target
material (X-ray fluorescence; see also the Section 2.3.2).

Figure 3.5: A generic scheme of the synchrotron facility (Figure from [136]). The electron
bunches injected into a closed storage ring emit the X-rays while under the three light sources(Bending magnet (BM), Wiggler, and Undulator), that are delivered through the X-ray optics anddiagnostics (BL) to the experimental chamber (EC). The partial electron energy lost during the
X-ray emission (few keVs out of few GeVs) is compensated by the radiofrequency (RF) cavity,
to keep the electrons in the orbit.

In the 1970s it was realized that the synchrotron radiation emitted from charged
particles circulating in storage rings, that had been originally constructed forhigh-energy nuclear physics experiments, was potentially a much more intense andversatile source of X-rays [3]. Synchrotron radiation is hence named after a specific
type of particle accelerator, but nowadays the synchrotron radiation has become
a generic term to describe radiation from charged particles travelling at relativistic
speeds in applied magnetic fields which force them to propagate in slalom paths.
Besides synchrotrons themselves, the synchrotron radiation is produced in storage
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55 of 171rings where electrons or positrons are kept circulating at constant energy of few GeVs[3]. In a storage ring (Fig. 3.5), the synchrotron radiation is generated either in thebending magnets (BMs) needed to keep the electrons in a closed orbit, or in theinsertion devices (IDs) such as wigglers or undulators situated in the straight sectionsof the storage ring.
The core of a synchrotron light source is an electron accelerator of the storage
ring class [165]. This consists of an injector (Fig. 3.5), where electrons are
generated and pre-accelerated to a high energy corresponding to a speed close
to that of light, c. From the injector, the electrons are sent into a closed pathinside an ultra-high-vacuum chamber. The electrons are then kept in the closed
orbit by a sophisticated system of bending and focusing magnets. Fundamental
electrodynamics however shows that a charge subjected to an acceleration must
emit electromagnetic waves, and this is the origin of the synchrotron radiation in thestorage ring. The electrons therefore partially lose their energy from the emission in
the magnetic systems (few keVs out of few GeVs), and this gets compensated by one
or more RF cavities applying an electric field to periodically accelerate the electrons in
the longitudinal direction to keep them circulating around the ring [165].
Among the three magnetic systems in the storage ring, we will focus on the undulator
light source that was employed for the experiments in Chapter 6, 7, and 8. The
undulator is a periodic series of alternating magnets, usually 1 to 5 m long, inserted
along an otherwise straight portion of the closed trajectory of the electrons (Fig. 3.6;
[165]). The Lorentz force caused by the magnets makes the electrons slightly oscillate
in the transverse direction, typically on the plane of the storage ring. Such oscillations(accelerations) thus lead to the emission of synchrotron light waves.

Figure 3.6: A representative illustration of the undulator light source with the electron bunchentering at a relativistic speed. The electron bunch accordingly experiences the undulator
magnetic field corrected for the Lorentz contraction (1/γ). The consequent dipole radiation fromthe oscillating and moving electrons gets further Doppler-shifted in the endstation frame (1/2γ).

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Lorentz factor (γ) of ∼104 , the dipole radiation from the oscillating and moving
electrons must be considered in a proper reference frame. Under the two relativistic
effects (Fig. 3.6), namely the Doppler effect and Lorentz contraction, the radiation
wavelength observed in the endstation frame becomes λ ∼ L/2γ 2 , where L is theundulator period and γ is the electron energy in m0 c 2 unit (γ = Ee /m0 c 2 , where m0 is
the electron rest mass and m0 c 2 is hence ∼0.5 MeV; [135]). Furthermore, the angular
distribution of the dipole radiation observed in the endstation frame becomes smaller√
than 1/γ Nu (angular collimation), where Nu is the number of undulator periods. Thatmeans, with the γ-factor of ∼104 , the emitted X-rays propagate within an angularspread of few tens of microradians.
In addition to this relativistic collimation effect, the total emitted power also becomes
proportional to ∼ γ 4 in the endstation frame, therefore the peak brilliance (brightness;[photons/s/mm2 /mrad2 /0.1% bandwidth]) of the undulator light source gets extremely
high compared to that of the X-ray tubes (Fig. 3.7; [165]). The magnetic field of
the undulator BL also affects the electron oscillations and thereby the radiation
wavelength as well, and this is commonly reflected in the K parameter of theundulator (K = eBL L/2πm0 c), which represents the deviation angle of the electron
beam with respect to the propagation axis [135]. As noted earlier, in the courseof dipole radiation the electrons partially lose their energy but only infinitesimally.
Accordingly, the γ-factor does not really change during the undulations.
The undulator emits not only the first-harmonic, fundamental photon energy
described above, but also higher-order harmonics with a certain spectral bandwidth(∆λ/λ ∼ 1/Nu ) that can be filtered out by the following X-ray optics. This originates
primarily from the stochastic nature of the synchrotron radiation, namely the electrons
are not an ideal point source but repelling each other (the electron "bunch") and haveindividual energies from slightly different trajectories in the storage ring and thus
different interaction with the magnetic systems (electron beam cross section; [165]).
When it comes to the time-resolved experiments however, the emitted X-rays in apulsed structure or a certain timing mode are crucial. The pulsed structure in factcomes from that of the bunched electrons, which are generated from the injector andcompressed into an ultrashort duration. Such a bunched structure is necessary toperiodically restore the electron energy lost from the synchrotron radiation by RFcavities, and the collective radiation of one electron bunch in the undulator hence
results in an ultrashort hard X-ray pulse [165]. With this, we will now move on to theactual parameters in the endstation at the synchrotron light source that are designed
for the time-resolved XAS and XSS experiments, e.g. 11-ID-D endstation at theAdvanced Photon Source (APS) storage ring facility.

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Figure 3.7: The evolution of the relative peak brilliance of the X-ray light sources over the last
14 decades, and the comparison with the increase in the number of transistors on a siliconprocessor chip (Moore’s law; Figure from European XFEL).

3.3.1 11-ID-D Endstation at APS
The 11-ID-D endstation is a beamline dedicated to optical pump hard X-ray probe
XAS and XSS experiments, receiving the hard X-ray probe pulses from the APS
storage ring through two in-line undulator sources (U2.3, U3.3). Specifically for ourtime-resolved EXAFS experiments in 2018 (Chapter 6), the hard X-ray pulses weregenerated from the electron bunches delivered at 6.52 MHz under the 24-bunch
timing mode (i.e. 153 ns equal bunch spacing; the "distance" between the bunches
to be re-accelerated depends on the length of the ring and the number of bunches
circulating around it), and had a pulse duration of ∼78.8 ps in FWHM [35; 128; 91].
The undulator radiation at ∼22 keV (Ru K-edge) was monochromatized by a Si(111)
double crystal monochromator (DCM; ∆E/E ∼ 10−4 ) to achieve an energy resolution

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58 of 171better than the Ru 1s core-hole lifetime-broadening [116; 74]. The afterwards
focused monochromatic X-rays consisted of ∼4*1012 photons per second, and had
a beam size of ∼500 µm (horizontal) by ∼70 µm (vertical) at the interaction point(FWHM). During the experiments the DCM was being scanned from the Bragg angle
corresponding to ∼22 keV to that for ∼22.47 keV (Fig. 2.15), and the difference inpropagated distance between the Bragg’s conditions was automatically compensatedto keep the probe pulse delay to the optical pump be constant.
Figure 3.8 illustrates the working principle of a single crystal monochromator inreflection geometry (Bragg’s law; the Laue equations in transmission geometry
will not be discussed in this thesis). In ideal case, the diffracted plane waves
satisfying the Bragg’s condition (Fig. 3.8 a), b)) will interfere constructively, leading
to the monochromatic (λB ) plane wave. In practical cases however, the undulator
radiation transported through the X-ray optics and diagnostics is divergent andsometimes induces a thermal deformation (heat load) on the crystal (Fig. 3.8 c);[165]). Therefore, the reflectivity curve of the single crystal monochromator withrespect to the incidence angle is not a delta function but has at least an inherent
bandwidth (Darwin width; [41; 140]) that is shifted due to the refraction. In otherwords, the interaction of the diffracted beam (λD ) with the crystal (multiple reflection)
as well as the interference between the diffracted and incident beam should be taken
into account (the dynamical diffraction theory; [11]).

Figure 3.8: Schematic illustration of the working principle of a single crystal monochromator in
ideal a), b) and practical c) cases.

Figure 3.9 shows the standard experimental setup at the 11-ID-D beamline for thetime-resolved EXAFS measurements. In a liquid sample environment the 25 mL
sample solution in the reservoir (kept at 5 °C and bubbled with N2 ) was flowed by theliquid jet system (yellow) through an injector with its inner diameter of 600 µm, toprevent compound precipitation and multiple photoexcitation during the measurement.
An additional dose of fresh acetonitrile was regularly refilled into the reservoir suchthat the sample concentration was kept stable (this was however not that critical forwater solvent). The optical pump pulse (blue) and the monochromatic X-ray probe
pulse (red) were then synchronized and intersected on the free-flowing sample jetspatially and temporally.

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Figure 3.9: Standard experimental setup at the 11-ID-D beamline for the time-resolved EXAFS
measurements in 2018. The sample reservoir was cooled down to ∼5 °C and refluxed under
nitrogen, and an additional dose of fresh acetonitrile was regularly refilled to keep the sample
concentration stable (yellow). The 400 nm pump pulse (blue) and the monochromatic X-ray
pulse (red) were spatially and temporally overlapped on the 600-µm-thick liquid jet.

The 400 nm pump pulse was the second-harmonic output from a 10 kHz Coherent
Legend Elite Ti:sapphire ultrafast amplifier laser system. The pump pulses were thenfurther stretched to ∼1.6 ps (FWHM) by a pair of prisms. The optical laser spot size
was ∼800 µm (horizontal) by ∼130 µm (vertical) at the interaction point (FWHM),
yielding a fluence of ∼75 mJ cm-2 .
The emitted X-ray photons from the sample compound were then recorded at 90°
with respect to the incident X-ray beam on both sides by two avalanche photodiodes(APDs; TFY mode). A combination of soller slits and a Mo filter, custom-designed forthis sample environment and the distance between the sample and detectors, wasinserted between the liquid jet system and the APDs. The first 12 X-ray pulses before
the optical photoexcitation were accumulated for the GS XAFS spectrum, whereas thelater X-ray pulses were used for the ES XAFS spectrum at 150 ps probe delay after
the 400 nm photoexcitation. Each energy point was integrated for 4 s.

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3.4 X-ray free-electron laser (XFEL)The modern accelerator science and thus the synchrotron light sources are so
sophisticated that the electron bunches are well under control in terms of the electronenergy, source size, and angular deflection, with a very high accuracy [165]. Yet,the electron bunches circulating around the ring do experience many different
perturbations in their journey to the light source (undulator in our case), such that theconsequent X-ray parameters cannot exceed the state-of-the-art values, e.g. thepulse duration, spectral bandwidth, coherence, and brilliance.
This confined destiny of the electrons has been liberated by the successful realizationof the XFELs [53; 168; 137; 92; 188; 146; 63; 109], whose unique characteristics
are represented by the electron bunch compressed into a few femtoseconds and its
self-amplified spontaneous emission (SASE) in a much longer undulator system.
In fact, now the well-defined femtosecond electron bunch without any previous
usage can be further optimized for the specific undulator parameters, and the linear
accelerator (Linac) can be stretched for the electron energy above 10 GeV.
These extraordinary electron bunches will however undergo the Lorentz interaction
maximum two or three times until they are dumped. In contrast to the electron
bunches in a storage ring delivering X-rays to more than 40 beamlines for days, such
an ultrashort lifetime of the electron bunches in XFEL for max. 2 or 3 beamlines
accordingly increases the price per pulse that can be only justified by its exceptional
beam quality.
First of all, the final electron energy at XFEL usually reaches a higher value thanthat at the storage ring, corresponding to the higher γ-factor and hence much
more angular collimation and higher brilliance. Furthermore, the fine electron
bunches propagating through a long undulator will generate quasi-monochromatic,
femtosecond, and almost spatially coherent X-rays [165; 137].
The coherence predominantly originates from the correlation between the electron
bunch and its synchrotron radiation waves in the advanced undulator system (Fig.
3.10). Here, both the electric and magnetic field of the initial X-rays work negatively(i.e. the electrons transfer energy to the X-rays) and force the "heavy" electrons
towards the nodes of the waves respectively. Such a microbunching process aligning
the relativistic electrons into a periodic structure equal to the emitted wavelength andcontinuous radiation in a correlated way therefore exponentially amplify the total wave
intensity and let the X-rays become more coherent (SASE; [53]).

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Figure 3.10: A simplified illustration of the SASE process by the femtosecond electron bunchat XFEL, undergoing the microbunching in a much longer undulator system (Figure from
European XFEL).

The microbunching requires not only the well-defined electron beam but also enough"time" to happen, so that the storage ring with its 5 to 10-m-long undulator cannot
result in the SASE [137]. As the negative work proceeds in an undulator longer
than 100 m, a continuous energy loss of the electrons causes an increasing phasemismatch between the electric field of the X-rays and electron oscillations, thus atsome point the work becomes positive. This in turn slightly accelerates the electrons,
and the energy in fact starts oscillating between the X-rays and electrons (saturation).
The advanced Linac system at XFEL supplies unprecedentedly robust electron
bunches for the SASE process, from which the femtosecond X-ray pulses with
∼1010 times higher peak brilliance than that at a storage ring are transported to thebeamline. Still, the stochastic nature of the SASE process involves the electronenergy spread, angular divergence, and diffraction of the waves, that affect boththe intensity amplification and saturation mechanism [165; 137]. Consequently, theso-called spatial and temporal jitter are inevitable, although the lateral coherence of
the X-rays is very high.
In this Section, the BL3EH2 endstation at SPring-8 Angstrom Compact free-electron
LAser (SACLA) and the femtosecond X-ray experiments (FXE) endstation at
European XFEL, both dedicated to the optical pump hard X-ray probe experiments,
will be introduced. The femtosecond-resolved XES and XSS experiments for Chapter
7 and 8 were conducted with von Hamos dispersive spectrometers in Bragg reflection
geometry and in the forward wide-angle X-ray scattering geometry respectively. The
characteristic repetition rates at each XFEL facility and their consequences will be
remarked as well.

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3.4.1 BL3EH2 Endstation at SACLA
SACLA, the world’s second hard X-ray free-electron laser in Japan (2011), consistsof a compact-scaled Linac system (8 GeV) and a short-period undulator (18 mm). A
high-density electron beam with a low transverse emittance is generated by a pulsed
thermionic electron gun with a single-crystal CeB6 cathode and accelerated by ahigh-gradient, C-band Linac system [92]. The electron beam with a high peak currentof >3 kA then propagates through the in-vacuum undulator system with the total
number of ∼5000 periods (max. K ∼2.2), resulting in the SASE X-ray pulses.

Figure 3.11: An overview of the X-ray optics and diagnostics for the pump-probe experiments
at BL3 of SACLA (Figure from [101]). TRM stands for a total reflection mirror, CC for achannel-cut crystal, AT for a silicon attenuator, and CRL for a compound refractive lens. The
focal point of the CRLs at the BL3EH2 endstation is the interaction point of the optical pump,
hard X-ray probe pulses with the target sample.

Figure 3.11 shows how the X-ray pulses are transported to the interaction point at theBL3EH2 beamline [188; 104; 101]. The higher-order harmonics from the undulator
are first filtered out by a certain configuration of so-called cutoff mirrors (TRM; thecritical angle for the total reflection in the X-ray regime depends on the photon energy
and the coating material), and the fundamental SASE spectrum can be checked byinserting and scanning the DCM (rocking curve). The diffraction grating then splits theincident beam into multiple branches, one of which (through M1 and M3) is exploitedfor the arrival-timing diagnostics between the X-ray and optical laser pulses (timing
tool; [104]). For our femtosecond-resolved XES and XSS experiments we used the
SASE X-ray pulses without monochromatization (pink beam; without CC1 and CC2),
and the pink beam was focused down to ∼5 µm by beryllium CRLs [101].
The focused pink beam enters the He-filled chamber specialized for the pump-probe
experiments (Fig. 3.12 left; [103]), whose scattering signal is recorded by two silicon
photodiodes (Hamamatsu, S3590-09) as its I0 transmissive intensity. At the interaction
point, both optical laser and hard X-ray pulses are overlapped spatially and temporally,
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Figure 3.12: (left) The He-filled chamber customized for the pump-probe experiments at
BL3EH2 endstation, and (right) two von Hamos dispersive spectrometers next to the chamber
in vertical (a) and backscattering horizontal (b) geometry (Figures from [103]).

and the corresponding X-ray fluorescence as well as the forward scattering signals
are measured on a per-shot basis.
Considering the stochastic X-ray intensities at XFELs, the point-to-line dispersive
spectrometer in Bragg reflection geometry (von Hamos spectrometer; Fig. 3.12
right) with cylindrically bent crystals is optimal for the XES [178]. The von Hamos
dispersive geometry to diffract the X-ray fluorescence signals from the sample onto a
2D position-sensitive detector is illustrated in Fig. 3.13 [85; 184].

Figure 3.13: Schematic illustration of the von Hamos dispersive geometry with a cylindricallybent crystal and 2D detector. Here, R stands for the radius of crystal curvature and ∆E for the
X-ray fluorescence energy range that can be covered by the crystal and 2D detector according
to the Bragg’s law (red = lower fluorescence energy, purple = higher fluorescence energy).

Basically, the geometry corresponds to the Bragg diffraction in a nearly perfect crystal
with a certain solid angle coverage, hence the Darwin width should be considered(Section 3.3.1). The Miller index and material of the cylindrically bent crystal andthe feasible Bragg angles onto the position-sensitive detector then determine the
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X-ray fluorescence energy range that can be covered (∆E). Another variable in thisgeometry is the radius of crystal curvature R, typically either 500 mm or 250 mm, thatchanges the Bragg angle coverage and the energy resolution of the spectrometer [80;2]. The energy resolution ∆EG (energy broadening due to geometrical contributions)means how wide range of the X-ray fluorescence energy is read as an energy point
per detector pixel in the dispersive plane (Fig. 3.13 (b)). A derivative of the Bragg’s
law describes this as,∆EG s + d≅
∗ cotθB
Ei
f

(3.1)

where Ei is the incident photon energy, s is the X-ray beam size, d is the detector
pixel size in the dispersive plane, and f is the diffraction path length from the sample
to the detector [80; 2].
The von Hamos geometry at SACLA for example has s ∼5 µm, d = 50 µm (multiportcharge-coupled device (MPCCD) detector), and f ∼500 mm. Thus, the θB ∼62.66° andEi ∼10 keV result in the ∆EG of ∼0.57 eV. There are still other energy broadening
factors in addition to the Darwin width, that are associated with any angular deviationfrom the exact Bragg condition, e.g. the lattice plane stress in the bent crystal andslightly tilted alignment of the crystal [80; 2]. However, the ∆EG is usually dominant
over all other possible energy broadening.
Currently 6 cylindrically bent crystals of Si(111) and Si(531) are available at SACLA(25 mm × 100 mm × 150 µm (dispersive × focusing × thickness); 2020). Two identical
MPCCD detectors with an active area of 25.6 mm × 51.2 mm are located according to
the two von Hamos spectrometers in vertical and backscattering horizontal geometry
respectively. During the femtosecond-resolved XES experiments, one in the verticalgeometry (Fig. 3.12 a) records certain emission lines at 90° with respect to theincident X-ray beam, while the other (Fig. 3.12 b) can record different emission lines.
On the other side of the interaction point, another silicon photodiode measures the
per-shot TFY at 90° with respect to the incident X-ray beam, that can be employed tonormalize the X-ray intensity fluctuations. For the XES measurements in general, it isextremely important to provide He or vacuum environment and reject any elasticallyscattered X-ray photons on the diffraction paths of the emission lines.
The femtosecond-resolved XSS measurements can be simultaneously performed witha MPCCD SWD (short working distance) detector in the forward wide-angle X-ray
scattering geometry (2θ up to 45°; [103]). Further information on the experimental
parameters as well as the data reduction and fitting analysis will be followed in
Chapter 7, 8, and Appendix B.

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3.4.2 FXE Endstation at European XFELThe first designs of the XFEL generate the respective electron bunch at a repetition
rate lower than 1 kHz [53; 92]. Accordingly, the XFELs with an unprecedented peakbrilliance cannot provide much higher SNRs than the contemporary storage rings thatcan deliver its synchrotron radiation at over 1 MHz. Intuitively, one can imagine thatthe free-electrons being accelerated at RF can be also operated at the same RF. A
difficulty here comes however from the fact that the free-electrons are only for a single
use, i.e. the electron bunches must be generated at the same RF by the electron gun.
Such a photocathode (Cs2 Te) RF-gun was successfully implemented at European
XFEL [18], reliably yielding the electron bunches up to 4.5 MHz.

Figure 3.14: Simulated image of the acceleration of the free-electrons in the superconducting
Linac system at European XFEL capable of the high-repetition rate operation up to 4.5 MHz(Figure from European XFEL).

The electron bunches are subsequently fed into a Linac system (Fig. 3.14), and the2-km-long superconducting cavities (resonators) made of niobium at European XFEL
can accelerate them up to ∼17.5 GeV [189]. This in turn enables a record-breaking
angular collimation and emitted power of the SASE X-rays radiated in the following
175-m-long advanced undulator system. Moreover, the photon energy up to ∼30 keV
is theoretically feasible, although the X-ray optics do not allow reaching that highphoton energy yet. The total 3.4-km-long European XFEL is therefore a researchfacility of superlatives, namely all its components are state-of-the-art.
The world’s most powerful SASE X-rays are then transported to the FXE endstation
dedicated to the optical pump hard X-ray probe experiments, via the specialized

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Figure 3.15: An overview schematic of the FXE endstation. The sketch includes major
instrumentation components installed in the photon beam transport tunnel and experimental
hutch (Figure from [189]).

X-ray optics and diagnostics (Fig. 3.15; [63]). The FXE beamline is equipped with astandard 4-crystal Si(111) monochromator and the subsequent photon diagnostics.
At the interaction point, a sample solution is flowed by a high-performance liquid
chromatography (HPLC) high-pressure-gradient pump system at a rather high speed,
to avoid multiple photoexcitation by the high-repetition rate optical and X-ray pulses.
Two X-ray emission spectrometers in von Hamos geometry and Johann geometry arelocated at 90° with respect to the incident X-ray beam on each side of the interaction
point respectively. The von Hamos dispersive spectrometer can accommodate up
to 16 cylindrically bent crystals with its focal length of 500 mm. The Bragg angles
between 67° and 83° are feasible, and the diffracted fluorescence signals are thenacquired at 10 Hz by 2D JUNGFRAU 1M and 500k detectors. The Johann focusing
spectrometer can accommodate up to 5 spherically bent crystals with its focal lengthof 1000 mm. Such a hybrid geometry will be especially efficient when it comes to theso-called photon-hungry experiments like RIXS or HERFD-XAS.
The femtosecond-resolved XSS experiments can be simultaneously performed in theforward wide-angle X-ray scattering geometry with a large pixel detector (LPD; 2θ up
to 65°), and the LPD is capable of recording the scattering signals on a per-shot basis(up to 4.5 MHz). After the interaction with the sample, the primary beam can further
proceed through the LPD central hole to the post-sample diagnostics. Figure 3.16
shows the 3D illustration of the sample interaction area at the FXE beamline.

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Figure 3.16: 3D illustration of the sample interaction area at the FXE beamline. The vonHamos and Johann spectrometers are located at 90° with respect to the incident X-ray beam,
whereas the LPD is in the forward wide-angle X-ray scattering geometry (Figure from [63]).

Even though the reduced data obtained from the FXE beamline are not shown in thisthesis, it is noteworthy to briefly mention some discrepancies in the time-resolved
XES and XSS measurements at between SACLA and European XFEL. First of all, thefront-end pulse energy at ∼9 keV is ∼0.6 mJ at SACLA, while is ∼3 mJ at European
XFEL. This makes a big difference in deploying the X-ray optics and diagnostics,
particularly in terms of the thermal deformation (heat load) on each component. Suchan extraordinary X-ray pulse is even delivered at around 1 MHz to the FXE beamline,
whereas only at 60 Hz to the BL3EH2 beamline. Hence, some of the components
inevitably either accumulate continuous fatigue or can break down at some point.
The high-repetition rate also directly affects the liquid jet flow-speed. The X-ray
vertical beam size of ∼10 µm for instance must be renewed within ∼1 µs (1 MHz),corresponding to the flow-speed of ∼10 m/s at slowest. This accompanies significant
amount of solvent vapor near the interaction point, especially if the solvent is volatile,that attenuates the X-ray signals and changes the sample concentration. On the other
hand, the SNR of the data can be prominently improved within the limited time of theexperiment.

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4 5d6 Ir(III)-Co(III) photocatalyst[A] being consumed does not mean [B] being produced.

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.3 TOAS on Ir(III) photosensitizers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.4 TOAS on Ir(III)-Co(III) triads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
The first chelates we will visit are 5d6 Ir(III) photosensitizers and their derivatives
with the cyclometalating ligands. The 5d6 Ir(III) photosensitizers are well-known
for their superior functionalities under the optical photoabsorption, acting as an
excellent reservoir of the incident photon energy to be converted into a chemicalenergy or a variable luminescence [191; 190; 132; 48; 111; 86; 15; 60; 119]. The very
advantage of the bis-cyclometalated [Ir(C∧N)2 (N∧N)]+ complexes is the tunability of
its photophysical properties by substituting either the monoanionic cyclometalating
ligand (C∧N), or the ancillary diimine ligand (N∧N). As noted in Section 2.1.1, thecyclometalating ligand (C∧N) as well as the dx 2 −y 2 orbital always correspond to the
HOMO orbitals, whereas the diimine ligand (N∧N) always corresponds to the LUMO
orbital (π* orbital) [132; 15].
With this, our working group has designed a new series of phen-ligands fusedwith thiophenes to improve the reactivity of the photocatalytic proton reduction in
the MLCT excited states [191; 192]. Furthermore, the [Ir(ppy)2 (5,7-disubstituted
thieno[3,4-f ]phen)]+ (ppy = monodeprotonated 2-phenylpyridine) was covalently
linked to two chloro(pyridine)cobaloxime(III) catalysts, to fully take advantage of the
MLCT states on the newly synthesized ligand after the directional charge separation.
The photocatalytic proton reduction showed higher performance with the triadic
system, and accordingly we conducted the TOAS experiments on both the Ir(III)photosensitizers and Ir(III)-Co(III) photocatalysts to visualize the ultrafast global
interaction in the molecular system. Upon 425 nm photoexcitation, a great portion of
the electron density in the 3 MLCT state on the diimine ligand got transferred to thecobaloxime(III) catalyst with a ∼100 ps time constant. Further investigation with thelocal hard X-ray probe will clearly show the formal oxidation state and spin densitychanges in the Co(III) moiety as well as its nuclear dynamics.

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4.1 Introduction

Figure 4.1: (left) A structural formula of [Ir(ppy)2 (bpy)]+ and (right) a simplified dipole transition
scheme in the [Ir(C∧N)2 (N∧N)]+ complexes upon optical photoabsorption (Figures from [14;132]). Both 1 MLCT and 1 LC states can be initially populated, followed by ISC and a mixture of
the 3 MLCT and 3 LC states, which will decay back to the GS either non-radiatively or radiatively.

[Ir(ppy)2 (bpy)]+ , one of the simplest [Ir(C∧N)2 (N∧N)]+ complexes, and its photophysics
have been researched over the last 4 decades to understand and hence to control
the nature of its excited states [110; 90; 199; 14; 190; 15]. Due primarily to its C2
point group symmetry, no symmetry-based selection rules can be applied, and theoptical dipole transitions mostly concern MLCT and LC excited states (Fig. 4.1). The
3

MC state is hardly accessible because of a sufficiently large ligand field splitting (∆)

in 5d-orbitals, but in few cases the thermal accessibility to the 3 MC state becomes
critical especially when it comes to the deactivation of the luminescence from the3

MLCT/3 LC states or the photodegradation [60].

Optical photoabsorption in the [Ir(C∧N)2 (N∧N)]+ complexes therefore often leads toeither the MLCT (dx 2 −y 2 → π*(N∧N)) or LC (π(C∧N) → π*(N∧N)) transition [132; 14;15], and the high spin-orbit coupling constant of the iridium (ζ = 431, 1042, 3909 cm-1
for Fe, Ru, and Ir respectively) facilitates ISC to energetically similar triplet states
within the first 100 fs [60; 132; 190]. Since the relative energy between the 3 MLCT
and 3 LC states can vary depending on nearby circumstances such as solvent or
temperature, a mixing between them can occur accordingly with sufficient overlap.
Thus, it is possible to control the relaxation dynamics by deliberately adjusting thevalence electronic structure, which can be achieved through substituent effects or bychanging the ligand parent strcture entirely [132].
In addition to the so far thorough experimental and computational studies, Tschierlei
et al. carried out the TOAS experiments on [Ir(ppy)2 (bpy)]+ to further specify the
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Figure 4.2: (left) Optical photoabsorption spectrum (above) of [Ir(ppy)2 (bpy)]+ in acetonitrileand (below) its TA spectra at probe delays of 0.5, 0.8, 1.25, 2, 5, and 50 ps after the 388 nm
photoexcitation perpendicularly polarized to the probe pulse. (right) Schematic representationof the relaxation processes in [Ir(ppy)2 (bpy)]+ (Figures from [190]).

relevant relaxation cascade leading to the luminescence [190]. Together with paralleland perpendicular polarization of the optical pump pulse relative to the probe pulse,they reported that both 388 nm and 460 nm photoexcitation resulted in the 3 MLCT
state on the bpy-ligand, and the interligand charge transfer (ILCT) from π*(ppy) toπ*(bpy) on the picosecond timescale followed the femtosecond vibrational cooling(Fig. 4.2). That means, the charge transfer nature involving a change of the transient
dipole moment in the system can be disclosed with the polarization anisotropy TOAS
measurements.
The same working group has recently introduced a phen-5,6-dithiolate ligand insteadof the bpy-ligand that can be readily linked to the second transition metal ion (Fig.
4.3), to achieve either a stable charge separation towards the second metal ion
with significant thermodynamic driving force, or an energy transfer by electronic
coupling between two active sites [55; 170]. Such dyadic systems are usefulmodels in understanding the extent of intermetallic cooperativity which results in an
effective luminescence quenching [58; 124; 96]. To trace back the excited electron
dynamics in the dyadic system, the TOAS experiments were performed on both theIr(III) photosensitizer (IrS2 ) and [Ir(ppy)2 (phen-5,6-dithiolate)Co(C5 H5 )]+ . The TA
spectra of IrS2 upon 388 nm photoexcitation showed pretty much similar features
bathochromically shifted compared to those of [Ir(ppy)2 (bpy)]+ , and this indicates thatthe lowest 3 MLCT state lies on the phen-5,6-dithiolate ligand.
In contrast, the TA spectra of [Ir(ppy)2 (phen-5,6-dithiolate)Co(C5 H5 )]+ revealed
completely different behaviours. Based on the fact that the quenching of theluminescent ES in this molecular system is exergonic towards the cobalt moiety, the

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Figure 4.3: (left) A structural formula of [Ir(ppy)2 (phen-5,6-dithiolate)Co(C5 H5 )]+ and (right) its
TA spectra (below) in comparison with those of IrS2 (above) at certain probe delays after the
388 nm photoexcitation (Figures from [55]).

absence of the 3 MLCT state features can be explained by either an excited electrontransfer or an energy transfer shortly after the photoabsorption. However, the TA
signatures were equally present upon 590 nm photoexcitation as well as from the solecobalt moiety complex, disproving a reasonable long-living charge separation in the
dyadic system. In the end, the Franck-Condon state branched out into two relaxation
pathways, one to the luminescent 3 MLCT state on the phen-5,6-dithiolate ligand, andanother to the singlet ES on the cobalt moiety via the singlet-singlet Dexter energy
transfer [7; 55]. This experiment nevertheless verified that the bridging ligand (BL) can
provide a highly effective coupling of linked metal centers.
A new series of phen-ligands fused with thiophenes by our working group were also
introduced to the Ir(III) photosensitizer as an alternative planar and rigid diimine ligand(N∧N) that is suitable for the electron or energy transfer facilitating the photocatalytic
proton reduction [191; 192]. Figure 4.4 shows two Ir(III) photosensitizers with the
5,7-disubstituted thieno[3,4-f ]phen ligands, namely Ir-O and Ir-th, together with
their optical photoabsorption spectra. The new ligands were then covalently linked
to two chloro(pyridine)cobaloxime(III) catalysts (Ir-O-Co and Ir-th-Co), to again
fully take advantage of the significant thermodynamic driving force towards thecobaloxime(III) (see also Appendix D.1). The reactivity of the photocatalytic proton
reduction, i.e. turnover number (TON), became higher with the presence of aneffective luminescence quenching, hence we carried out the TOAS experiments onthese 4 molecular systems to visualize the ultrafast electronic coupling between two
active sites.
Cobaloxime(III) originally refers to the cobalt(III)-dimethylglyoximato unit with twobidentate anions of dimethylglyoxime occupying the axial positions in coordination

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Figure 4.4: Optical photoabsorption spectra of Ir-O (yellow), Ir-O-Co (green), Ir-th (blue),Ir-th-Co (red), and chloro(pyridine)cobaloxime(III) (purple). The corresponding dipole
transitions to the absorption bands are denoted (LLCT = Ligand-to-Ligand Charge Transfer).
The TON of the photocatalytic proton reduction was higher with the triadic system (Ir-O-Co
and Ir-th-Co) than with the two-component system (Ir-O + cobaloxime(III) and Ir-th +cobaloxime(III)). The TOAS experiments were performed accordingly with the pump laser
centered at 425 nm.

(Co(III)(dmgH)2 ), and was for the first time used as a water reduction catalyst in 1983
with [Ru(bpy)3 ]2+ as a photosensitizer [172; 191; 79; 8; 131]. Among its derivatives,the chloro(pyridine)cobaloxime(III) (purple in Fig. 4.4) has been studied for overa century and widely applied as a proton reduction catalyst [87; 200; 99]. With
its relatively simple synthesis and high efficiency, the cobaloxime(III) and Ir(III)photosensitizer with cyclometalating ligands have been covalently linked and showed
in general enhanced hydrogen production compared to the separate entities [58; 96;124]. In these Ir(III)-Co(III) photocatalysts, the coordination of cobaloxime(III) to theIr(III) photosensitizer appeared to be a crucial factor for the efficiency and stabilityof the photocatalytic proton reduction. In other words, the spatial interaction of two
active sites through a coordination bond enhanced an electron transfer and therefore
stabilized the system.
In line with this, the Ir-O-Co and Ir-th-Co are two new photocatalysts also withenhanced hydrogen production compared to the two-component system [191], thus
their ultrafast dynamics upon MLCT photoexcitation were investigated in comparison
with those of the Ir-O and Ir-th.

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4.2 Experimental details
The Ir-O, Ir-th, Ir-O-Co, and Ir-th-Co were synthesized [191], and dissolved indegassed acetonitrile at the concentration of 1.544 mM, 2.541 mM, 4.03 mM, and
4 mM respectively for the optical density (OD) between 0.1 and 0.3 at 425 nm
photoabsorption and 200 µm optical path length. The 425 nm pump pulse waschosen to selectively excite the Ir(III) moiety and populate its MLCT state accordingly(Fig. 4.4). To prevent compound precipitation and multiple photoexcitation during the
TOAS measurements, each solution was flowed by a micro-gear pump (mzr-2505)
through a microfluidic flat jet system.
Fundamental pulses at 850 nm were first generated by a commercial NOPA (Topas
White by Light conversion) pumped with 800 nm pulses in 0.5 mJ pulse-1 and ∼100 fs(FWHM) from a 5 kHz Ti:sapphire chirped pulse amplifier (Legend Elite by Coherent).
Afterwards the pump pulses at 425 nm (∆λ ∼3 nm) were generated by focusing
the collimated 850 nm pulses into a BBO crystal (250 µm thickness) and by meansof the second-harmonic generation (SHG) at the right phase matching angle. The
residual 850 nm pulses were then filtered out by a BG38 filter, and the pump pulses
were recollimated and focused at the interaction point (∼60 µm diameter in 1/e2 ). The
pump laser power was set to be less than 6 mJ cm-2 , and the power-dependence
measurement was regularly carried out to ensure that the experiments are conductedin a linear absorption regime.
The probe pulse was a broadband supercontinuum covering from 320 nm to 720 nm,
generated by focusing a fraction of the Ti:sapphire laser source into a 5-mm-thick
CaF2 crystal. To avoid thermal damage, the crystal was mounted on a motorizedstage horizontally wobbling. The residual 800 nm pulses after the supercontinuum
generation were then filtered out by a BG40 filter, and the probe pulses were split by
a 50/50 beam-splitter and focused at two spots (∼30 µm diameter in 1/e2 ) on thesample. One of them was spatially overlapped with the pump pulses, and the otherpassed through an unexcited sample position to be used as a reference.
After the interaction with the sample, both supercontinuum pulses were collected
and collimated by a parabolic mirror and then dispersed by a CaF2 prism onto two
512-pixel CMOS sensors (S11105 series by Hamamatsu). The overall time resolutionof the measurements was ∼30 fs (σ). More details on the experimental setup andcalculation of the TA signal in a single-shot detection scheme can be found in theliterature [150; 156].

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4.3 TOAS on Ir(III) photosensitizers
The correction of the probe pulse GVD inside the sample solution as well as the
singular value decomposition (SVD) and global fit (GF) analysis on the reduced TOAS
data to further exclude any irrelevant noises are well-described elsewhere [23; 68;
150; 156]. For the following analysis only the first two singular values were taken
into account, and the reconstructed data successfully resembled the original ones.
The sequential first-order rate equation was then used to model the corresponding
relaxation dynamics.
In this section, we will first start with interpreting the TA spectra from the Ir-O and Ir-th
in comparison with those from the aforementioned Ir(III) photosensitizers. Basicallyall the Ir(III) photosensitizers introduced in this thesis contain two ppy-ligands as
cyclometalating ligands (C∧N), and a different diimine ligand (N∧N) whose π* orbital
hence always corresponds to the LUMO orbital in the molecular system [15].

Figure 4.5: (a) TA spectra of Ir-O at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse, (b)transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF results(solid line) up to 10 ps, (c) up to 100 ps, and (d) the corresponding decay-associated spectra(DAS) with two time constants considered. The ∼9 ps time constant represents the vibrationalcooling in the 3 MLCT state. The time constants of the radiative (τr ) and non-radiative (τnr ) GS
recovery were calculated from the quantum yield and lifetime of the phosphorescence.

Figure 4.5 summarizes the results of the SVD-GF analysis on the reduced TOAS datafrom the Ir-O. The TA spectra up to 450 ps probe delay shows an ESA band centered
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75 of 171at ∼450 nm with a weak but broad absorption tail at the longer wavelengths, and thisESA band looks quite similar to that from the 3 MLCT state of the [Ir(ppy)2 (bpy)]+ andIrS2 (Fig. 4.2, 4.3). Therefore this ESA band is attributed to the 3 MLCT state on thediimine ligand, which is initially in a higher vibronic state (3 MLCT hot) and while being
cooled its absorption band becomes narrower and blue-shifted. Thus, the ∼9 ps time
constant corresponds to the vibrational cooling in the 3 MLCT state, and neither GSB
nor SE signals were identified.

Figure 4.6: (a) TA spectra of Ir-th at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse,(b) transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF
results (solid line) up to 10 ps, (c) up to 100 ps, and (d) the corresponding DASs with three time
constants considered. The ∼0.2 ps and ∼9 ps time constants represent the π-extension andvibrational cooling in the 3 MLCT state respectively. The time constants of the radiative (τr ) andnon-radiative (τnr ) GS recovery were calculated from the quantum yield and lifetime of thephosphorescence.

The TA spectra of Ir-th shows an ESA band centered at ∼510 nm with again a broad
absorption tail at the longer wavelengths (Fig. 4.6). The ESA band is however almost
absent right after the photoexcitation, and evolves with two time constants that are
well-separated (∼0.2 ps and ∼9 ps). The increasing cross section can be explained by
the increasing electron density in the 3 MLCT state on the diimine ligand. Comparedto the Ir-O, the Ir-th has three thiophenes connected to each other via σ-bonds thatcan be rotated. In the GS, the angle between two neighboring thiophene planes is∼70° [191], and in the 3 MLCT state the π* orbital on the diimine ligand can be further
delocalized to the neighboring thiophenes and eventually up to the pyridines, forming

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76 of 171a much larger π-conjugation on the diimine ligand. In other words, the 0.14 ps TA
spectrum represents the 3 MLCT state before the π-extension, and the ESA band
centered at ∼510 nm originates from the larger π-conjugation which develops in two
steps, finally reaching the vibrationally cooled state. Here again, neither GSB nor SE
signals were identified from the TA spectra.
The polarization anisotropy TOAS measurements were also conducted with theparallel and perpendicular polarization of the pump pulse with respect to the probe
pulse, to further investigate the charge transfer nature involving a change of thetransient dipole moment in the system. The results are summarized in Appendix D.1.

4.4 TOAS on Ir(III)-Co(III) triads
The same TOAS experiments were carried out on the Ir-O-Co and Ir-th-Co to provethe electronic coupling between the Ir(III) and Co(III) moiety.

Figure 4.7: (a) TA spectra of Ir-O-Co at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse,(b) transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF
results (solid line) up to 10 ps, (c) up to 500 ps, and (d) the corresponding DASs with threetime constants considered. The ∼9 ps time constant represents the vibrational cooling in the3

MLCT state, whereas the ∼100 ps decay time constant represents an electron transfer to thecobaloxime(III). The time constants of the radiative (τr ) and non-radiative (τnr ) GS recovery
were calculated from the quantum yield and lifetime of the phosphorescence.

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In contrast to the [Ir(ppy)2 (phen-5,6-dithiolate)Co(C5 H5 )]+ case (Fig. 4.3; [55]), the TA
spectra of Ir-O-Co apparently resemble those of the Ir-O in line shape after the 425
nm photoexcitation, and the ESA band from the 3 MLCT state on the diimine ligand
decreases effectively but not completely with the time constant of ∼100 ps (Fig. 4.7).
Since the DAS of the ∼9 ps time constant looks quite similar to that from the Ir-O,
this dynamics is again attributed to the vibrational cooling process. The following∼100 ps decay dynamics therefore proceeds in the 3 MLCT state, leaving the 450 ps
TA spectrum in almost identical line shape. This can be interpreted as a decreaseof the excited electron density, namely a great portion of the excited electron gets
transferred to the cobaloxime(III). This then results in two co-existing ESs, the 3 MLCT
state and a Co(II) ES [26], in accordance with the well-known catalytic mechanism of
the cobaloxime(III) [200; 99; 8; 131]. Nevertheless, we cannot explicitly indicate theelectron transfer nature on rather long timescale, because the cobaloxime(III) does
not reveal any transient signals in the optical regime ([87]; see also Appendix D.1).

Figure 4.8: (a) TA spectra of Ir-th-Co at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse,(b) transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF
results (solid line) up to 10 ps, (c) up to 500 ps, and (d) the corresponding DASs with four time
constants considered. The ∼0.2 ps and ∼9 ps time constants represent the π-extension andvibrational cooling in the 3 MLCT state respectively, whereas the ∼100 ps decay time constantrepresents an electron transfer to the cobaloxime(III). The time constants of the radiative (τr )and non-radiative (τnr ) GS recovery were calculated from the quantum yield and lifetime of thephosphorescence.

The same interpretation applies to the TA spectra of Ir-th-Co (Fig. 4.8). Both DASs of

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the ∼0.2 ps and ∼9 ps time constants are very similar to those from the Ir-th, and thefollowing ∼100 ps decay dynamics again leads to the 450 ps TA spectrum in almost
identical line shape. Hence, regardless of the number of thiophenes in the diimine
ligand, the electron transfer to the cobaloxime(III) occurs in the Ir(III)-Co(III) triadic
system effectively but not completely after the 425 nm photoabsorption with a ∼100 pstime constant.
In addition to the Ir(III)-Co(III) photocatalysts with the 5,7-disubstituted
thieno[3,4-f ]phen ligands, our working group has synthesized another Ir(III)-Co(III)dyadic system with a bpy-derivative BL [143], and the TOAS experiments were
performed on this dyad as well as its respective Ir(III) photosensitizer. The results are
summarized in Appedix D.1.

4.5 Conclusion and outlook

Figure 4.9: A summary of the TOAS experiments on Ir-O-Co and Ir-th-Co photocatalysts.
Upon 425 nm photoabsorption, a great portion of the 3 MLCT excited electron density gets
transferred to the cobaloxime(III) with a ∼100 ps time constant (dashed red), and the rest
decays back to the GS via phosphorescence (blue). A time-resolved X-ray study is foreseen to
close-up the localized dynamics in both Ir(III) and Co(III) moieties (purple) as well as the BL.

Two Ir(III)-Co(III) photocatalysts, i.e. the Ir-O-Co and Ir-th-Co (Fig. 4.9), with the new
5,7-disubstituted thieno[3,4-f ]phen BLs were synthesized in order to improve theirphotocatalytic proton reduction. The ultrafast dynamics in these triadic systems
were investigated in the optical regime upon 425 nm photoexcitation, to visualize thecorresponding global interaction via the electronic coupling between the Ir(III) and
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Co(III) moiety. The TOAS experiments disclosed that a great portion of the electron
density in the 3 MLCT state on the diimine ligand gets transferred to the cobaloxime(III)catalyst with a ∼100 ps time constant, regardless of the number of thiophenes in theBL.
Such an intramolecular electron transfer in a covalently linked bimetallic system isknown to be much faster than ∼100 ps [87; 26; 157], although its thermodynamic
viability does not necessarily mean that its kinetics is favorable [7]. That means, a
new charge distribution in the Ir(III)-Co(III) photocatalysts can trigger for instance a
structural reorganization that takes time, which is in this experiment not clearly visible
with the optical probe window. Besides, the decrease of the 3 MLCT ESA band alsodoesn’t necessarily mean that the electron density has moved to the cobaloxime(III).
Since there was no transient signal appearing from the Co(III) moiety as a response, it
is still not evident that the electron density has arrived in the Co(III) moiety.
As we noted in Chapter 3, the local hard X-ray probe pulse can be exploited tounravel the formal oxidation state and spin density changes in the Co(III) moiety
upon the same 425 nm photoexcitation. In addition, the nuclear dynamics in the
Co(III) moiety or the thiophenes as well as the GS recovery in the Ir(III) moiety canbe monitored. Thereby the TOAS experiments and time-resolved X-ray studies candeliver complementary information on the ultrafast dynamics in inorganic chelates.

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5 4d6 Ru(II)-Mo(VI) photocatalyst

Picosecond "electron" transfer in a dyadic system.

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.3 TOAS on Ru(II) photosensitizers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.4 TOAS on Ru(II)-Mo(VI) dyad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Unlike 5d6 Ir(III) photosensitizers, 4d6 Ru(II) photosensitizers often show their GSB
as well as ESA signals in the optical probe window upon MLCT photoexcitation [157;180; 201; 185]. In other words, the GS recovery dynamics can be easily recognizedfrom the 4d6 Ru(II) photosensitizers, and therefore disentangled from the relaxation
dynamics in the ES manifold.
In accordance with the previous chapter, herein the TOAS methodology with the 425
nm photoexcitation was again exploited to visualize the ultrafast global interaction inanother dyadic system with the Ru(II) photosensitizer. A so-called photo-induced
intramolecular electron transfer (PIET) in a bimetallic system can promote various
catalytic reactions, such as oxygen atom transfer (OAT) by Ru(II)-Mo(VI) photocatalyst[50]. The picosecond PIET dynamics to activate the Mo(VI) moiety were thenidentified as the oxidation of a non-innocent phenol-thiosemicarbazone ligand, bycomparing the difference absorption spectrum from the spectroelectrochemical
oxidation of the catalytic Mo(VI) complex [MoO2 (LMe )MeOH] with the results of the
SVD-GF analysis. This let us to propose a revised mechanism of the PIET-enhanced
OAT catalysis on our biomimetic Ru(II)-Mo(VI) dyad, [Ru(bpy)2 (L2 )MoO2 (solv)]2+ ,in which the 3 MLCT state of the Ru(II) moiety spontaneously perturbed the Mo(VI)moiety to be activated, resulting in two co-existing charge separations.
Since the ultrafast dynamics of the Ru(II)-Mo(VI) dyad within the probe delay windowof 500 ps were dominated by the Ru(II) moiety, explicit designation of the oxidation of
the chromophoric part and the concomitant reduction in the Mo(VI) moiety was notpossible. This chapter was written primarily based on our peer-reviewed work [157].

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5.1 Introduction
Metalloenzymes often regenerate their active sites via efficient electron transfer
between spatially-separated electron transfer units [76]. Thus, the comprehensionof electronic relays between these components in bio-inspired redox catalysts is
attracting increasing interests [198; 44].
Electrochemical approaches to mimic such a process include for instance theattachment of reversible electron transfer components to metalloenzyme-mimics[71; 22; 187]. PIET processes have also been explored to harness solar energy andthereby to accelerate redox catalysis. For example, Ru(II) photosensitizers covalently
linked to Mn-based photosystem II models were introduced to mimic the four-electronoxidation of two water molecules by light-induced electron transfer [82]. Similarly,hydrogenase-mimics with reversible photosensitizers attached have been investigatedwith the aim of catalysing the light-driven production of H2 [155; 183].
Photo-redox processes have also been utilized to catalyse oxygenation reactions withenvironmentally-benign O-atom sources [120; 5; 162], in particular 3 O2 or H2 O [89].
Enemark and Kirk et al. for instance demonstrated that oxo-molybdenum(V) can be
photo-activated via an antenna-mediated electron transfer process by covalently
linking the oxo-molybdenum(V) to porphyrin-Fe(III) or Zn(II) complexes [197; 12].
Recently, Duhme-Klair et al. have developed biomimetic cis-dioxo Mo(VI) complexes
with appended Ru(II) photosensitizers to facilitate the OAT catalysis via the PIET
(Fig. 5.1; [51]). They reported that the attachment of a bis(bipyridyl)-phenanthroline
Ru(II) complex to a cis-dioxo Mo(VI) thiosemicarbazone allows the OAT from dimethyl
sulfoxide (DMSO) to triphenyl phosphine (PPh3 ) to be accelerated upon irradiation
with visible light. It was therefore proposed that the oxidation of the bpy- -ligand
upon MLCT photoexcitation by an oxidizing agent (OxA) leads to a highly reactive,
one-electron oxidized phenol-thiosemicarbazone ligand in the catalytic Mo(VI) moiety
via the PIET ((3) in Fig. 5.1), being the key to accelerate the rate-determining step of
the OAT catalysis [50].
Since the oxidative quenching process of the bpy- -ligand is diffusion-limited, thehighly reactive one-electron oxidized Mo(VI) moiety is expected to be present
on rather microsecond to millisecond timescale. It is worth noting that the formaloxidation state of the molybdenum is already 6+ and cannot increase further.
Thus, the electron transfer following the oxidative quenching is most likely from thenon-innocent phenol-thiosemicarbazone ligand in the catalytic Mo(VI) moiety, towards

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Figure 5.1: Light-induced activation of cis-dioxo Mo(VI) catalytic moiety in Ru(II)–Mo(VI) dyad
[Ru(bpy)2 (L2 )MoO2 (MeOH)]2+ , as proposed in [50]: (1) The MLCT photoexcitation to one of the
bpy-ligands, (2) the oxidative quenching of the bpy- -ligand by a diffusion-limited reaction with an
OxA, followed by (3) the activation of the Mo(VI) moiety by an electron transfer to the Ru(II)moiety. (4) An OAT catalysis taking place to a reducing agent (ReA) from the activated cis-dioxo
Mo(VI) moiety.

the Ru(II) moiety. As this kinetic model was proposed based on catalytic studies andsteady-state characterization methods, a further validation with the time-resolved
techniques is highly desirable.
With this motivation we carried out the TOAS experiments on the Ru(II)-Mo(VI)photocatalyst upon 425 nm photoexcitation. Additionally, we performed control
experiments on the Ru(II) photosensitizer [Ru(bpy)2 (phen-NH2 )]2+ and [Ru(bpy)3 ]2+ todisentangle the ultrafast dynamics in the Ru(II) moiety from those in the Mo(VI) moiety,and ultimately to identify the initial steps of the PIET-enhanced OAT catalysis.

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5.2 Experimental details
The [Ru(bpy)2 (phen-NH2 )]2+ , [Ru(bpy)3 ]2+ , and Ru(II)-Mo(VI) dyad were synthesized[50], and dissolved in degassed acetonitrile at the concentration of ∼1.2 mM, ∼1.3 mM,and ∼0.5 mM respectively for the OD between 0.1 and 0.3 at 425 nm photoabsorption
and 200 µm optical path length. The 425 nm pump pulse was chosen to selectively
excite the Ru(II) moiety and populate its MLCT state accordingly. To prevent
compound precipitation and multiple photoexcitation during the TOAS measurements,
each solution was flowed by a micro-gear pump (mzr-2505) through a microfluidic flatjet system.
Fundamental pulses at 850 nm were first generated by a commercial NOPA (Topas
White by Light conversion) pumped with 800 nm pulses in 0.5 mJ pulse-1 and ∼100 fs(FWHM) from a 5 kHz Ti:sapphire chirped pulse amplifier (Legend Elite by Coherent).
Afterwards the pump pulses at 425 nm (∆λ ∼3 nm) were generated by focusing
the collimated 850 nm pulses into a BBO crystal (250 µm thickness) and by meansof the second-harmonic generation (SHG) at the right phase matching angle. The
residual 850 nm pulses were then filtered out by a BG38 filter, and the pump pulses
were recollimated and focused at the interaction point (∼60 µm diameter in 1/e2 ). The
pump laser power was set to be less than 6 mJ cm-2 , and the power-dependence
measurement was regularly carried out to ensure that the experiments are conductedin a linear absorption regime.
The probe pulse was a broadband supercontinuum covering from 320 nm to 720 nm,
generated by focusing a fraction of the Ti:sapphire laser source into a 5-mm-thick
CaF2 crystal. To avoid thermal damage, the crystal was mounted on a motorizedstage horizontally wobbling. The residual 800 nm pulses after the supercontinuum
generation were then filtered out by a BG40 filter, and the probe pulses were split by
a 50/50 beam-splitter and focused at two spots (∼30 µm diameter in 1/e2 ) on thesample. One of them was spatially overlapped with the pump pulses, and the otherpassed through an unexcited sample position to be used as a reference.
After the interaction with the sample, both supercontinuum pulses were collected
and collimated by a parabolic mirror and then dispersed by a CaF2 prism onto two
512-pixel CMOS sensors (S11105 series by Hamamatsu). The overall time resolutionof the measurements was ∼30 fs (σ). More details on the experimental setup andcalculation of the TA signal in a single-shot detection scheme can be found in theliterature [150; 156].

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5.3 TOAS on Ru(II) photosensitizers
The correction of the probe pulse GVD inside the sample solution as well as the
SVD-GF analysis on the reduced TOAS data to further exclude any irrelevant noises
are well-described elsewhere [23; 68; 150; 156]. For the following analysis the first
five singular values were taken into account, and the reconstructed data successfully
resembled the original ones. The sequential first-order rate equation was then used tomodel the corresponding relaxation dynamics [157].

Figure 5.2: A structural formula of (A) [Ru(bpy)3 ]2+ and (B) [Ru(bpy)2 (phen-NH2 )]2+ together
with their optical absorption spectra.

First of all, both [Ru(bpy)3 ]2+ and [Ru(bpy)2 (phen-NH2 )]2+ (Fig. 5.2) were subjectedto the TOAS experiments to serve as a basis set for the Ru(II)-Mo(VI) dyad. The
corresponding ultrafast dynamics have been extensively researched over the last few
decades [201; 25; 68; 180; 47; 185], and will be revisited in this section with the 425
nm photoexcitation. The following comparison of the TA spectra and their kinetics willthen allow us to distinguish the transient signal from the Mo(VI) moiety out of theothers. Figure 5.3 summarizes the results of the SVD-GF analysis on the reduced
TOAS data from the [Ru(bpy)3 ]2+ , [Ru(bpy)2 (phen-NH2 )]2+ , and Ru(II)-Mo(VI) dyad.
The TA spectra of the [Ru(bpy)3 ]2+ (Fig. 5.3 A) shows its prominent ESA (∼365 nm)
and GSB (∼450 nm) signals. This ESA occurs at the bpy- -ligand [185], and the GSB
matches well with the inverted OA in line shape. The femtosecond DAS (Fig. 5.3 B)
can be rationalized as the modulation of the electron density in the MLCT state due tothe solvation [23], whereas the picosecond DAS is assigned to the vibrational cooling.
The TA spectra of the [Ru(bpy)2 (phen-NH2 )]2+ (Fig. 5.3 C) show slightly different
behaviours especially in the ESA band, due predominantly to the GS absorption
band of the phen-NH2 -ligand at the similar wavelength [202]. In other words, thesuppressed ESA band on the femtosecond timescale that differs from the [Ru(bpy)3 ]2+corresponds to the ESA at the phen-NH2 - -ligand, and ILCT follows to the bpy-ligand,fully recovering its ESA band line shape on the picosecond timescale [180; 47].

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Figure 5.3: A representative selection of the TA spectra from the (A) [Ru(bpy)3 ]2+ , (C)[Ru(bpy)2 (phen-NH2 )]2+ , and (E) Ru(II)-Mo(VI) dyad, together with their inverted opticalabsorption (OA) spectra and (B, D, F) DASs. For the comparison of the results, the number ofspectral components was kept to 5 and the ones not used in the SVD-GF analysis are labelledwith "-". The spectral range contaminated with pump scattering is masked with a grey box.

That means, the 425 nm photoexcitation resulted in the prompt population of thephen-NH2 - -ligand as well as the bpy- -ligand, and the 3 MLCT state became localized
onto the latter. This is now reflected in the DAS of the ∼3 ps time constant (Fig. 5.3 D),
and the other DASs can be interpreted as before.
The same ILCT dynamics is also observed in the TA spectra of the Ru(II)-Mo(VI)dyad (Fig. 5.3 E), and detailed interpretation of the DASs (Fig. 5.3 F) as well as anadditional signal appearing at λprobe >500 nm will follow in the next section. During the
TOAS experiments, no SE signal was observed in agreement with its non-allowed
character [25].
Figure 5.4 again compares the TA spectra at representative probe delays of 110 fs, 2

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Figure 5.4: Comparison of the TA spectra at certain probe delays after the 425 nm
photoexcitation between (above) [Ru(bpy)3 ]2+ and [Ru(bpy)2 (phen-NH2 )]2+ , and between(below) [Ru(bpy)2 (phen-NH2 )]2+ and Ru(II)-Mo(VI) dyad. The spectral range contaminated withpump scattering is masked with a grey box.

ps, and 200 ps between the [Ru(bpy)3 ]2+ and [Ru(bpy)2 (phen-NH2 )]2+ , and between[Ru(bpy)2 (phen-NH2 )]2+ and Ru(II)-Mo(VI) dyad. From the first comparison, weclearly notice a transition in the ESA band from 110 fs to 2 ps by the ILCT dynamics.
This transition is also present in the Ru(II)-Mo(VI) dyad, although two additional
dynamics at λprobe >500 nm on the femtosecond timescale and at λprobe <500 nm on
the picosecond timescale will be further analysed in the next section.

5.4 TOAS on Ru(II)-Mo(VI) dyad
Figure 5.5 is a reprint of the Fig. 5.3 E and F as well as the difference TA (DTA)
spectra of the Ru(II)-Mo(VI) dyad at certain probe delays after the 425 nm
photoexcitation with respect to the 200 fs TA spectrum. Here we will further interpret
two distinctive dynamics appearing at λprobe >500 nm on the femtosecond timescale
and at λprobe <500 nm on the picosecond timescale.
Both dynamics are visible in presence of the catalytic Mo(VI) moiety, and byassuming that the dyadic system contains only the 3 MLCT state at 200 fs probe
delay, we can separate the contribution from the Mo(VI) moiety by subtracting the
200 fs TA spectrum. Among the obtained DTA spectra, it is undoubtful that the(200 ps-200 fs) DTA spectrum and the inverse sum of the DAS6.8ps and DAS42ps
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Figure 5.5: (left) A representative selection of the TA spectra from the Ru(II)-Mo(VI) dyad (A),together with its inverted OA spectrum and (B) DASs. (right) DTA spectra at certain probe
delays after the 425 nm photoexcitation with respect to the 200 fs TA spectrum (A), and acomparison of the difference spectrum from the spectroelectrochemical oxidation of thecatalytic Mo(VI) complex [MoO2 (LMe )MeOH] with the (200 ps-200 fs) DTA spectrum and theinverse sum of the DAS6.8ps and DAS42ps [50]. A relative shift of 0.65 eV was applied in theenergy domain and the spectral range contaminated with pump scattering is masked with agrey box.

show strong similarities at any wavelength with the difference spectrum from thespectroelectrochemical oxidation of the catalytic Mo(VI) complex [MoO2 (LMe )MeOH][50], after applying a relative shift of 0.65 eV (Fig. 5.5 right). This spectral shift isjustified since the global charge of the electrochemically oxidized Mo(VI) catalyst[MoO2 (LMe )MeOH] and the photoexcited dyad are different. Hence, we can deducethat (1) the femtosecond signals at λprobe >500 nm originate from the LC transitionsin the neutral Mo(VI) moiety, which are just red-shifted by the strong electrostatic
field from the transient dipole moment in the Ru(II) moiety [115; 144], and (2) the
picosecond signals are an oxidative process of the chromophoric part in the Mo(VI)moiety, i.e. the conjugated phenolate-thiosemicarbazone ligand [179].
In other words, the GS absorption signal of the neutral Mo(VI) moiety at λprobe >500
nm starts disappearing on the picosecond timescale, in accordance with a new
charge distribution from the Mo(VI) coordination ligand sphere towards acceptor

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orbitals on the non-innocent ligand close to the BL or on the BL itself. The dyadic
system is also not as flat and stretched as in Fig. 5.1 since the linker is very flexible,
thus a folded structure is possible. That means, we cannot exclude a (partial) charge
localization on the cis-dioxo Mo(VI) or Ru(II) moiety, but this did not cause any
detectable distortion in the associated TA spectra.
The inverse sum of the DAS6.8ps and DAS42ps from the SVD-GF analysis hence
describes the oxidation dynamics of the Mo(VI)-coordinated chromophoric ligand
spanning from 2 to 60 ps. Therefore, we can understand that upon the MLCT
photoexcitation the oxidation of the non-innocent phenol-thiosemicarbazone ligand
in the catalytic Mo(VI) moiety occurs on the picosecond timescale, while the 3 MLCT
state in the Ru(II) moiety is still intact. The complete bleaching of the GS absorption
signal of the neutral Mo(VI) moiety at λprobe >500 nm can be then interpreted as theunitary quantum yield of the second ES in the dyadic system.
In addition to the 425 nm photoexcitation, we conducted the same TOAS experiments
and SVD-GF analysis with 450 nm photoexcitation to rule out that the observed
dynamics resulted from the direct photoabsorption by the Mo(VI) moiety. The results
are consistent and can be found in Appendix D.2.

5.5 Conclusion and outlook
In conclusion, our study revealed that the enhanced OAT on a Mo(VI) oxotransferase
model in the Ru(II)-Mo(VI) dyad stems from the picosecond PIET in the Mo(VI)moiety resulting in the second ES upon the MLCT photoexcitation. An oxidative
process of the chromophoric part in the Mo(VI) moiety, namely the conjugatedphenolate-thiosemicarbazone ligand, spans from 2 to 60 ps and leads to the catalytic
activation of the Mo(VI) moiety. These dynamics do not accompany any GS recovery
in the Ru(II) moiety, indicating that the dynamics in Ru(II) and Mo(VI) moieties are notcoupled within our probe delay window.
Accordingly we propose a revised mechanism of the PIET-enhanced OAT catalysis
as in Fig. 5.6. Without any diffusion-limited process considered, the simultaneouspresence of both photo-activated states in the Ru(II) and Mo(VI) moieties unveiled an
unexpected scenario, where the second excited electron from the non-innocent ligand
is not transferred to the Ru(II) moiety but only to the acceptor orbitals close to the BL,
or on the cis-dioxo Mo(VI) moiety.
This novel and ultrafast photocatalytic trigger in such a small supramolecular dyadic
system has not been reported before, and therefore it will have an important impact

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Figure 5.6: Proposed mechanism for the initial steps leading to the light-induced activation of
the catalytic Mo(VI) moiety.

on modelling photocatalytic processes in bimetallic dyads, not only concerning theultrafast timescale but also much longer timescales, such as µs or even to ms which isway slower than the expected lifetime of the bi-activated dyadic system. For instance,
one of the first implications is that both the neutralization of the bpy- -ligand by an OxAsuch as MV2+ and the reduction of the catalytic Mo(VI) moiety by a ReA such as PPh3
can potentially proceed in parallel. On the other hand, the OAT can be much slower(seconds to minutes; [83]) than the oxidative quenching process (typically µs).
In fact the PIET-enhanced OAT was only feasible in the presence of MV2+ [50], andthe OAT to the PPh3 was an extremely slow process (several minutes). This discloses
that the oxidative quenching by the MV2+ prolongs the lifetime of the second ES in theMo(VI) moiety beyond the microsecond timescale, letting the PIET-enhanced OAT
process be feasible. Remarkably, despite the slow catalytic activity at the Mo(VI)moiety cannot be directly correlated to the ultrafast formation of the bi-activated state,
the presence of this state is still crucial for the catalytic function.

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While this study revealed the formation of two photo-activated states and theirdynamics, it raises yet more questions relating to the role of the solvent or specificnuclear coordinates in modulating the sequence of the reactions, as well as the local
electronic and structural changes involved in the process. In perspective it will be
crucial to carry out the time-resolved XAS and XSS experiments with the hard X-ray
probe pulse, together with the polarization anisotropy TOAS experiments.

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6 4d6 Ru(II) photosensitizer

We know, that we don’t know.

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
6.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.3 Time-resolved EXAFS on Ru(II) photosensitizers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.4 Lifetime studies in acetonitrile and water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
As noted in Chapter 4 and 5, a long-lived 3 MLCT state (>100 ps) of the inorganic
chelates upon optical photoabsorption is crucial to the light-harvesting applications[141; 190; 191; 157]. Compared to the 5d6 Ir(III) photosensitizers, the 4d6 Ru(II)photosensitizers have weaker ligand field splitting, letting the 3 MC state be placed
closer to the 3 MLCT state (∆E ∼0.5 eV; [60]). This opens up a thermal population of
the 3 MC state in the second relaxation pathway, often lowering the phosphorescencequantum yield and causing the photodegradation [194].
The 3 MLCT state population and depopulation kinetics with respect to the solvent,temperature, and presence of oxygen have been therefore extensively studied
starting with the [Ru(bpy)3 ]2+ and [Ru(phen)3 ]2+ [59; 194; 139; 193; 127; 68]. The
localization of the excited electron and the following transient dipole moment within
the first picosecond are also of great importance to direct the excited electron to aparticular ligand [201; 180; 193; 127], especially in the heteroleptic complexes with adipyrido[2,3-a:3’,2’-c]phenazine (dppz) ligand known for its "light-switch" effect [171;153]. With a highly π-deficient ligand such as 1,4,5,8-tetraazaphenanthrene (TAP)
instead of bpy or phen however, the [Ru(TAP)2 (dppz)]2+ becomes again strongly
luminescent in water by having the excited electron localized on the TAP-ligand [154].
Complementary to the thorough researches so far, herein we report the results ofthe time-resolved EXAFS experiments on the light-switching and photo-oxidizing
Ru(II) complexes, i.e. [Ru(phen)3 ]2+ , [Ru(phen)2 (dppz)]2+ , [Ru(TAP)2 (dppz)]2+ , and[Ru(TAP)3 ]2+ in degassed acetonitrile upon 400 nm photoexcitation. In addition, theirtransient absorption signal kinetics measured in degassed water and acetonitrile
further confirmed the non-radiative deactivation processes previously reported fromthe luminescence lifetime studies [127; 152; 139; 154].

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6.1 Introduction
Based on the Fourier analysis of the EXAFS at its K- and L3 -edge (Section 2.3.1), thecoordination shell structures of the [Ru(bpy)3 ]2+ and its derivatives have shown theirchanges with respect to the surrounding elements [88], different ligands [16; 72; 166;
167], and photoabsorption [70; 169]. In general the first and second coordination shellstructure differ only on the picometer-scale regardless of the chemical environment,
and this predominantly comes from a structural reorganization by different charge
distribution in the system [182]. When it comes to the 3 MLCT state structure, acontraction of Ru-N bonds by ∼0.03 Å on average is caused by the strong transient
dipole moment with the localized excited electron.
The localized excited electron in the 3 MLCT state has its own lifetime and usuallydecays radiatively back to the GS. The oxidative quenching as well as the thermal
population of the 3 MC state are however two representative non-radiative decay
pathways present at the same time [194; 59; 139], and depending on its chemical
environment the Ru(II) photosensitizer can be either "switched-on" or off [148]. Sucha luminescent probe with its readily tunable photophysical and electrochemical
properties has been accordingly introduced to some biological applications [147; 30].

Figure 6.1: (left) A structural formula of phen-, TAP-, and dppz-ligand and (right) opticalphotoabsorption spectra of [Ru(phen)3 ]2+ , [Ru(phen)2 (dppz)]2+ , [Ru(TAP)3 ]2+ , and[Ru(TAP)2 (dppz)]2+ in acetonitrile.

The Ru(II) photosensitizers (Fig. 6.1) that recognize and react with nucleic acids
have been reported since the 1980s [78; 30], affording a rich vein of insight into
the relationship between biomolecular structure and photophysical dynamics.
The early work on the interaction of the ∆ and Λ enantiomers of [Ru(phen)3 ]2+with double-stranded DNA showed that the ∆ enantiomer bound more strongly
to B-DNA than the Λ one, by insertion of a phen-ligand between the base-pairsof the polynucleotide (intercalation; [30]). A clear way to increase the strength of

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93 of 171binding interactions through intercalation is thus to incorporate a more extended
heteroaromatic ligand, such as a dppz-ligand (Fig. 6.2; [78; 152]).

Figure 6.2: Intercalated [Ru(L)2 (dppz)]2+ in double-stranded DNA fragment (CG-GC; Figure
from [4]).

A striking property of [Ru(phen)2 (dppz)]2+ for example is that it is non-emissive inaqueous solution but luminesces strongly when it binds to DNA [30]. The first TOAS
measurement on the [Ru(phen)2 (dppz)]2+ in aqueous media was made by the groupof Barbara with the 400 nm photoexcitation [152]. The results unveiled two ultrafast
dynamics, one with the lifetime of 3 ps and the other with 250 ps. In D2 O however,the lifetime was determined with a single time constant of 560 ps, indicating that the
non-radiative deactivation process strongly depends on the -OH or -OD vibration.
From a subsequent study, it was also concluded that in the ES both phenazine
nitrogens (N9 and N14) of the dppz-ligand were coordinated to water [151].
It is well-known that the electrochemical properties of the Ru(II) photosensitizers canbe modified by change of ligands. One system that has been largely investigatedin this regard is TAP, where the electron-withdrawing nature of the ligand makes
the complex a much stronger OxA. A consequence of this is, for instance,that [Ru(TAP)3 ]2+ causes significantly more photo-damage to DNA than does[Ru(phen)3 ]2+ , as was first demonstrated using plasmid DNA to monitor single strand
breaks [108]. However, later work using 32 P-labelled DNA showed that a more
important reaction is the formation of photo-adducts [57]. Subsequently this adductwas isolated and characterized, and shown to involve covalent attachment of the
TAP-ligand to the exocyclic 2-N of guanine [97].
It is clear from these early studies that TAP-complexes such as [Ru(TAP)3 ]2+ makean interesting system to study the photo-oxidation of DNA, but they suffer from thedisadvantage that the binding to DNA is relatively weak and the mode of binding
is uncertain. A much better option is to study complexes which can be expected tobind via intercalation. Such a complex is [Ru(TAP)2 (dppz)]2+ , which was found to

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photo-induced electron transfer (PET) from guanine to the ES, as is the case for othercomplexes having at least two TAP-ligands [121].
To further investigate the primary processes of these photo-oxidation reactions,
TOAS experiments were carried out. Initial experiments were performed with
nucleotides, and showed that the ES of [Ru(TAP)3 ]2+ was efficiently quenched by
5’-guanosine-monophosphate (GMP) at rates close to diffusion-controlled but muchless efficiently by 5’-adenosine-monophosphate (AMP), and not measurably by the
pyrimidine nucleotides [122]. In the case of GMP, quenching caused the productionof the reduced species [Ru(TAP)2 (TAP⋅- )]+ or [Ru(TAP)2 (TAPH⋅ )]2+ (pKa = 7.6) andthe guanine radical (which is formed by deprotonation of the guanine radical cation atneutral pH as it has a pKa of 3.9). The back reaction of the reduced species and theguanine radical proceeds with a rate constant close to that of diffusion control. Below
we summarize the results of TOAS and luminescence lifetime studies on both thelight-switching and photo-oxidizing complexes (Table 6.1).
Lifetimes

MeCN

H2 O

Ru(phen)3 [127]320 ns920 nsRu(phen)2 (dppz)

180 ns [147; 148]

3 ps, 250 ps [152]Ru(TAP)2 (dppz)

N/A [161]

820 ns [154]

Ru(TAP)3 [139]68 ns

223 nsTable 6.1: The 3 MLCT state lifetimes of the Ru(II) light-switching and photo-oxidizing
complexes in acetonitrile and water.

The homoleptic [Ru(TAP)3 ]2+ , famous for its photo-oxidation on the π-deficient ligand
via the PET, is also famous for its peculiar photophysics in the 3 MLCT state [139;100]. From the temperature dependence studies [194], the lower energy barrier tothe thermal population of the 3 MC state makes the non-radiative deactivation roughly
twice faster than the [Ru(bpy)3 ]2+ , responsible for the photolability of the [Ru(TAP)3 ]2+ .
This is now reflected in its relatively short phosphorescence lifetimes (Table 6.1),
arising a question on its ES geometric structure.
Even shorter lifetime has been reported from the heteroleptic [Ru(phen)2 (dppz)]2+ inwater [152; 153; 39], and this extraordinary dynamics was then understood as thelowest 3 MLCT state on the phenazine is induced by an exergonic ILCT (Fig. 6.3),
where its two nitrogens (N9 and N14) can be coordinated to water molecules and

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efficiently quenched by the partial proton donation [151]. On the other hand, the[Ru(TAP)2 (dppz)]2+ with its prominently enhanced photo-oxidizing ability showed thatits lowest 3 MLCT state is on one of the TAP-ligands acting as the electronic "sink"electrochemically and photophysically [147; 38; 52; 173]. Their geometric structurechanges in both 3 MLCT states on different ligands have been yet visited so far.

Figure 6.3: Simplified Jablonski diagrams of [Ru(TAP)2 (dppz)]2+ (left) and [Ru(bpy)2 (dppz)]2+(right) with their dipole transitions upon MLCT photoexcitation denoted (Figure from [173]).

In this Chapter, the results of the time-resolved EXAFS experiments on [Ru(phen)3 ]2+ ,[Ru(phen)2 (dppz)]2+ , [Ru(TAP)2 (dppz)]2+ , and [Ru(TAP)3 ]2+ dissolved in acetonitrile
will be discussed. The experiments were carried out at the 11-ID-D beamline of the
APS upon 400 nm photoexcitation, and the sample solution at 278 K was always
bubbled with nitrogen gas during the measurements.

6.2 Experimental details
The time-resolved EXAFS experiments on the [Ru(phen)3 ]2+ , [Ru(phen)2 (dppz)]2+ ,[Ru(TAP)2 (dppz)]2+ , and [Ru(TAP)3 ]2+ dissolved in degassed acetonitrile were
performed at the 11-ID-D beamline of the APS upon 400 nm photoexcitation. The
sample concentration was set to 4 mM except [Ru(phen)3 ]2+ (2.5 mM), and flowed in
the liquid jet system through an injector with its inner diameter of 600 µm, to prevent
compound precipitation and multiple photoexcitation during the measurement.
The 400 nm pump pulse was the second-harmonic output from a 10 kHz Coherent
Legend Elite Ti:sapphire ultrafast amplifier laser system. The pump pulses were thenfurther stretched to ∼1.6 ps (FWHM) by a pair of prisms. The monochromatized X-ray
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probe pulses (∆E/E ∼ 10−4 ) were delivered at 6.5 MHz with ∼78.8 ps (FWHM) fromthe APS storage ring under the 24-bunch operation mode with ∼4*1012 photons
sec-1 . The optical laser spot size (FWHM) was ∼800 µm (horizontal) by ∼130 µm(vertical) at the interaction point, yielding a fluence of ∼75 mJ cm-2 . The X-ray beam
size (FWHM) was ∼500 µm (horizontal) by ∼70 µm (vertical) at the interaction point,
hence completely covered by the optical laser.
The emitted X-ray photons from the Ru(II) photosensitizer were recorded at 90°
with respect to the incident X-ray beam on both sides by two APDs (TFY mode). A
combination of soller slits and Mo filter, custom-designed for the sample chamberconfiguration and the distance between the sample and detectors, was inserted
between the liquid jet system and the APDs. The first 12 X-ray pulses before theoptical photoexcitation were collected for the GS XAFS spectrum, whereas the later
X-ray pulses for the ES XAFS spectrum at 150 ps probe delay after the 400 nm
photoexcitation. Each energy point was integrated for 4 s.

6.3 Time-resolved EXAFS on Ru(II) photosensitizers
First of all, detailed EXAFS data reduction and fitting procedures are well-described
elsewhere [163; 195; 19]. The molecular structures of each complex in its GS
and 3 MLCT state were obtained by applying the density functional theory (DFT;PBE0/def2-svp), and these structural models were then imported as an initial guess tosimulate and fit the corresponding EXAFS spectra using FEFF8 and IFEFFIT codes
respectively (Fig. 2.16; [6]).
Upon optical laser irradiation, the molecules that absorb optical photons as well as
those not will be both exposed to the X-ray probe pulse. That means, what X-ray
photons see is a mixture of the GS and ES molecules. The resulting transient signal
T (E, t) can be therefore described as follows [17];T (E, t) = f (t) ∗ (ES(E, t) − GS(E)) = LaserOn(E, t) − GS(E)(6.1)

where f (t) is the ES fraction that is presumed to be constant during each EXAFS
measurement at probe delay of t, but can differ for each complex. So once wemeasure T (E, t) and know f (t) value for it, we can extract the "pure" ES EXAFS
spectrum ES(E, t). Since it is well-known that a charge redistribution in the 3 MLCT
state of Ru(II) photosensitizer will cause ∼2 eV shift of its XAFS spectrum [88; 16;72; 166; 167; 169], by ranging the f (t) value from 0 % to 80 % we found the energy
shift at three different features; the K-edge, white line, and peak position of the first
derivative (Fig. 6.4).

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Figure 6.4: The energy shift at the K-edge, white line, and peak position of the first derivative ofeach Ru(II) XAFS spectrum with respect to the presumed f (t) value ranging from 0 % to 80 %.
The f (t) value for each complex was then determined at ∆E ∼2 eV.

Hereby we determined the f (t) value of each Ru(II) photosensitizer at probe delay of
150 ps and thus their pure ES EXAFS spectra. The χ(k ) spectra of [Ru(phen)3 ]2+ ,[Ru(phen)2 (dppz)]2+ , [Ru(TAP)2 (dppz)]2+ , and [Ru(TAP)3 ]2+ in their GS and 3 MLCT
state weighted by k 2 are shown in Fig. 6.5. In their comparison, there seems nodifference in coordination shell structure between the GS and 3 MLCT state at 150 psprobe delay, nor in scattering pathways between the Ru(II) photosensitizers. In otherwords, the localized excited electron on the dppz-ligand, TAP-ligand, or even in the3

MC state did not trigger any significant changes in their geometric structures.

This is also confirmed from a sanity checking the transient oscillatory structure by justcomparing with the subtraction of the GS EXAFS spectrum from the 2-eV-shifted
one (Fig. 6.6). Such a sanity check easily proves that the oscillations in the transient
signal T (E, t) predominantly originated from the energy shift of the spectrum by the
charge redistribution in the system, not from the geometric structure change.
Now to verify the first and second coordination shell structure changes in the ES,

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Figure 6.5: The χ(k) spectra of the Ru(II) photosensitizers in their GS and 3 MLCT state at theprobe delay of 150 ps weighted by k 2 .

the EXAFS equation (2.4) was fitted to each EXAFS spectrum. Based on the
DFT-optimized molecular structures of the Ru(II) photosensitizers in their GS and
3

MLCT state, the calculations of scattering amplitudes and phases for the simulated

EXAFS spectra were carried out using the FEFF8 code. The simulated spectra werethen fitted to the k 2 -weighted χ(k ) spectra in R-space (∣χ(R)∣; see also the Section
2.3.1) with Artemis software (part of the IFEFFIT). The fitting results are summarizedin Table 6.2 and Appendix D.3.
As expected, the coordination shell structure of the Ru(II) complexes did not changethat much in their 3 MLCT states. By setting two independent fitting parameters for theR(Ru-NMLCT ) and R(Ru-N), we were however able to increase the spatial resolution of
the experiment down to the picometer-scale. In general the bond lengths between
the photo-oxidized metal ion and photo-reduced ligand became shorter than thosein the GS, and others in return longer. The changes were most pronounced in the[Ru(phen)3 ]2+ and least in the [Ru(TAP)2 (dppz)]2+ , although their Debye-Waller factors
are too large to confirm these changes. If the only one fitting parameter is set for the
ES R(Ru-NAVR ), the bond lengths do not change in the 3 MLCT state.
As noted in Section 6.1, at 150 ps probe delay after the 400 nm photoexcitation, thelocalized excited electron in the [Ru(phen)2 (dppz)]2+ should be on the phenazine
and the [Ru(TAP)3 ]2+ may start populating its 3 MC state on the way back to the GS.

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Figure 6.6: A sanity checking the transient oscillatory structures by comparing with thesubtraction of the GS EXAFS spectrum from the 2-eV-shifted one (pGS). The close
resemblance proves that the transient signal T (E, t) primarily originates from the energy shift ofthe spectrum by the charge redistribution in the system.
400 nm, 150 ps

GS R(Ru-N)

ES
R(Ru-NMLCT )ES
R(Ru-N)ES R(Ru-NAVR )GS
σ 2 (N)ES
σ 2 (N)Ru(phen)3

2.06 Å

2.03 Å

2.11 Å

2.06 Å

0.00210.0028Ru(phen)2 (dppz)

2.06 Å

2.045 Å2.096 Å2.06 Å

0.00190.0019

Ru(TAP)2 (dppz)

2.06 Å

2.06 Å

2.06 Å

2.06 Å

0.00260.0026

Ru(TAP)3

2.06 Å

2.037 Å2.08 Å

2.06 Å

0.00310.0056

Table 6.2: Fitting results of the EXAFS equation (2.4) to the k 2 -weighted χ(k) spectra of theRu(II) photosensitizers in their GS and 3 MLCT state at the probe delay of 150 ps after the 400
nm photoexcitation (Fig. 6.5).

Apparently from our EXAFS fitting analysis with the picometer-scale spatial resolution,
both ultrafast dynamics were not able to be distinguished at the probe delay of 150
ps. Hence, we additionally measured the transient absorption signal kinetics of each
Ru(II) photosensitizer in degassed acetonitrile and water, to monitor how the energy
shift of the XAFS spectrum decays back to 0 depending on the solvents.

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6.4 Lifetime studies in acetonitrile and water
The transient absorption signal kinetics were measured with the monochromatized
X-ray pulse (∆E/E ∼ 10−4 ) at the peak position of each transient XAFS spectrum, andfitted with the exponential function convoluted with the Heaviside step-function andinstrumental response function (IRF; Gaussian profile). The fitting results as well as
the model equation T (Epeak , t) are shown in Fig. 6.7 and (6.2) respectively,
T (Epeak , t) =T0 (Epeak )t − t0σ2t − t0σ∗ exp( 2 −) ∗ [1 + erf ( √ − √ )]
2
2τ
τ
2τ
2σ

(6.2)

where T0 (Epeak ) is an amplitude factor, σ is the standard deviation of the IRF, τ is the
decay time constant, and t0 is the instrumental time-zero.

Figure 6.7: Fitting analysis of the transient absorption signal kinetics with the first-order rateequation (6.2). The Ru(II) complexes were dissolved in either degassed acetonitrile or water.
The kinetics were measured with the monochromatized X-ray pulse (∆E/E ∼ 10−4 ) at the peak
position of each transient XAFS spectrum, and the IRF(σ) was evaluated to be ∼50 ps. The
error bars were calculated in propagated standard error.

All the kinetics were successfully fitted with one decay time constant each, and theresults are consistent with the previous luminescence lifetime studies (Table 6.1).
Small deviations predominantly originate from different molar concentrations (e.g. 10
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µM and 4 mM) and the fact that the transient signals in both studies involve different
dipole transitions (e.g. phosphorescence (PL) and TFY), and thus probe the same
ultrafast dynamics from different point of view. The comparison of the lifetime studies
are summarized in Table 6.3.
Lifetimes (TFY)

MeCN (PL)H2 O (PL)Ru(phen)3

303 ns (320 ns)

362 ns (920 ns)

Ru(phen)2 (dppz)

199 ns (180 ns)

375 ps (250 ps)

Ru(TAP)2 (dppz)

320 ns (N/A)

429 ns (820 ns)

Ru(TAP)3

68 ns (68 ns)

N/A (223 ns)

Table 6.3: Comparison of the 3 MLCT state lifetimes of the Ru(II) light-switching andphoto-oxidizing complexes in acetonitrile and water, seen by TFY and phosphorescence.

The transient absorption signal kinetics of the Ru(II) photosensitizers upon
400 nm photoexcitation therefore further confirmed that the 3 MLCT state of the[Ru(phen)2 (dppz)]2+ in water gets efficiently quenched by the coordination of thewater molecule to the phenazine nitrogens (N9 and N14; [152; 151]), and the thermal
population of the 3 MC state in the [Ru(TAP)3 ]2+ is in fact dominant in its second
relaxation pathway [139; 100].
Moreover, the time-resolved EXAFS experiments on the Ru(II) complexes indegassed acetonitrile at the probe delay of 150 ps revealed that there is a minimalchange in their geometric structures due primarily to the charge redistribution in thesystem.

6.5 Conclusion and outlook
The Ru(II) light-switching and photo-oxidizing complexes, namely the [Ru(phen)3 ]2+ ,[Ru(phen)2 (dppz)]2+ , [Ru(TAP)2 (dppz)]2+ , and [Ru(TAP)3 ]2+ were subjected to thetime-resolved EXAFS measurements in degassed acetonitrile at the probe delay of
150 ps after the 400 nm photoexcitation. With the picometer-scale spatial resolution
achieved, the localized excited electron on a different ligand disclosed a minimalchange in the first coordination shell structure mainly from the charge redistribution in
the molecular system.
Additional measurements of the transient absorption signal kinetics at the peak
position of each transient XAFS spectrum showed consistent results with the previous
luminescence lifetime studies, further confirming the non-radiative deactivation

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pathways in [Ru(phen)2 (dppz)]2+ and [Ru(TAP)3 ]2+ as well as the 3 MLCT state in[Ru(TAP)2 (dppz)]2+ localized on one of the TAP-ligands.
A following EXAFS experiment on the [Ru(TAP)3 ]2+ at rather long probe delay of ∼50
ns could show a more pronounced change in the geometric structure owing to the
populated 3 MC state. Furthermore, the time-resolved EXAFS experiments on theintercalated complexes between the base-pairs of the polynucleotide can reveal
how the coordination shell structure accommodates itself in the light-switching or
photo-oxidizing mechanism, as the extensive number of structure determinations has
been carried out on the DNA-intercalated species.

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7 3d10 Cu(I) photosensitizer

Strongly coupled dynamics for the first picosecond.

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.3 Femtosecond-resolved Kα and Kβ1,3 XES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.4 Femtosecond-resolved Kβ2,5 XES and XSS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
The 3d-chelates such as [Fe(bpy)3 ]2+ or [Co(terpy)2 ]2+ exhibit fairly different ultrafast
electronic and nuclear dynamics with those of the 4d- and 5d-chelates upon opticalphotoexcitation [204; 123; 13]. As noted earlier, due primarily to the typically weakligand field (∆) in the 3d-chelates (Fig. 2.3; see also the Tanabe-Sugano diagram)the HS state lies energetically lower than the 3 MLCT state (Fig. 2.8, [123; 9]). Thus,
the SCO becomes favorable in the second relaxation pathway (Section 2.2.2) andaccordingly the molecular symmetry gets significantly distorted [125; 113; 13], which
is not really desirable for the light-harvesting application.
The 3d10 [Cu(dmp)2 ]+ photosensitizer however shows a comparable lifetime of the3

MLCT state (>40 ns) thanks to its fully occupied 3d-orbitals in the GS (Fig. 2.12)[94]. Therefore, the ultrafast dynamics in the [Cu(dmp)2 ]+ has been intensively
studied with the time-resolved optical and X-ray absorption spectroscopies as wellas theoretical calculations [176; 34; 175; 93; 95; 65; 66; 49; 158; 29; 27; 28; 107;
102; 126]. Nevertheless, the strongly coupled electronic and nuclear dynamics in thephotoexcited [Cu(dmp)2 ]+ within the first picosecond were not clearly unraveled.
In this chapter, we will discuss the femtosecond-resolved XES and XSS data fromthe [Cu(dmp)2 ]+ simultaneously measured at SACLA upon 550 nm photoexcitation[37]. Particularly for the first picosecond, one time constant of 400-700 fs wasfound consistently from the Cu Kα, Kβ1,3 , and XSS transient signal kinetics, which
is hence attributed to the pseudo Jahn-Teller (PJT) distortion [94]. The fingerprintof the ISC from 1 MLCT to 3 MLCT state was not able to be identified from the Cu
Kβ2,5 fluorescence signals with respect to the probe delay, due predominantly to therelatively low SNR [37].

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7.1 Introduction

Figure 7.1: A geometric structure of the [Cu(dmp)2 ]+ illustrated by Chimera (center brown: Cu+ ,blue: Nitrogen, beige: Carbon, white: Hydrogen).

[Cu(dmp)2 ]+ (Fig. 7.1) is one of the most promising candidates for the future
light-harvesting agents with its long-lived 3 MLCT charge-separated state (>40ns) upon visible light absorption and excellent chemical tunability [94]. Thus, thephotophysics of the [Cu(dmp)2 ]+ has been studied a lot over the last two decades inboth empirical and quantum chemical manners [175; 27; 28; 95; 102; 126; 37].
In its GS, the Cu(I) metal ion with its electronic configuration of [Ar]3d10 , iscoordinated with two dmp ligands via the σ-character overlaps between thevacant sp3 -hybrid orbitals and the nonbonding sp2 -hybrid orbitals, as well as thecorresponding π-character overlaps (Fig. 2.12). The molecule therefore forms atetrahedral symmetry (D2d ) with the dihedral angle (dha; the angle between the two
dmp ligands) of 90° [93].
In its 1,3 MLCT state however, the molecular symmetry becomes weaker due to now
the formal electronic configuration of 3d9 in the metal ion, and hence reduced to D2
with the dha of ∼70° [126]. Such a structural change to lower the molecular symmetry
and thus the overall potential energy of the system is understood as the PJT distortion(the second-order Jahn-Teller effect, [93]), and has been characterized by manydifferent methodologies [34; 175; 94; 107; 126]. For example, Iwamura et al. reported
the ultrafast photoemission dynamics of the [Cu(dmp)2 ]+ after the photoexcitation at
420 nm, which partially visualized its PESs [93].
Figure 7.2 summarizes the results and interpretation reported by Iwamura et al.
The photoexcitation to the 1,3 MLCT state manifold was immediately followed bya very intense fluorescence signal centered at ∼500 nm with its time constant of∼45 fs, which originates from the Franck-Condon state, i.e. the S2 state, by directly
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Figure 7.2: (left) The ultrafast photoemission dynamics of the [Cu(dmp)2 ]+ upon photoexcitation
at 420 nm and (right) its corresponding PESs (Figures from [93]). Two relevant parameters of
the system, namely the dha and molecular spin multiplicity, are also properly indicated in the
PESs.

decaying back to the GS. Here the dipole transition to the lowest singlet MLCT state(S1 ) is formally symmetry-forbidden, whereas that to the higher singlet MLCT stateis formally symmetry-allowed, as reflected in their molar attenuation coefficients(Fig. 7.2, [175]). The resulting S2 state can afterwards undergo the first relaxation
dynamics either to the S1 state (internal conversion; IC) or to the T1 state (ISC). The
relative quantum yields of these two competing pathways were then evaluated fromthe fluorescence intensity comparison between upon the 420 nm excitation (S2 ) andthe 550 nm excitation (S1 ) (Fig. 7.2, [93]).
After the S2 state had disappeared with its decay time constant of ∼45 fs, the second
emission dynamics showed up as an intermediate transition from a higher emission
energy to a lower one, with its decay time constant of ∼660 fs (Fig. 7.2, [93]). Thisrelaxation is attributed to the so-called flattening motion (PJT distortion) of the[Cu(dmp)2 ]+ where the dha changes from 90° (D2d ) to ∼70° (D2 ). The assignment is
not only based on the lower potential energy in the flattened state compared to theFranck-Condon geometry (Fig. 7.2), but also on the fact that this time constant is infact dependent on the solvent viscosity and the substituents at 2,9-positions of theligands [93; 94]. In other words, the flattening motion proceeds either on the S1 state
manifold or on the T1 state manifold.
The last emission dynamics appearing at ∼700 nm with its decay time constant of ∼7.4

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ps is then assigned to the "delayed" ISC from the S1 state to the T1 state, primarily
owing to the weakened spin-orbit coupling by the flattening motion [93; 175]. In theend, the lowest long-lived 3 MLCT state decays back to the GS via phosphorescence
after few tens of nanoseconds.

Figure 7.3: (left) The coherent nuclear wavepacket motion in the [Cu(dmp)2 ]+ appearing on
the transient optical absorption signal kinetics upon photoexcitation at 540 nm, and (right) a
relevant PES diagram (Figures from [95; 175]). The vibrational coherence is coupled with the
PJT distortion, revealing the same damping time constant of 770 fs.

The understanding didn’t change much from the TOAS experiments. Shaw et al.
reported a very similar PES diagram to the previous one (Fig. 7.3), describing thatthe optical methods are probing the structural changes indirectly, and the opticalsignals originating from different transient species often overlap with one another[175]. That means, it is still complicated to identify the ultrafast electronic and nuclear
dynamics even from the thorough examinations in the optical regime, especially when
the degrees of freedom in the system are strongly coupled to each other.
Iwamura et al. however found an additional oscillatory component appearing on thetransient optical absorption signal kinetics at the probe wavelength of 650 nm, upon
the photoexcitation at 540 nm directly to the S1 state (Fig. 7.3, [95]). Right after thetime-zero, the instantaneous transient absorption intensity keeps increasing with its
time constant of 770 fs, which corresponds to the PJT distortion. The oscillation of
the transient absorbance was then extracted by fitting the exponential function to thedata, and the damping time constant of the oscillation was found to be same with the

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time constant of the structural change. This implies that the oscillation arises from thecoherent nuclear motion in the S1 Franck-Condon state, and the vibrational coherence
gets damped as the flattening motion proceeds [95]. In other words, the excessive
vibrational energy in the S1 Franck-Condon state dissipates effectively into other
degrees of freedom through the anharmonic coupling between the ligand twisting
motion (ν25 ) and the flattening distortion.
The coherent nuclear wavepacket was also observed by our working group fromthe femtosecond-resolved XAS experiment at SACLA [102]. The time-dependent
Fourier transform analysis on the wavepacket unveiled the central component of Cu-N
breathing mode (ν8 ) together with the ligand twisting mode (ν25 ), consistent with theprevious results from Iwamura et al. [95]Despite the rigorous empirical studies so far, the ultrafast dynamics in [Cu(dmp)2 ]+photosensitizer is still under active debate in the literature [175; 94; 107; 27; 28; 102;126]. To further get insights into its electronic and nuclear dynamics, specificallythe ISC from 1 MLCT to 3 MLCT state and the PJT distortion, our working group has
conducted the femtosecond-resolved XES and XSS experiments simultaneously at
SACLA, upon photoexcitation at 550 nm [37]. By employing these complementary
hard X-ray techniques the structural changes can be disentangled from the spin statechanges [109; 113; 117], especially also by covering the Cu Kβ2,5 emission lines [27].

7.2 Experimental details
The femtosecond-resolved XES and XSS experiments were simultaneously carriedout at BL3EH2 of SACLA [188; 92; 101; 103]. The SASE X-ray pulse centered at ∼10
keV was focused down to ∼5 µm by beryllium CRLs [101]. [Cu(dmp)2 ](PF6 ) salt (molar
mass 625 g mol-1 ) was dissolved in acetonitrile at the concentration of 100 mM (OD∼0.5 with 50 µm optical path length at 550 nm). During the whole measurement the
sample solution was under magnetic stirring and purged with Ar gas. The solution
was circulating in the liquid jet system inside the He-filled chamber [103], through aninjector with its inner diameter of 50 µm.
At the interaction point, [Cu(dmp)2 ]+ was excited directly to the S1 state by the 550 nm
pump pulse (∼45 fs FWHM) from a Ti:sapphire laser system and OPA (HE-TOPAS,
Coherent). The optical laser fluence was set to ∼209 mJ cm-2 that was slightly lower
than the onset of the two-photon absorption process. The arrival-timing between X-ray
and optical laser pulses was recorded by the timing diagnostics based on the X-ray
beam splitting scheme [104]. For the temporal jitter correction, the interval bin width of
50 fs was used. The overall time resolution of the experiment was evaluated to be ∼80

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fs (FWHM) by fitting a Gaussian profile to the first derivative of both Cu Kα and Kβ1,3transient emission signal kinetics [102].
The XSS data at each probe delay point were measured in the forward wide-angle
X-ray scattering geometry, and the XES data at each probe delay point were
measured with two von Hamos dispersive spectrometers in Bragg reflection geometry.
6 Si(111) analyzer crystals were aligned for the Cu Kβ1,3 and Kβ2,5 emission lines invertical direction, and 6 Si(531) analyzer crystals for the Cu Kα emission lines inhorizontal direction [103].

7.3 Femtosecond-resolved Kα and Kβ1,3 XES
In this section, we will now focus on interpreting the reduced data (see also Appendix
B and C) of the Cu Kα and Kβ1,3 fluorescence signal with respect to the probe delay.
The Voigt profile fits to the GS emission lines were already discussed in the Section
2.3.2 [45; 134; 1; 37; 106], to grasp the origin of the line shapes. The changes
in spectral features along the probe delay will be then understood by fitting thesequential first-order rate equation to the transient emission signal kinetics [102]. The
kinetics were extracted by integrating the positive parts of the transient signals. We
found however no difference in between integrating the negative transient signals andpositive transient signals for the kinetics, as depicted in Fig. 7.4.

Figure 7.4: Comparison of the kinetics extracted by integrating the positive transient signals(blue) and negative transient signals (red) of the Cu Kα (left) and Kβ1,3 (right) fluorescencefrom [Cu(dmp)2 ]+ upon 550 nm photoexcitation. The error bars were calculated in propagatedstandard error.

Before we start analysing the kinetics, we need to first identify the instrumental
time-zero and its response function (IRF) defining the overall time resolution of theexperiment, by fitting a Gaussian profile to the first derivative of both the Kα and Kβ1,3transient signal kinetics [102]. Figure 7.5 summarizes the results.

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Figure 7.5: Instrumental time-zero and its response function (IRF) of this experiment found by
the Gaussian fits to the first derivative of both Cu Kα (above) and Kβ1,3 (below) transient signal
kinetics from [Cu(dmp)2 ]+ upon 550 nm photoexcitation [102]. The time-zero and IRF were
evaluated to be t0 = -13 fs ± 2.6 fs and IRF(σ) = 44 fs ± 2.8 fs for Kα, and t0 = -8 fs ± 2.4 fs andIRF(σ) = 34 fs ± 2.4 fs for Kβ1,3 . Hereafter we will use these values to convolute the IRF(σ, t) =A0 *exp[−(t − t0 )2 /2σ 2 ] and the rate equation. The interval bin width of 30 fs was used for the
temporal jitter correction.

The origin of such a Gaussian broadening at time-zero, namely the convolution of a
Gaussian profile and Heaviside step-function, is that in time domain both optical and
X-ray pulses in Gaussian line shape convolute inside the cylindrical liquid jet stream,
where the refractive index changes to ∼1.3 at 550 nm (acetonitrile), so that the GVD
of 550 nm pulse also contributes to the Gaussian broadening [102]. In addition thejitter correction precision of the timing tool affects the actual time resolution as well√[104], so the final IRF(FWHM) can be estimated as 452 + 62 + 502 + 162 ∼69 fs. Herethe pulse duration of the pump laser (∼45 fs) was measured with an autocorrelator
before the experiment, and that of the X-ray pulse (∼6 fs) was estimated by anautocorrelation measurement.
Both IRF values from the Gaussian fit (∼80 fs FWHM) and instrumental estimation(∼69 fs FWHM) are in good agreement, even though the Gaussian fit may turn out tobe incorrect if the initial population would take much longer than the calculated IRF.
The corresponding population in the Franck-Condon state occurs however faster

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than ∼100 fs as we noted earlier from the ultrafast dynamics of [Cu(dmp)2 ]+ reported
by Iwamura et al [95]. Therefore, the instrumental time-zero and response function
determined by the Gaussian fits will be applied to the following analysis.
The sequential first-order rate equation for the fitting analysis was built based on
the proposed photophysical mechanism so far in the [Cu(dmp)2 ]+ after 550 nm
photoexcitation [94; 102; 37].
k1

S 0 + hv Ð←ÐÐ→Ð MLCT hot (90 °)

(7.1)

k2

MLCT hot (90 °) ÐÐ→ MLCT hot (70 °)

(7.2)

k3

MLCT hot (70 °) ÐÐ→ MLCT cool (70 °)

(7.3)

k4

MLCT cool (70 °) ÐÐ→ S 0(7.4)

Since all reactions throughout the photocycle are unimolecular, each reaction rate is
proportional to the time-dependent quantum yield of the reactant (first-order reaction).
Only the (7.1) is a reversible reaction, but the reaction (7.2) and (7.3) also haveanother competing radiative decay back to the GS (S0 ) as reported by Iwamura etal [93]. The rate constants of non-radiative decay pathways (k2 , k3 ) are however
much larger than those of radiative ones [25; 69; 24; 94; 102], hence we don’t takeaccount of the radiative decay pathways into our model equation for the picosecond
time window. The Heaviside step-function convoluted with the IRF(σ, t) can therefore
describe the forward reaction of (7.1), whose rate constant (k1 ) must be >1013 s-1
upon the 550 nm photoexcitation, as remarked earlier [102; 94].
From (7.2) onward we should be careful, because some relaxation pathways do notappear in the kinetics depending on the ultrafast methodologies [94; 102; 26] (seealso the Section 3.3). The X-ray emission lines are obviously sensitive to the effective
charge density change in the system, i.e. Cu(I) 3d10 → Cu(II) 3d9 (7.1), and thus arealso sensitive to the structural rearrangement such as the flattening motion (7.2,[117]). The vibrational cooling process however (7.3, [102]) does not affect the X-ray
fluorescence as the process occurs in one electronic state.
Accordingly the sequential first-order rate equation for the transient emission signal
kinetics was formulated to reflect the corresponding photophysical mechanism (7.1,
7.2, and 7.4). The ISC from 1 MLCT to 3 MLCT state was also excluded from the model
equation, since both the Cu Kα and Kβ1,3 emission lines were not sensitive to theprocess, as we will see soon from the data.

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[MLCThot,90° ](t) = [MLCThot,90° ]0 ∗ exp(−k2 ∗ t)

(7.5)

[MLCThot,70° ](t) = B0 ∗ [MLCThot,90° ]0 ∗ {1 − exp(−k2 ∗ t)} ∗ exp(−k4 ∗ t)

(7.6)

= B0 ∗ [MLCThot,90° ]0 ∗ exp(−k4 ∗ t) − B0 ∗ [MLCThot,90° ]0 ∗ exp{−(k2 + k4 ) ∗ t} (7.7)
The first-order rate equation (7.5) is the non-radiative decay of the quantum yield of
MLCThot, 90° state, where [MLCThot, 90° ]0 appears with the k1 >1013 s-1 in the forward
reaction of (7.1). As the [MLCThot, 90° ](t) decays (7.2), the [MLCThot, 70° ](t) arises
accordingly with [MLCThot, 70° ]0 (t) = [MLCThot, 90° ]0 *{1-exp(-k2 *t)}, which at the sametime starts decaying in the first-order with the rate constant of k4 (7.4). In (7.6) a
constant value of B0 is multiplied to consider the different X-ray fluorescence yields
from different excited states. The value for the rate constant k4 , which corresponds
to the GS recovery from the lowest long-lived 3 MLCT state, was adopted from theliterature [94; 102]. Hence, in (7.7) we have three unknowns (B0 , [MLCThot, 90° ]0 , k2 ) tobe fitted to the kinetics. The sequential first-order rate equation was then convoluted
with the IRF(σ, t), and the following equation (7.8) was used to analyse the transientemission signal kinetics from [Cu(dmp)2 ]+ upon 550 nm photoexcitation.
∆IXES (t) = [MLCThot,90° ](t)∗H(t)⊗IRF (σ, t)+[MLCThot,70° ](t)∗H(t)⊗IRF (σ, t) (7.8)
where H(t) is the Heaviside step-function and ⊗ is the convolution operator.
Figure 7.6 shows the results from the Cu Kα fluorescence signals with respect tothe probe delay. First of all, we compared the emission spectra with and without the
optical photoexcitation at the probe delay of 19 ps (Fig. 7.6 (a), (b)), where we knowthat the system is in the lowest long-lived 3 MLCT (T1 ) state [94; 107]. The differenceis quite small (∼1 %), and looks dominated by the effective charge density change inthe system (Cu(I) 3d10 → Cu(II) 3d9 ), which made the spectrum be red-shifted [27].
In addition to this, the discrepancy between the positive and negative transient line
shape of Kα1 originates from the slightly more broadened emission spectrum in the
ES due to the spin density change [74].
Both kinetics with the time window of ∼3 ps (Fig. 7.6 (c)) and ∼20 ps (Fig. 7.6 (d))
were obtained by integrating the blue-shaded areas of the transient emission signals.
The fitting analysis with the sequential first-order rate equation (7.8) disclosed a decay
time constant of ∼410 fs (1/k2 ), and no additional dynamics was noticed within thetime window of ∼20 ps. This femtosecond dynamics was then tentatively attributed to
the flattening motion, because the so-called core-to-core transitions from 2p to 1s (Kαfluorescence; Fig. 2.12) cannot detect a spin-flip on the ligand, but rather can probe
the structural rearrangement as discussed before [117].

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Figure 7.6: (a) Cu Kα emission spectra of [Cu(dmp)2 ]+ in its GS (blue) and T1 state (red) at theprobe delay of 19 ps as well as (b) their difference. The blue shaded areas were integrated to
extract the kinetics (c) up to 2.5 ps and (d) 20 ps. The sequential first-order rate equation (7.8)
consisting of the [MLCThot, 90° ](t) (green) and [MLCThot, 70° ](t) (orange) was fitted to the kinetics(fitting range up to 2.2 ps), revealing a decay time constant of ∼410 fs (1/k2 ). The error bars
were calculated in propagated standard error.

From the Cu Kβ1,3 fluorescence signals we obtained the very similar results as shownin Fig. 7.7. Again, we first compared the emission spectra with and without the opticalphotoexcitation at the probe delay of 19 ps (Fig. 7.7 (a), (b)), where we know that thesystem is in the lowest long-lived 3 MLCT (T1 ) state [94; 107]. The difference is quite
small (∼1 %), and both the spectator-hole satellite lines gained higher transition rates
in the ES, while the diagram transitions lost theirs (Section 2.3.2).
The kinetics with the time window of ∼3 ps (Fig. 7.7 (c)) and ∼20 ps (Fig. 7.7 (d)) wereextracted by integrating the blue-shaded areas of the transient emission signals. The
fitting analysis with the sequential first-order rate equation (7.8) again disclosed thesame decay time constant of ∼410 fs (1/k2 ), and no additional dynamics was noticed
within the time window of ∼20 ps. Here we confirmed that the femtosecond dynamics
in fact corresponds to the flattening motion, which affects both the Cu Kα and Kβ1,3
fluorescence signals consistently [117].
The core-to-core emission lines, namely the Cu Kα (2p to 1s) and Kβ1,3 (3p to1s), could not capture the molecular spin multiplicity nor the coherent molecular

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Figure 7.7: (a) Cu Kβ1,3 emission spectra of [Cu(dmp)2 ]+ in its GS (blue) and T1 state (red) at
the probe delay of 19 ps as well as (b) their difference. The blue shaded areas were integrated
to extract the kinetics (c) up to 2.5 ps and (d) 20 ps. The sequential first-order rate equation(7.8) consisting of the [MLCThot, 90° ](t) (green) and [MLCThot, 70° ](t) (orange) was fitted to thekinetics (fitting range up to 2.2 ps), revealing a decay time constant of ∼410 fs (1/k2 ). The error
bars were calculated in propagated standard error.

vibrations. To further facilitate disentangling the strongly coupled electronic andnuclear dynamics in the photoexcited [Cu(dmp)2 ]+ within the first picosecond, we will
now analyse the simultaneously-collected XSS data as well as the reduced data of
the Cu Kβ2,5 fluorescence signal with respect to the probe delay. Additional analysis
on the Cu Kα and Kβ1,3 data can be found in Appendix D.4.

7.4 Femtosecond-resolved Kβ2,5 XES and XSS
Figure 7.8 shows the results from the Cu Kβ2,5 fluorescence signals with respect tothe probe delay. The Voigt profile fits to the GS emission lines were already discussed
in the Section 2.3.2 [54], to grasp the origin of the line shapes [138; 134; 159; 43;196]. The emission spectra with and without the optical photoexcitation at the probe
delay of 19 ps (Fig. 7.8 (a), (b)) were compared, where we know that the system is in
the lowest long-lived 3 MLCT (T1 ) state [94; 107]. The difference is there, but the SNR
is less than 1 even though ∼1129422 SASE X-ray pulses were invested in the Laser
Off and On emission spectra respectively. Nevertheless we gave a try on extracting

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Figure 7.8: (a) Cu Kβ2,5 emission spectra of [Cu(dmp)2 ]+ in its GS (blue) and T1 state (red) at
the probe delay of 19 ps as well as (b) their difference. The blue shaded areas were integrated
to extract the kinetics (c) up to 2.5 ps. The DFT calculations (d) suggested that the shoulder
peak from the occupied π* molecular orbital could be sensitive to the ISC. The error bars were
calculated in propagated standard error.

the kinetics by integrating the blue-shaded area, which corresponds to the transientemission signal from the occupied π* molecular orbital (Section 2.3.2).
As shown in Fig. 7.8 (c), the kinetics up to ∼2.5 ps clearly unveiled the femtosecond
dynamics in three different interval bin widths, surviving for longer than 19 ps. The
kinetics were however not able to be fitted with the sequential first-order rate equation(7.8) due predominantly to the limited number of data points, therefore the quantitative
fitting analysis as well as our interpretation was also restricted. In other words, there
are still uncertainties in identifying this femtosecond dynamics, although the DFT
calculations suggested that the shoulder peak from the occupied π* molecular orbital
could be sensitive to the ISC (Fig. 7.8 (d)).
Another factor to be considered is the flattening motion which can also cause thetransient signals of the Cu Kβ2,5 fluorescence [27; 42; 138]. To unravel this complexity
in the femtosecond dynamics, the transient XSS signals with respect to the probe
delay were simultaneously recorded in the forward wide-angle X-ray scatteringgeometry during the experiment (Fig. 7.9).
The preliminary analysis on the XSS data by just integrating the low-q signals (0.3 ∼XFEL.EU THESIS-2021-002

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Figure 7.9: (left) Transient XSS signals at selected probe delays and (right) the transient signal
kinetics obtained by integrating the ∆Ssolute response (0.3 ∼ 0.8 Å-1 ) from the [Cu(dmp)2 ]+ upon
550 nm photoexcitation. The sequential first-order rate equation (7.8) was fitted to the kinetics,
returning a time constant of ∼705 fs (1/k2 ). The residual signals (Σ|∆Ssolute | − fit) showed thecoherent vibrational motion with a period of ∼330 fs (Cu-N breathing mode, ν 8 ), which has its
damping time constant of ∼540 fs [102].

0.8 Å-1 ; ∆Ssolute response) revealed the consistent femtosecond dynamics originatingfrom the PJT distortion (400-700 fs). Without yet sophisticated data processes, theresidual signals (Σ|∆Ssolute | − fit) clearly retained the coherent vibrational motion witha period of ∼330 fs (Cu-N breathing mode, ν 8 ), which has its damping time constant of∼540 fs [102]. As noted earlier, this wavepacket motion is believed to be coupled to
the flattening distortion, i.e. the symmetry breaking in photoexcited [Cu(dmp)2 ]+ [95].
Comprehensively speaking, within the first picosecond after the 550 nm
photoexcitation, the nuclear dynamics with the time constant of 400-700 fs seemdominant in all XES and XSS transient signal kinetics. That means, what we noticed
from the Cu Kβ2,5 transient emission signal kinetics probably also corresponds to the
PJT distortion. Much higher SNR is thus required to explicitly identify the femtosecond
dynamics from the Cu Kβ2,5 fluorescence signals, which can only be accomplished at
the high-repetition rate XFEL facilities such as European XFEL and LCLS-II [63; 109].

7.5 Conclusion and outlook
Complementary to the previous studies, herein we reported the femtosecond nuclear
dynamics in photoexcited [Cu(dmp)2 ]+ investigated with the femtosecond-resolved
XES and XSS methodologies available at SACLA. With the overall time resolution of∼94 fs (FWHM) achieved, we observed that the molecular structure flattening (PJT
distortion) proceeds with the time constant of 400-700 fs after 550 nm photoexcitation,affecting both the Kα and Kβ1,3 core-to-core fluorescence. The XSS data further
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confirmed this dynamics, and were additionally sensitive to the coherent vibrationalmotion of [Cu(dmp)2 ]+ primarily from the Cu-N breathing mode (ν8 ). The ISC from
1

MLCT (S1 ) to 3 MLCT (T1 ) state was not identified from the femtosecond-resolved

Kβ2,5 XES, however we expect much better resolution on the line shapes of thevalence-to-core fluorescence as well as the pre-edge absorption structure at thehigh-repetition rate XFEL facilities such as European XFEL and LCLS-II.

Figure 7.10: The summarized PESs of the [Cu(dmp)2 ]+ with its representative femtosecond
relaxation dynamics upon 550 nm photoexcitation. Following the two branching PJT distortions(blue), the coherent nuclear wavepacket decays with 540 fs time constant (red).

Figure 7.10 shows the summarized PESs of the [Cu(dmp)2 ]+ along the dha coordinate
and the Cu-N bond length coordinate. The photoabsorption at 550 nm promotes
the system to the S1 Franck-Condon state where the coherent molecular vibrations
start dissipating their energy into different nuclear and electronic degrees of freedom(540 fs, ν8 ), as the PJT distortion proceeds (400-700 fs, ν21 , ν25 ). Whether the PJT
distortion occurs on the 1 MLCT state manifold or the 3 MLCT state manifold is still an
open question.

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8 3d6 Fe(II)-Co(III) photocatalyst

Femtosecond "energy" transfer in a dyadic system.

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
8.3 Femtosecond-resolved XES on Fe(II)-Co(III) dyad. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.4 Femtosecond-resolved XES on Fe(II) photosensitizer . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
8.5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
The 3d6 chelates have exhibited their rich dynamics on the ultrafast timescale as wellas their intriguing spectral features in the hard X-ray domain [84; 74; 24; 123; 177;
160; 113; 61; 13; 174; 204]. Many of them however have been only spotted over thelast two decades, in accordance with the emergence of femtosecond light sourcesthat allow us to clock the very initial step of the light-harvesting mechanism. The
typically weak ligand fields in the 3d6 chelates let their electronic structures undergo
the SCO within the first picosecond after the MLCT photoexcitation, leading to theirown interesting photophysical properties.
These unique characteristics are unfortunately not favorable to the light-harvesting, as
the electronic and geometric structure in the MC excited state cannot possess thedesired transient dipole moment nor the long enough lifetime. Hence, there have beenenormous chemical efforts to destabilize the MC states or alternatively much morestabilize the MLCT states [114; 77; 33; 56; 207; 32; 205; 186; 117; 87; 206; 105; 208;
129; 130; 62]. One of the most effective modifications is to replace the nearest atoms
which directly form valence orbital overlaps with the metal ion (Fig. 2.9, Section 2.1.1).
Herein, one of the newly synthesized Fe(II) NHC chelates by our working group andits ultrafast dynamics will be introduced and discussed [205; 206; 87]. The prototype
Fe(II) photosensitizer and its derivative with a cobaloxime(III) attached showed theiractivity in the photocatalytic proton reduction, and the performance was roughly
twice higher in case of the dyadic system [87]. The femtosecond-resolved XES
measurements were conducted at SACLA upon 515 nm photoexcitation to explore
their ultrafast dynamics, and disclosed the femtosecond energy transfer occuring fromFe(II) to Co(III) moiety that was not unveiled from the TOAS measurements.

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8.1 Introduction
Iron, the element right above Ruthenium, is the fourth most abundant element
on Earth, and therefore is very tempting to be introduced to the light-harvesting
applications. [Fe(bpy)3 ]2+ for example (Fig. 2.8, 8.1), has been extensively studied
over the last few decades as an analogy to the [Ru(bpy)3 ]2+ [204; 24; 69; 67; 68; 113;123]. Even though the complex has similar structural parameters in the GS and highphotostability, its relaxation dynamics is quite different with the Ru(II) analogy upon
the MLCT photoexcitation. Due primarily to the weak ligand field (small ∆; Fig. 2.3)
the electronic configuration of the 3 MC or 5 MC state becomes energetically morestable than that of the 3 MLCT state, letting the SCO in the second relaxation pathway
be dominant (Section 2.2.2), and accordingly the molecular symmetry get significantly
distorted (Fig. 8.1; [204; 123; 113]).

Figure 8.1: (left) A structural formula of [Fe(bpy)3 ]2+ and (right) its projected PESs together withabsorption bands (Figures from [9]). The complex has been introduced as an alternative to theRu(II) photosensitizers with its high photostability and symmetric structure. Its rich dynamics
on the femtosecond timescale however lead to an instantaneous extinction of the MLCT state,whose lifetime can be prolonged by altering the nitrogen coordinations to the carbon ones.

To detour the femtosecond SCO, there have been many different approaches suchas extending the π-conjugation or adding some electron-withdrawing groups andso on [129; 130; 77; 56], but our working group has decided to introduce the NHC
ligand to the complex [207; 205; 206; 208; 87]. As discussed in Section 2.1.1, thestabilized carbon atom via the σ-coordination bond from the NHC ligand cannot form
any π-character bond, i.e. any π-donation backbond with the metal ion (Fig. 8.2).
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Figure 8.2: (left) A model complex with the NHC ligands and (right) its projected PESs (Figure
from [62]). The carbene forms three sp2 -hybrid orbitals but no π orbital, thus the π-donation
backbond to compensate the σ-coordination bond cannot be properly followed. This results in a
prolonged lifetime of the MLCT state [129; 77; 205; 206; 117; 87].

Figure 8.2 shows a model complex with the NHC ligands and its projected PESs asan example [129; 62; 77; 125; 117]. Although the coordinated carbon may have the
π-character bond shared from the nonbonding electrons of the nearest nitrogens,still the excessive electron density on the metal ion cannot be balanced out, suchthat the MC states become energetically higher and less probable to be populated,
whereas the MLCT state on the NHC ligand (π* orbital) becomes relatively lower
and more probable to be populated. Hence, it has been reported experimentally andcomputationally that the model complex in the MLCT state decays to the GS with atime constant of ∼9 ps via the 3 MC state upon 400 nm photoabsorption.
In line with the previous achievements our working group has designed another model
complex with the extended π-conjugation and thus the enhanced steric hindrance (Fig.
8.3; [207; 205; 206; 87]), which is readily attached to the cobaloxime(III) thanks to
the heteroleptic NHC ligands. Our hypothesis here is that after the charge separation
upon the MLCT photoexcitation, the excited electron has now sufficient time toproceed further to the cobaloxime(III), where the chloride ion departs away from thephotocatalyst and two protons get reduced to a hydrogen molecule on the samesite. In this way the Fe(II)-Co(III) dyadic system ([Fe-BL-Co]) would show higher
performance in the photocatalytic proton reduction compared to the two-component
system ([Fe-BL]+[Co]). To examine this intramolecular electron transfer process weperformed the TOAS measurements on both [Fe-BL] and [Fe-BL-Co] with the pumplaser centered at 515 nm, where the Fe(II) moiety will be selectively excited and its
MLCT state will be populated [87].

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Figure 8.3: (left) The Fe(II)-Co(III) photocatalyst synthesized by our working group and (right)
its optical absorption bands together with those of the two separate components [87]. The closeabsorption bands of [Fe-BL] and [Fe-BL-Co] indicate that the attenuation cross sections are
predominantly from the Fe(II) moiety. The pump laser was therefore centered at 515 nm to
exclusively excite the Fe(II) moiety and populate its MLCT state.

Figure 8.4 summarizes the results of the TOAS measurements, and both TA signals
resemble each other a lot because their GS absorption spectra also look very similar
to each other within our probe wavelength window. What we clearly see first from thereference system (a) and our dyadic system (b) is a ∼10 ps decay time constant (bluearrow) that corresponds to the GS recovery. There seems otherwise no additional
dynamics appearing in the reference system, whereas another dynamics with a ∼2
ps time constant is clearly noticed in the dyadic system at the probe wavelength
longer than ∼600 nm (red arrow). According to our assumption this dynamics should
correspond to the excited electron transfer from the Fe(II) to Co(III) moiety, before thesystem returns back to the GS.
The optical probe window was however not enough to convince the electron transfer
process, as the signal at the probe wavelength longer than ∼600 nm can be anything(see also the Section 2.2.1). Hence we decided to employ another probe pulsethat is locally sensitive to the electronic and geometric structure change in boththe [Fe-BL] and [Fe-BL-Co]. With the SASE X-ray pulse centered at ∼9 keV, we
successfully obtained both Fe Kα and Co Kα1 emission spectra simultaneously fromthe [Fe-BL-Co], in addition to the XSS data at SACLA. Besides, we measured Fe
Kβ1,3 emission spectra from the [Fe-BL] and Co Kβ1,3 emission spectra to get further
selectivity and sensitivity to the local electronic structure. In the following sections,the reduced data (see also Appendix B and C) with respect to the probe delay will beinterpreted step-by-step, and thereby show the resolving power of the X-ray probe
pulse in this research field.

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Figure 8.4: TA spectra of the [Fe-BL] (a) and [Fe-BL-Co] (b) at certain probe delays upon 515
nm photoexcitation. Only one dynamics of the GS recovery is present in the [Fe-BL], whereas
an additional dynamics with a shorter time constant has appeared in the [Fe-BL-Co]. The TOAS
experiments were however not conclusive.

8.2 Experimental details
The femtosecond-resolved XES and XSS experiments were simultaneously carriedout at BL3EH2 of SACLA [188; 92; 101; 103]. The SASE X-ray pulse centered at∼9 keV was focused down to ∼10 µm by beryllium CRLs [101]. The [Fe-BL] and[Fe-BL-Co] were dissolved in acetonitrile at the concentration of 9.4 mM and 11.3 mM
respectively (OD <1 with 100 µm optical path length at 515 nm). During the wholemeasurement the sample solution was refilled with acetonitrile and purged with Argas. The solution was circulating in the liquid jet system inside the He-filled chamber[103], through an injector with its inner diameter of 100 µm.
At the interaction point, the sample molecule was excited to the MLCT state by
the 515 nm pump pulse (∼45 fs FWHM) from a Ti:sapphire laser system and OPA(HE-TOPAS, Coherent). The optical laser fluence was set to ∼80 mJ cm-2 that wasslightly lower than the onset of the two-photon absorption process. The arrival-timing
between X-ray and optical laser pulses was recorded by the timing diagnostics basedon the X-ray beam splitting scheme [104]. For the temporal jitter correction, theinterval bin width of 50 fs was used. The overall time resolution of the experiment wasevaluated to be ∼113 fs (FWHM) by fitting a Gaussian profile to the first derivative ofboth Fe Kα1 and Co Kα1 transient emission signal kinetics [102].

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The XSS data at each probe delay point were measured in the forward wide-angle
X-ray scattering geometry, and the XES data at each probe delay point were
measured with two von Hamos dispersive spectrometers in Bragg reflection geometry.
6 Si(531) analyzer crystals were aligned for the Co Kα1 , Fe Kβ1,3 , and Co Kβ1,3emission lines in vertical direction, and 6 Si(111) analyzer crystals for the Fe Kαemission lines in horizontal direction [103].

8.3 Femtosecond-resolved XES on Fe(II)-Co(III) dyad

Figure 8.5: Fe Kα (left) and Co Kα1 (right) emission spectra of the [Fe-BL-Co] in its GS (blue)
and the lowest ES (red) at the probe delay of 10 ps after the 515 nm photoabsorption, and theirdifference (green).

Now we know from the TOAS results that the GS will be recovered with the time
constant of ∼10 ps after the 515 nm photoexcitation [87]. That means, the transientemission signals at 10 ps probe delay should come from the lowest ES of the system.
Figure 8.5 shows the Fe Kα and Co Kα1 emission spectra and their transientsfrom the Fe(II)-Co(III) photocatalyst. It is already quite impressive that the Co Kα1
fluorescence signals disclosed any difference, since it indicates that in fact something
had happened between the Fe(II) and Co(III) moiety and was still present at 10 psprobe delay.
By first looking at the line shapes of the transients we can deduce the change inthe local electronic structure [117], for instance the Fe Kα1 emission lines were
not only blue-shifted in the ES owing to the formal oxidation state change, but alsobroadened due to the spin density change. On the other hand, the Co Kα1 emission
lines were primarily blue-shifted because of the formal oxidation state change. During
the measurements the Co Kα2 emission lines were not able to be recorded due
predominantly to the limited Bragg angle (up to ∼77°) for the von Hamos dispersive
geometry at SACLA.

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To figure out the interaction between the Fe(II) and Co(III) moiety, we obtained thetransient signal kinetics by integrating the positive parts of the Fe Kα1 and Co Kα1transients. As compared in Fig. 8.6, there was however no critical difference inbetween integrating the negative transient signals and positive transient signals.

Figure 8.6: Comparison of the kinetics extracted by integrating the positive transient signals(blue, green) and negative transient signals (red) of the Fe Kα1 (left) and Co Kα1 (right)
fluorescence from the [Fe-BL-Co] upon 515 nm photoexcitation. The error bars were calculated
in propagated standard error.

Before we start analysing the kinetics, we need to again specify the instrumental
time-zero and its response function (IRF) determining the overall time resolution of theexperiment, by fitting a Gaussian profile to the first derivative of both the Fe Kα1 andCo Kα1 transient signal kinetics [102]. Figure 8.7 summarizes the results.
The origin of such a Gaussian broadening at time-zero, namely the convolution of a
Gaussian profile and Heaviside step-function, is that in time domain both optical and
X-ray pulses in Gaussian line shape convolute inside the cylindrical liquid jet stream,
where the refractive index changes to ∼1.3 at 515 nm (acetonitrile), so that the GVD of
515 nm pulse also contributes to the Gaussian broadening [102]. In addition the jittercorrection precision of the timing tool affects the actual time resolution as well [104],√
so the final IRF(FWHM) can be estimated as 452 + 62 + 992 + 162 ∼110 fs. Herethe pulse duration of the pump laser (∼45 fs) was measured with an autocorrelator
before the experiment, and that of the X-ray pulse (∼6 fs) was estimated by anautocorrelation measurement.
Both IRF values from the Gaussian fit (∼113 fs FWHM) and instrumental estimation(∼110 fs FWHM) are in good agreement, even though the Gaussian fit may turn out tobe incorrect if the initial population would take much longer than the calculated IRF.
The corresponding population in the Franck-Condon state occurs however faster than∼100 fs as we noted earlier from the ultrafast dynamics of Fe(II) NHC complexes [77;205; 117]. Therefore, the instrumental time-zero and response function determined by

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Figure 8.7: Instrumental time-zero and its response function (IRF) of this experiment defined by
the Gaussian fits to the first derivative of both Fe Kα1 (above) and Co Kα1 (below) transient
signal kinetics from the [Fe-BL-Co] upon 515 nm photoexcitation [102]. The time-zero and IRF
were evaluated to be t0 = -21.5 fs and IRF(σ) = 48 fs for Fe Kα1 , and t0 = 16.3 fs and IRF(σ) =58 fs for Co Kα1 . Hereafter we will use t0 = -2.6 fs and IRF(σ) = 48 fs to convolute the IRF(σ, t)= A0 *exp[−(t − t0 )2 /2σ 2 ] and the rate equation. The interval bin width of 50 fs was used for the
temporal jitter correction. The error bars were calculated in propagated standard error.

the Gaussian fits will be applied to the following analysis.
At the moment we don’t know yet how many excited states are involved in the kinetics
nor whether in parallel or sequence, we will thus first assume that only one ES showsup, i.e. only the first term of the first-order rate equation (7.8) will be used to analyse
the transient emission signal kinetics.
Figure 8.8 shows the fitting results of the Fe Kα1 transient signal kinetics with thefirst-order rate equation, and this revealed a decay time constant of 8.4 ps whichcorresponds to the GS recovery [87]. The time constant alone was however not
sufficient to also cover the femtosecond dynamics, so we need another term for thesequential first-order rate equation like (7.8).
With two excited states in sequence considered, now the fitting analysis showsmuch better results (Fig. 8.9), and the femtosecond time constant was attributed
to the transition from the 3 MLCT state to the 3 MC state [205; 206; 186; 117; 87].

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Figure 8.8: Fitting analysis of the Fe Kα1 transient signal kinetics with the first-order rateequation. One decay time constant of 8.4 ps (GS recovery) was not enough to cover the
femtosecond dynamics (τ2 ). The error bars were calculated in propagated standard error.

This assignment was based on another fitting analysis as well, namely the FWHM
of the Voigt profile fits to the Fe Kα1 Laser On emission spectra was fitted with thesequential first-order rate equation (7.8) and revealed the same ∼110 fs time constant.
Since the FWHM of the Fe Kα1 emission line is dependent on the spin density of themetal ion [74], hereby we confirmed that the 3 MC state (spin multiplicity = 3) follows
the 3 MLCT state (spin multiplicity = 2) after the 515 nm photoabsorption and recovers
back to the GS.

Figure 8.9: Fitting analysis of the Fe Kα1 transient signal kinetics (left) and the FWHM of the
Voigt profile fits to the Fe Kα1 Laser On emission spectra (right) with the sequential first-order
rate equation (7.8). The ∼110 fs time constant (τ2 ) was hence assigned to the transition fromthe 3 MLCT state to the 3 MC state. The error bars were calculated in propagated standard error.

For the Co Kα1 transient signal kinetics we start again back from the simplest rateequation, and the fitting results unveiled a decay time constant of ∼2.5 ps for the GS
recovery, which was also able to explain the dynamics on the femtosecond timescale(Fig. 8.10). This indicates that in fact only one ES is involved in the kinetics, andbecause the GS of the cobaloxime(III) is in LS (see also Appendix D.5), the ES could
be either Co(II) LS state or Co(III) HS state.

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Figure 8.10: Fitting analysis of the Co Kα1 transient signal kinetics with the first-order rateequation. One decay time constant of ∼2.5 ps (GS recovery) was already sufficient to explain
the femtosecond dynamics. The error bars were calculated in propagated standard error.

To clarify the spin density of the metal ion, we further measured the Co Kβ1,3 emission
spectra and their transients from the [Fe-BL-Co] at certain probe delays (Fig. 8.11).
Although the Co Kβ1,3 fluorescence was quite effectively absorbed by the Fe(II)moiety, we were still able to see that at least for the first picosecond the Co(III) moiety
was not in HS. In comparison with the reference spectra from the Co(II) HS and Co(III)HS states, the fingerprint of the HS state which is the shoulder peak of Kβ ′ is now
missing in our case [74]. Therefore we confirmed that the Co(II) LS state appears in
the [Fe-BL-Co] upon 515 nm photoexcitation, and decays to the Co(III) LS state withthe time constant of ∼2.5 ps. But how can the Co(II) LS state be induced?

Figure 8.11: (left) The Co Kβ1,3 transient emission signals at certain time delays and (right) the
Co Kβ1,3 emission spectra of the [Fe-BL-Co] in its GS and ES at the probe delay of 0.5 ps after
the 515 nm photoabsorption, together with the reference spectra from the Co(II) HS, Co(III) HS,
and Co(III) LS states (courtesy of Florian Otte).

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In the end, the fitting analysis of the Co Kα1 transient signal kinetics as well as the
FWHM of the Voigt profile fits to the Co Kα1 Laser On emission spectra with thesequential first-order rate equation (7.8) unveiled the second time constant of ∼110 fs(Fig. 8.12). Thus, the Fe(II) and Co(III) moieties were coupled for the first picosecond
after the 515 nm photoabsorption, and the 3 MLCT state of the Fe(II) moiety promptly
perturbed the Co(III) moiety such that the second excited electron from one of thedmgH ligands was transferred onto the Co(III) metal ion (pseudo-Förster energy
transfer; [7]). Afterwards, both the 3 MC state of the Fe(II) moiety and the Co(II) LS
state of the Co(III) moiety decayed back to the GS with their own time constant of ∼8
ps and ∼2.5 ps respectively. That means, not the excited electron but the energy wastransferred from the Fe(II) to Co(III) moiety on the femtosecond timescale.

Figure 8.12: Fitting analysis of the Co Kα1 transient signal kinetics (left) and the FWHM of the
Voigt profile fits to the Co Kα1 Laser On emission spectra (right) with the sequential first-order
rate equation (7.8). The ∼110 fs time constant was hence attributed to the perturbation from the3

MLCT state of the Fe(II) moiety to the Co(III) moiety, which induced the Co(II) LS state. The

error bars were calculated in propagated standard error.

8.4 Femtosecond-resolved XES on Fe(II) photosensitizer
From the previous section now we understand the ∼2 ps time constant appearing at
the probe wavelength longer than ∼600 nm in TA spectra of the [Fe-BL-Co]. Moreover
we understand the electronic structure change in both the Fe(II) and Co(III) moieties
primarily based on the fitting analysis of the FWHM changes in the Kα1 emission line
[74]. We can therefore already guess the ultrafast dynamics of the [Fe-BL] upon 515
nm photoexcitation.
Figure 8.13 shows the Fe Kα and Kβ1,3 emission spectra and their transients from the
Fe(II) photosensitizer at 10 ps probe delay after the 515 nm photoabsorption. The
transients are again from the lowest ES of the system, and the line shape of the Fe
Kα1 transient looks very similar with that from the [Fe-BL-Co] (Fig. 8.5). The line

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Figure 8.13: Fe Kα (left) and Kβ1,3 (right) emission spectra of the [Fe-BL] in its GS (blue) andthe lowest ES (red) at the probe delay of 10 ps after the 515 nm photoabsorption, and theirdifference (green).

shape of the Fe Kβ1,3 transient is quite distinct, and proves that the lowest ES of thesystem is not the 5 MC state because of the missing shoulder peak of Kβ ′ [74; 204;
113; 117]. Thus we can deduce that the ultrafast dynamics of the [Fe-BL] will be samewith that of the Fe(II) moiety in [Fe-BL-Co].
The transient signal kinetics were obtained by integrating the positive parts of the
Fe Kα1 and Kβ1,3 transients, and there was again no critical difference in between
integrating the negative transient signals and positive transient signals (Fig. 8.14,
see also Appendix D.5). As we expected, a decay time constant of ∼7 ps for the GS
recovery was not sufficient to also cover the femtosecond dynamics, so we needed
another term for the sequential first-order rate equation like (7.8).
With two excited states in sequence considered, now the fitting analysis showsmuch better results (Fig. 8.15), and the femtosecond time constant was attributed
to the transition from the 3 MLCT state to the 3 MC state [205; 206; 186; 117; 87],
in accordance with the FWHM changes of the Fe Kα1 emission line [74]. The
femtosecond dynamics is however less pronounced than that in the Fe(II) moiety ofthe [Fe-BL-Co], i.e. the fitting errors are much larger than the previous case. This wasunderstood as the dynamics becomes less prominent without the coupling with thecobaloxime(III). Nevertheless it is certain that the 3 MC state (spin multiplicity = 3)
follows the 3 MLCT state (spin multiplicity = 2) after the 515 nm photoexcitation, andthe cobaloxime(III) does not much influence the ultrafast dynamics in the [Fe-BL].
The same interpretation applies to the fitting results of the Fe Kβ1,3 transient signal
kinetics, and an additional kinetics extracted by integrating the transient signals at
7054±1 eV of the fluorescence energy can be another evidence of the 3 MC state
appearing right after the 515 nm photoexcitation [204; 113; 117] (see also Appendix

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Figure 8.14: Fitting analysis of the Fe Kα1 (above) and Kβ1,3 (below) transient signal kinetics
with the first-order rate equation. One decay time constant of ∼7 ps (GS recovery) was notenough to cover the femtosecond dynamics (τ2 ). The error bars were calculated in propagatedstandard error.

D.5).
The Fe(II) NHC complex studied here is a close derivative of the model complex
introduced in Fig. 8.2 [129; 62; 77; 125; 117]. Due predominantly to their similar
chemical structure and optical absorption bands (Fig. 8.3 and [117]), we expect thatour [Fe-BL] would have analogous PESs to those described in Fig. 8.2, and therefore
follow the analogous relaxation pathways. Even though we successfully interpreted
the Fe Kα1 and Kβ1,3 transient signal kinetics with the simplest model including the3

MLCT and 3 MC states in sequence, the model complex shown in Fig. 8.2 decays

along slightly different pathways upon the MLCT photoexcitation [117].
The most critical difference between two experiments and interpretation originatesfrom the discrepancy in the optical pump wavelength, namely a bit higher photon
energy can open up other relaxation pathways that are otherwise not accessible [94;102; 129; 117]. The heteroleptic NHC ligands with two additional ring substituentson one side and a pyridine pendant on the other side in [Fe-BL] can be also critical[87], but in general both systems decay back to the GS within ∼10 ps after the MLCT
photoexcitation via the 3 MC state (see also Appendix D.5).

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Figure 8.15: (above) Fitting analysis of the Fe Kα1 transient signal kinetics (left) and the
FWHM of the Voigt profile fits to the Fe Kα1 Laser On emission spectra (right) with thesequential first-order rate equation (7.8). The ∼110 fs time constant (τ2 ) was hence assigned
to the transition from the 3 MLCT state to the 3 MC state. (below) Fitting analysis of the Fe
Kβ1,3 transient signal kinetics (left) with the sequential first-order rate equation (7.8), and acomparison with the kinetics obtained by integrating the transient signal at 7054±1 eV of thefluorescence energy (right). The error bars were calculated in propagated standard error.

8.5 Conclusion and outlook
Complementary to the previous studies, in this chapter we report the femtosecond
intramolecular energy transfer in a new Fe(II)-Co(III) photocatalyst observed with the
femtosecond-resolved XES methodologies available at SACLA. With the overall time
resolution of ∼113 fs (FWHM) achieved, we found the same arising time constant of∼110 fs present in both Fe Kα1 and Co Kα1 transient signal kinetics upon 515 nm
photoexcitation. This early coupling between Fe(II) and Co(III) moiety is understoodas a pseudo-Förster energy transfer from the Fe(II) to Co(II) moiety [7], causing thesecond ES of Co(II) LS state as well as the 3 MC state in the Fe(II) moiety. Eventually
two ESs in our dyadic system recover back to the GS independently.
The Fe(II) photosensitizer without the cobaloxime(III) disclosed almost identicalrelaxation dynamics, and the line shapes of the Fe Kβ1,3 flourescence with respect to
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the probe delay further proved the absence of the 5 MC state. The conclusion of thischapter is now summarized in Fig. 8.16.

Figure 8.16: (left) The ultrafast dynamics in the Fe(II)-Co(III) dyadic system and (right)
the relaxation pathways depicted in the projected PESs of the Fe(II) moiety upon 515 nm
photoabsorption (Figures from [87; 117]). After the perturbation of the 3 MLCT state, thefollowing 3 MC state and Co(II) LS state via an energy transfer return to the GS independently.

The Fe(II) NHC chelates have exhibited unprecedented tunability of their ligands fields
and thus the extended lifetime of the MLCT state, primarily depending on the numberof carbene coordinations [129; 62; 56; 130; 205; 206; 117; 87]. Our working group
has linked the Fe(II) NHC complex directly to the cobaloxime(III) and shown that thecatalytic activity became higher than that of the two-component system. Although the
femtosecond "crosstalk" between the Fe(II) and Co(III) moiety seems not relevantto the prolonged lifetime, to the best of our knowledge this is the first-observed
intramolecular energy transfer by exploiting the femtosecond X-ray pulses.

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9 Outlook and perspective

It is all about electrons.

Chemical reactions are driven by the statistical thermodynamic factors, in most of thecases without any single photon involved. Still, many of the non-light-driven ultrafast
dynamics in chemistry can be in fact mimicked by triggering them with optical photons[36]. Therefore, a broad range of the photo-induced "reactions" dealt with in thisthesis can be generally applicable to the relevant dark chemistry.
The femto-chemistry and femto-biology for example determine the material properties
on our daily timescale, thus their importance are particularly significant in the 21st
century since A. H. Zewail was awarded the 1999 Nobel Prize in Chemistry for his
studies of the transition states of chemical reactions using femtosecond spectroscopy[203]. A remarkable fact here is that the ratio of few hundreds of femtoseconds to asecond is quite similar to that of a year to the age of the universe. In other words,
every single femtosecond dynamics in a molecular system can be seen as a cycle of
one universe!
Infinite number of microscopic dynamics are still appearing and disappearing, andthe main character in this femtosecond world is of course an electron. Since we havesuccessfully described a bound electron in various states, we may now notice thatit resembles ourselves quite closely. After being promoted a new state distorts thesystem and perturbs another system in distance, eventually exchanging either anenergy or itself. Sometimes the system forces it rather to come back to the originalstate, but it is noteworthy that a single electron’s excitation and relaxation pathways, tosome extent, determine the universe’s identity (the butterfly effect).
Electron’s wavelike nature and its interaction with the electromagnetic field force
carrier (photon) literally govern the whole universe. Although the studies in thisthesis are restricted to the transition metal complexes under the optical or hard X-ray
irradiation, how the photon energy absorption transforms on the ultrafast timescale
gives us valuable insights into the light-harvesting applications. The fitting analysis
with the sequential first-order rate equation was especially impressive, because thechemical kinetics for molecular systems can in fact be also applied to the ultrafast

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kinetics of the excited electron.
The shortest time resolution achieved with the light sources employed for the thesis
was ∼30 fs (σ) that is enough to resolve the coherent nuclear wavepacket in amolecular system. Even shorter light pulses with only few periods are in principle
also available [40; 64; 21], and the so-called attosecond science intrinsically covers
the most fundamental processes in nature. The orbital period of an electron in the
GS hydrogen in Bohr’s model for instance amounts to 150 as, hence the attosecond
timescale mostly concerns the electronic motion on an atomic scale. In a molecularsystem however, an electron is moving in the combined potential of the nuclei and allother electrons, and undergoes a dipole transition in a strongly entangled way. Thus,
the attosecond-resolved studies involve correlated multi-electron dynamics and higher
frequency vibrations as well as what we have assumed to happen "instantaneously"(e.g. Franck-Condon principle).
Besides the attosecond-resolved experiments, the optical pump tender X-ray probe
spectroscopies at between 2 and 5 keV are another milestone for the future researchon the ultrafast dynamics in light-harvesting molecules. From instrumental point ofview, the tender X-rays are transported in a similar way as for the hard X-rays, and still
the fluorescence yield is dominant over the Auger electron yield [145]. Even though
the attenuation cross section in light-element-environment becomes larger, at thesame time the scattering cross section of the tender X-rays becomes also larger sothat the so-called single-particle diffractive imaging benefits for example.
From spectroscopic point of view, the K-edges (e.g. phosphorus or sulfur) andL-edges (e.g. Mo or Ru) concern dipole-allowed X-ray absorptions and fluorescencesthat accordingly reveal their fine features and changes with high SNRs [178]. The
same geometries as for the hard X-ray absorption and emission spectroscopies
can be exploited (Fig. 3.1), namely that the von Hamos spectrometer with different
analyzer crystals can be set for the fluorescence energies between 2 and 5 keV.

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A PETRA III P01 data reduction
This chapter describes how to reduce the raw data obtained at P01 beamline in
PETRA III storage ring with a Pilatus 100k detector from the XES measurements invertical von Hamos dispersive geometry. The corresponding python code is shared inAppendix C.
After each run, a .fio file is created and contains all relevant information of the run.
From this .fio file the values of the monochromator energy and all PIN diodes will be
imported.

Figure A.1: An image from the Pilatus 100k detector in vertical von Hamos dispersive
geometry exposed to the emitted and scattered X-ray photons. The Cu Kβ1,3 fluorescencesignals diffracted by the Ge(400) crystals were recorded. The x-axis corresponds to thedispersive plane (lower value = lower Bragg energy) and the y-axis to the focusing plane(Darwin width).

Figure A.1 shows an image of the Cu Kβ1,3 fluorescence signals recorded on the
Pilatus 100k detector as an example. This image consists of 5 signals from 5 analyzer
crystals, and background noises from electronics and scattered photons. Here weassume that the experimental setup was well-aligned that on the Pilatus 100k detector
mostly the emitted X-ray photons were recorded.
Several region-of-interests (ROIs) are first defined as the background noises andaveraged along the y-axis, and subtracted from other 5 ROIs containing the emission
signals. Each of the 5 ROIs is then also averaged along the y-axis, and aligned toeach other e.g. by matching the max. intensity x-values, to be averaged into an 1-D
array. If available the obtained spectrum can be normalized by the corresponding
incident photon flux I0 or TFY, otherwise the spectrum is normalized by its area.
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For the x-axis calibration (fluorescence energy), a so-called elastic scan is performedby recording the scattered X-ray photons from the analyzer crystals while scanningthe monochromator energy across the expected fluorescence energy range. Oncethe monochromatized X-rays are scattered at the interaction point, those satisfying
the Bragg’s condition will be reflected by the analyzer crystals and therefore appear
on the detector as a peak (Fig. A.2). By scanning the monochromator energy in the
expected fluorescence energy range, we can obtain the linear relationship between
the monochromator energy and the x-value on the detector.

Figure A.2: 5 monochromatized X-ray photons scattered at the interaction point andreflected by the analyzer crystals. The x-axis is thereby calibrated from the x-values to themonochromator energies. The FWHM of each peak (Gaussian profile) is dominated by theenergy resolution of the Si(111) monochromator (∆E/E ∼ 10−4 ).

Figure A.3 shows some of the reference Cu Kβ1,3 and Co Kα emission spectra
measured at P01 beamline. Apart from the difference in their covalency, there isno significant change in the line shapes between the Cu(I) and Cu(II) compounds,
whereas a remarkable shift of the emission lines is noticed between the Co(II) andCo(III) compounds. This result already reveals a very difference between the copperand cobalt chelates, namely the SCO in the Co(II) chelates dramatically affects thecore-to-core fluorescence.
With the same experimental setup, the RIXS spectroscopy can be carried out by
scanning the monochromator energy in the vicinity of the K-edge of target element.
As discussed in section 2.3.3, a 1s2p RIXS plane like in Fig. 2.23 can be plotted andinterpreted. Besides, a XANES spectrum like in Fig. 2.13 can be simultaneously
obtained by following a specific transition channel, in addition to that measured by PIN
diodes either in transmission mode or TFY mode.

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Figure A.3: Cu Kβ1,3 (left) and Co Kα (right) emission spectra from several reference samples
measured at P01 beamline in vertical von Hamos dispersive geometry. The SCO in the Co(II)chelates dramatically changes the line shapes.

The data reduction for the RIXS plane follows the same procedure for the XES except
the spectrum area normalization, but when it comes to the XAFS measurement intransmission mode or TFY mode a different process is applied.

Figure A.4: XANES spectra of cobaloxime(III) in solution phase, cobalt foil (left), and iron foil(right) measured at P01 beamline in TFY mode.

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B SACLA XES data reduction

Figure B.1: Raw images from the MPCCD detectors in horizontal (left) and vertical (right) von
Hamos dispersive geometries exposed to the emitted and scattered X-ray photons. The left
image contains the Fe Kα emission lines diffracted by the Si(111) crystals, whereas the rightimage contains the Co Kα and Fe Kβ1,3 emission lines diffracted by the Si(531) crystals. The
y-axis corresponds to the dispersive plane (lower value = higher Bragg energy) and the x-axis
to the focusing plane (Darwin width).

Basically the same logic as in Appendix A applies to the SACLA XES data reduction.
Each MPCCD detector image recorded at 30 Hz is first reduced into an 1-D array byintegrating 6 ROIs along the x-axis with proper background noise subtraction (Fig.
B.1). If available the obtained spectrum can be normalized by the corresponding
incident photon flux I0 or TFY, otherwise the spectrum is normalized by its area.
The emission spectra without optical photoexcitation are then all summed up andaveraged after filtering out some outliers (Laser Off). The emission spectra with
optical photoexcitation are first sorted out into the 50 fs interval bins after the temporal

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jitter correction [149; 104], and then the emission spectra in each interval bin aresummed up and averaged after filtering out some outliers (Laser On).
X-ray fluorescence energy axis (the y-axis) can be calibrated in comparison with theprevious measurements at the storage ring. Transient signals, i.e. Laser On – Laser
Off = (excited fraction f)*(ES spectrum – GS spectrum), can be further normalized bytheir corresponding averaged optical pump laser power.

Figure B.2: Per-shot diagnostics of the raw data to filter out bad signals. Certain criteria(details can be referred to Appendix C) are set to exclude the outliers, e.g. from the liquid jet
fluctuation. Counts on the right distribution stand for the integrated values over a fixed range of
y-axis on the MPCCD detectors.

Figure B.2 shows the per-shot diagnostics to filter out bad signals. The outliers
are primarily from the liquid jet fluctuation, and the same plot was used during theexperiments to check the jet stability.

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C Python data analysis codes
The python data reduction and analysis codes written by Tae-Kyu Choi and used forthis thesis can be found in the following links.

∎

TOAS experiments at University of Bernhttps://github.com/choi-t/thesis/bern2.py∎

XES and RIXS at PETRA III P01
https://github.com/choi-t/thesis/petra.py∎

Time-resolved EXAFS at APS 11-ID-D
https://github.com/choi-t/thesis/aps.py∎

Femtosecond-resolved XES at SACLA BL3EH2
https://github.com/choi-t/thesis/1dsacla.py

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D Supplementary information
D.1 Chapter 4

Figure D.1: Cyclic voltammogram of the Ir-O-Co measured in degassed acetonitrile at 1 mM
[191]. A Pt working electrode, Ag/AgNO3 reference electrode, and a Pt wire as a counterelectrode were used (vs = 0.1 V s-1 ).

Cyclic voltammetry was performed on both the Ir-O-Co and Ir-th-Co in degassed
acetonitrile to confirm that the excited electron transfer is exergonic towards thecobaloxime(III) catalyst (Fig. D.1; [191; 55]). The cyclic voltammogram of the Ir-th-Co
showed similar redox potentials, but this thermodynamic driving force does notnecessarily mean that its kinetics will be favorable [7].

Figure D.2: (left) The anisotropic absorption r(t) of Ir-O (red), Ir-th (green), Ir-O-Co (orange),
and Ir-th-Co (blue) up to 1 ps, (middle) up to 100 ps, and (right) DASs of the Ir-O with paralleland perpendicular polarization of the optical pump pulse relative to the probe pulse. The probe
wavelength for the r(t) was chosen to be the peak of the corresponding ESA band [201; 47].

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The polarization anisotropy TOAS measurements can distinguish not only thedegeneracy of the transient dipole moment right after the MLCT photoexcitation [201;47; 180], but also a change of the transient dipole moment in the system such as
ILCT [190]. In accordance with their directional charge separation character, the fourbis-cyclometalated Ir(III) complexes were subjected to the polarization anisotropy
TOAS experiments (Fig. D.2).
The anisotropic absorption r(t) of all Ir(III) complexes were calculated based on[201] at the probe wavelength of the peak of each ESA band, and thus the major
contribution to the anisotropy signal was ESA. There was no significant difference inbetween the measurements with parallel and perpendicular polarization except for the
Ir-O, whose r(t) shows an exponential decay with the time constant of ∼250 fs. The
DASs of the Ir-O revealed however quite similar line shapes and time constants (∼20ps), which is again assigned to the vibrational cooling in the 3 MLCT state that causes
its absorption band becoming narrower and blue-shifted.
The nevertheless pronounced r(t) of the Ir-O is understood as an ultrafast incoherent
electron-hole hopping process (vibronic resonance) among the degenerate 3 MLCT
hot states within the first picosecond. Other three Ir(III) complexes have also shown aweak decay of their r(t), and these are understood as a hole transfer in dπ levels.

Figure D.3: (left) A new Ir(III)-Co(III) dyadic system with a bpy-derivative BL synthesized by
our working group, and (right) its optical absorption bands together with those of the two
separate components [143]. The close absorption bands of [Ir-L] and [Ir-L-Co] indicate thatthe attenuation cross sections are predominantly from the Ir(III) moiety. The pump laser was
therefore centered at 425 nm to exclusively excite the Ir(III) moiety and populate its MLCT state.

In addition to the triadic systems introduced in Chapter 4, our working group has
synthesized a new Ir(III)-Co(III) dyadic system (Ir-L-Co) with a bpy-derivative BL (Fig.
D.3). In contrast with the triadic systems, the photocatalytic proton reduction was
not improved compared to the two-component system (Ir-L + Co), hence the TOAS
experiments were performed to investigate the corresponding ultrafast dynamics.
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The Ir-L and Ir-L-Co were dissolved in degassed acetonitrile at the concentrationof ∼6.9 mM and ∼4.8 mM respectively for the OD between 0.1 and 0.3 at 425 nm
photoabsorption and 200 µm optical path length.

Figure D.4: a) TA spectra of Ir-L at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse,b) transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF
results (solid line) up to 10 ps, c) up to 500 ps, and d) the corresponding DASs with three time
constants considered. The ∼0.4 ps time constant represents the ILCT from π*(ppy) to π*(bpy),
and the ∼30 ps time constant represents the vibrational cooling in the 3 MLCT state.

Figure D.4 summarizes the results of the SVD-GF analysis on the reduced TOAS datafrom the Ir-L. Interestingly, the 0.14 ps TA spectrum shows three ESA bands centeredat ∼385 nm, ∼450 nm, and ∼570 nm among which the ∼450 nm ESA band disappears
afterwards while the others develop. The line shapes of the TA spectra up to 300 ps
probe delay then resemble those from the [Ir(ppy)2 (bpy)]+ (Fig. 4.2), confirming thatthe lowest 3 MLCT state lies on the diimine ligand. The ∼450 nm ESA band in the earlystage is thus attributed to the ESA of the π*(ppy) orbital, and the corresponding ∼0.4
ps time constant is assigned to the ILCT process (π*(ppy) → π*(bpy), [190]). The
DAS of the ∼30 ps time constant has a quite similar line shape to that of the ∼9 pstime constant from the Ir-O (Fig. 4.5), so this dynamics corresponds to the vibrationalcooling in the 3 MLCT state on the diimine ligand. The ∼385 nm and ∼570 nm ESA
bands are however also present in the 0.14 ps TA spectrum, unveiling the ultrafast
incoherent electron hopping process among three ligands followed by the excitedelectron localization onto the diimine ligand within the first picosecond. Otherwise
neither GSB nor SE signals were identified from the TA spectra.

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Figure D.5: a) TA spectra of Ir-L-Co at certain probe delays after the 425 nm photoexcitation
measured at magic angle of the probe pulse polarization with respect to the pump pulse,b) transient signal kinetics at different probe wavelengths (dot) together with the SVD-GF
results (solid line) up to 10 ps, c) up to 500 ps, and d) the corresponding DASs with three time
constants considered. The ∼0.4 ps time constant represents the ILCT from π*(ppy) to π*(bpy),
and the ∼124 ps time constant represents the vibrational cooling in the 3 MLCT state.

Figure D.5 summarizes the results of the SVD-GF analysis on the reduced TOAS
data from the Ir-L-Co. Similar to the Ir-L, the ∼450 nm ESA band in the early stage
corresponds to the ESA of the π*(ppy) orbital and the ∼0.4 ps time constant is again
assigned to the ILCT process (π*(ppy) → π*(bpy)). The following dynamics retains
its TA spectrum from 0.5 ps to 300 ps with a slight decrease in intensity, hence thisis attributed to the vibrational cooling in the 3 MLCT state on the diimine ligand. In
contrast to the Ir-O-Co and Ir-th-Co, there is no significant decrease of the excitedelectron density within the picosecond time window, namely there is no clue on theelectron transfer to the cobaloxime(III) in Ir-L-Co. This is however in line with thesame performance of both dyadic and two-component system in photocatalytic proton
reduction [143].
The polarization anisotropy TOAS measurements were carried out on both Ir-L and
Ir-L-Co to further study the charge transfer nature involving a change of the transient
dipole moment in the system (Fig. D.6). The first DAS (red) in all configurations
corresponds to the ultrafast incoherent electron hopping process among three ligands
and therefore is less dependent on the relative polarization. The second DAS (green)
however, that shows negative signals in parallel but positive signals in perpendicular

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Figure D.6: The DASs of Ir-L (above) and Ir-L-Co (below) upon photoexcitation at 425 nm withmagic angle, parallel, and perpendicular polarization of the probe pulse to the pump pulse.

polarization, corresponds to the vibrational cooling in the 3 MLCT state on the diimine
ligand. Since the transient dipole moment is now aligned to the diimine ligand, theparallel probe pulse can add more electron density into the π* orbital while theperpendicular probe pulse can reduce the electron density by inducing an ILCT.

Figure D.7: TA spectra of the cobaloxime(III) at certain probe delays after the 425 nm
photoexcitation measured at magic angle of the probe pulse polarization to the pump pulse.

The same TOAS experiment was also conducted on the cobaloxime(III) (Fig. D.7),
and its TA spectra showed negligible signals in optical probe window [87], so that the
ESA bands from the cobaloxime(III) are ignored in this thesis.

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D.2 Chapter 5

Figure D.8: Comparison of the difference spectrum from the spectroelectrochemical oxidationof the catalytic Mo(VI) complex [MoO2 (LMe )MeOH] (blue) with the (200 ps-200 fs) DTA
spectrum (green) and the inverse sum of the DAS6.8ps and DAS42ps (red) as well as the inverse
sum of the DAS2ps , DAS6.8ps , and DAS42ps (yellow) upon 450 nm photoexcitation [50]. A relative
shift of 0.65 eV was applied in the energy domain.

To further prove that the spectral evolution on the picosecond timescale stems
from the Mo(VI) moiety, we performed the same TOAS measurements and
SVD-GF analysis with the 450 nm phototoexcitation (Fig. D.8). The results
consistently reproduced the spectral features from the oxidation dynamics of theMo(VI)-coordinated chromophoric ligand spanning from 2 to 60 ps (Fig. 5.5).
Thus, we confirmed that the catalytic Mo(VI) moiety was not directly photoexcitedduring the TOAS experiments but indirectly photo-activated due primarily to thespontaneous perturbation by the transient dipole moment in the Ru(II) moiety.

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D.3 Chapter 6
Figure D.9 shows the normalized XANES spectra of the Ru(II) photosensitizers in their
GS and 3 MLCT state at 150 ps probe delay. In general, the GS XANES features are
simply shifted by ∼2 eV in the ES as also noticed in the EXAFS sanity check (Fig. 6.6),
although a tiny pre-edge structure has appeared additionally.

Figure D.9: XANES spectra of the Ru(II) photosensitizers in their GS and 3 MLCT state at theprobe delay of 150 ps.

Figure D.10 shows the fitting results of the ES χ(k ) spectra weighted by k 2 with the
EXAFS equation (2.4) using FEFF8 and IFEFFIT codes. The fitting was successfully
performed in Fourier transform R-space ∣χ(R)∣, and the fitted parameters are
summarized in Table 6.2.
By having two independent fitting parameters for the R(Ru-NMLCT ) and R(Ru-N),
we were able to visualize the picometer-scale changes in the first coordination
shell structure, that were most pronounced in [Ru(phen)3 ]2+ and least in[Ru(TAP)2 (dppz)]2+ . However as we noticed from Fig. 6.5 and 6.6, there is hardly adifference in the geometric structure between the Ru(II) photosensitizers nor between
the GS and 3 MLCT state at 150 ps probe delay. In other words, with the degassed
acetonitrile as solvent the localized excited electron on the dppz-ligand, TAP-ligand, oreven in the 3 MC state did not result in any distinct signals.

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Figure D.10: Fitting analysis of the ES χ(k) spectra of the Ru(II) photosensitizers weighted
by k 2 with the EXAFS equation (2.4). The fitting was performed in Fourier transform R-space∣χ(R)∣.

The ∣χ(R)∣ spectra in Fourier transform R-space show the first peak of thecorresponding pseudo-radial distribution function (PDF) at around 1.5 Å that isshorter than the actual value of 2.06 Å. According to [181], such a shift (∼0.3 to∼0.5 Å) to a shorter value can be due to the core-hole lifetime-broadening which
attenuates the amplitude of high-R side peaks more than the low-R side ones.

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D.4 Chapter 7

Figure D.11: (left) Comparison of the kinetics extracted from the positive transient signalsof Kα1 (green) and Kα2 (pink). (right) Comparison of the kinetics extracted from the positivetransient signals of Kβ1,3 at higher fluorescence energy (green) and at lower fluorescenceenergy (pink). The error bars were calculated in propagated standard error.

To further verify that there was no difference between the kinetics obtained from thepositive transient signals of Kα1 and Kα2 as well as the left and right positive parts of
the Kβ1,3 transient signals, we compared them together in Fig. D.11. Within the error
range there was no significant difference, thus we integrated all the positive transient
signals, to increase the SNR for the fitting analysis in the main text.

Figure D.12: Comparison of the transient signals of the Cu Kα (left) and Kβ1,3 (right)
fluorescence at the probe delay of 0.1 ps and 1 ps.

The reduced data of the Cu Kα and Kβ1,3 fluorescence signals consistently revealed
the femtosecond nuclear dynamics with a decay time constant of ∼410 fs. This veryclear change in the kinetics originates from the overall intensity decrease of thetransient signals (Fig. D.12). In other words, there is no significant change in the line
shapes of the transient signals during the PJT distortion.
In addition to the main analysis, we also performed the SVD-GF analysis on the
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Figure D.13: 2-D plots of the Cu Kα and Kβ1,3 fluorescence signals with respect to the probe
delay before (left) and after (middle) the SVD analysis and their difference (right). Only the
first singular value was taken into account to exclude any other background noises, and thereconstructed data successfully resembled the original one.

reduced data of the Cu Kα and Kβ1,3 fluorescence signals to further exclude any
irrelevant noises (Fig. D.13). The first singular value and the corresponding DAS as
well as the singular vector of kinetics were already enough to reconstruct the data
without missing any important signal. The first singular vectors of kinetics from the Kα
and Kβ1,3 data were then fitted with the sequential first-order rate equation (7.8), andthe fitting analysis returned a decay time constant of ∼400 fs consistently (Fig. D.14).

Figure D.14: The first singular vectors of kinetics from the SVD analysis on the Kα (left) andKβ1,3 (right) data as well as the fitting results with the sequential first-order rate equation (7.8)
consisting of the [MLCThot, 90° ](t) (green) and [MLCThot, 70° ](t) (orange), revealing a decay time
constant of ∼400 fs (1/k2 ) consistently.

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D.5 Chapter 8
Prior to the femtosecond-resolved XES experiments at SACLA, the Co Kβ1,3 emission
spectrum of the cobaloxime(III) in solution phase was measured at the P01 beamline
in the PETRA III storage ring (Fig. D.15) and disclosed that the spin density is in LS.

Figure D.15: Co Kβ1,3 emission spectrum of the cobaloxime(III) in solution phase measured at
the P01 beamline. The spin density in the GS is LS.

To further verify that there was no difference between the kinetics obtained from thedifferent parts of the Fe Kα1 and Kβ1,3 transient signals from the [Fe-BL] upon 515 nm
photoexcitation, we compared them together in Fig. D.16. Within the error range there
was no critical difference in between integrating the negative transient signals andpositive transient signals. The kinetics below the time-zero show abnormal behaviours
compared to Fig. 8.6, and this comes from the lack of shot numbers, i.e. the low
SNR. The kinetics obtained by integrating the transient signals at 7054±1 eV of thefluorescence energy can be an evidence of the 3 MC state appearing right after the
515 nm photoexcitation.
As noted in the main text, a comparison of the Fe Kβ1,3 transient line shapes from the[Fe-BL] and the model complex in [117] upon the MLCT photoexcitation is essential tounderstand the slightly different interpretation between them. Figure D.17 shows the
Fe Kβ1,3 transient line shapes from the [Fe-BL], [117], and [204] at the probe delay of
0.1 ps and 10 ps. Both line shapes from the [Fe-BL] and [117] at 0.1 ps probe delay
and 10 ps probe delay do not show any difference, and from the reference line shapes
it is not clear in which state the complex resides. Nevertheless, it is certain that both

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Figure D.16: Comparison of the kinetics extracted from the positive transient signals (blue,
green) and negative transient signals (red) of the Fe Kα1 (left) and Kβ1,3 (right) fluorescencefrom the [Fe-BL] upon 515 nm photoexcitation. Owing to the lack of shot numbers, the kinetics
below the time-zero behave abnormally. The error bars were calculated in propagated standard
error.

the [Fe-BL] and the model complex in [117] disclosed the same transient line shapes
at 0.1 ps and 10 ps probe delay, i.e. the same ESs populated and hence the same
relaxation dynamics.

Figure D.17: Comparison of the Fe Kβ1,3 transient line shapes from the [Fe-BL], [117], and[204] at the probe delay of 0.1 ps and 10 ps.

Figure D.18 shows the preliminary analysis of the XSS data from the [Fe-BL-Co]
measured at certain probe delays simultaneously with the XES, and thehigh-frequency component for the first picosecond indicates relatively long-distancenuclear dynamics, which is in line with the femtosecond coupling between the Fe(II)and Co(III) moiety.

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Figure D.18: Transient XSS signals from the [Fe-BL-Co] at selected probe delays after the 515
nm photoexcitation. The high-frequency component for the first picosecond indicates relatively
long-distance nuclear dynamics, which is in line with the femtosecond coupling between the
Fe(II) and Co(III) moiety.

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E List of hazardous substancesThis chapter lists the hazardous substances that were handled in the experiments
for this thesis. Their precautionary (P) and hazard (H) statements are listed together
with the corresponding globally harmonized system of classification and labelling ofchemicals (GHS) pictograms.

Substance

CAS Nr.

H-statement(s)P-statement(s)Pictogram(s)Acetonitrile

75-05-8

225, 302, 312,
332, 319

210, 280, 301,
312, 303, 361,
353, 304, 340,
312, 305, 351,
338

Bis(2-phenylpyridine)(dtbbpy) iridium(III)hexafluorophosphate*

676525-77-2

N/A

N/A

Chloro(pyridine)cobaloxime(III)23295-32-1

315, 319, 335

261, 305, 351,
338

Tris(2,2-bipyridine)ruthenium(II)hexafluorophosphate60804-74-2

N/A

N/A

N/A

Bis(2,2-bipyridine)(5-aminophenanthroline)
ruthenium(II)hexafluorophosphate

84537-86-0

N/A

N/A

N/A

Tris(1,10-phenanthroline)
ruthenium(II) chloride*

23570-43-6

N/A

N/A

N/A

(1,10-phenanthroline)
copper(II) chloride*14783-09-6

315, 319

302, 352, 305,
351, 338

N/A

Table E.1: List of the hazardous substances used in the experiments for this thesis. * denotes
the closest compound to those newly synthesized and consumed. All the statements refer tothe respective safety data sheets from Sigma-Aldrich.

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Eidesstattliche Versicherung

Hiermit versichere ich an Eides statt, die vorliegende Dissertation selbst
verfasst und keine anderen als die angegebenen Hilfsmittel benutzt zu haben.
Die eingereichte schriftliche Fassung entspricht der auf dem elektronischen
Speichermedium. Ich versichere, dass diese Dissertation nicht in einem
früheren Promotionsverfahren eingereicht wurde.

Tae Kyu Choi
Hamburg, den 23.07.2021

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