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Ultrafast photoinduced dynamics of the electron donor-acceptor type
metal-organic framework

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BANUSamiranAuthor(English)

Samiran Banu

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12666号,
授与年月日:2024年3月26日,
学位の種別:課程博士,
審査員:腰原 伸也,沖本 洋一,大島 康裕,河野 正規,谷口 耕治

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12666号,
Conferred date:2024/3/26,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

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Doctoral Thesis

種別(和文)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)

Ultrafast photoinduced dynamics of the electron
donor-acceptor type metal-organic framework

By

Samiran Banu
Submitted to the Department of Chemistry, School of Science
Doctor of Philosophy

at theTOKYO INSTITUTE OF TECHNOLOGYSupervisors: Professor Shinya Koshihara and Professor Yoichi Okimoto

Koshihara-Okimoto Laboratory1

2

Abstract

The photo-control of macroscopic physical properties in the solid state is widely known as
photoinduced phase transition (PIPT). Ultrafast switching phenomena caused by PIPT havebeen particularly important because of their potential applications in optical data storage, fastswitching devices, sensors, etc.
Over the past two decades, a family of porous materials known as metal-organic frameworks(MOFs) has been developed significantly, opening a new avenue in molecular materials
science. Because of their porous nature, MOF materials have shown potential applications forelectrocatalysis, energy storage devices, gas sensor equipment, etc. Regrettably, ultrafast
photocontrol of physical properties in MOF systems has rarely been reported. Nevertheless,
MOFs with charge-transfer (CT)-type phase transitions may provide an ideal system forobserving fast photo-responses of electronic states.
In the present study, as a target material for PIPT, we focused on the DA-type (where D =donor and A = acceptor) layered MOF system, (NPr 4)2[Fe2(Cl2An)3] (prepared by Prof.
Miyasaka's group, IMR, Tohoku Univ.), which exhibits two-step CT phase transition (Tc = 317
and 354 K). At Tc, CT occurs between Fe and Cl 2An ions, which magnetic, structural, andspectroscopic measurements have confirmed. This system's mechanism of phase transitions
has been discussed as multi-stability due to the balance between the valence and the Coulombinteraction, like the neutral-ionic (NI) phase transition. It should be noted that there is noapparent structural change at each Tc, probably due to the rigid bond nature between Fe andCl2An ions, contrary to the existence of dimerization of the lattice in the typical NI system,
TTF-CA. Therefore, the phase transition of this system can be viewed as a purely electronic
one. Then, an efficient photoinduced CT phase transition can be expected without lattice
structural deformation by weak photoexcitation.
Here, we observed highly efficient photoinduced CT dynamics in the DA-type layered MOF:
(NPr4)2[Fe2(Cl2An)3] by using time-resolved spectroscopy with a pulse width of 90-fs at room
temperature (RT: 300 K). The photoinduced spectral changes are similar to the thermally3

induced spectral change at around 2.6 eV, suggesting photoinduced valence change of theCl2An ions. However, we observed a considerable difference in the spectral shapes in the CT
transition and mid-IR regions. We found two unexpected new photoinduced absorption bands
from the spectra analysis, one at the higher-energy side of the main CT band and the other inthe mid-IR energy range—just immediately after photoexcitation. The observed spectralchange strongly implies the disappearance of the local inversion center on the Cl 2An ion
instantly upon photoexcitation, causing an ultrafast change in the lattice structure due to thesoftening of rigid bonds. This has never been realized in a thermal phase transition. Thesefindings demonstrate that a new electronic state with a unique lattice structure—i.e., aphotoinduced hidden state—appears in this MOF system at ultra-high speed (within 110 fs)
upon photoexcitation at RT.
Early-stage dynamics for the formation of structurally modulated photoinduced hidden state,discussed in the previous section, is still unclear, and we thought of a way to visualize them inthis study. Herein, we used pump-probe spectroscopy with nearly single-cycle optical, infrared
pulses (pulse width ~ 6 fs) to investigate early-stage dynamics for photoinduced structurally
modulated state formation. This study was done in collaboration with Prof Iwai's group at
Tohoku University. After the initial large spectral change due to the photoinduced CT at ∆t =0 fs, there is a clear spectral change in the relaxation process (redshift of zero-crossing point)
from 0 to 50 fs, suggesting sequential structural change corresponding to the "hidden state" weobserved. Meanwhile, there is no further spectral change up to at least 500 fs. In addition, we
observed coherent oscillations in the time evolution of ∆R/R. According to the analysis of thecoherent oscillations, we found the IR active and Raman inactive mode in the thermal
equilibrium phases ~ 494 cm -1 becomes Raman active in the photoinduced state at least after
50 fs, which implies a photoinduced symmetry breaking of this material. Hence, we assumed
that the formation timescale of the hidden state from the initial purely electronic excited statemust be ~50 fs.
We have successfully demonstrated an efficient photoinduced CT conversion between Fe andCl2An ions (Cl2An3- + Fe3+→ Cl2An2- + Fe2+) at RT. But now we intend to observe thephotoinduced reverse CT (Fe 2+ + Cl2An2-→ Fe3+ + Cl2An3-), i.e., conversion from hightemperature (HT) to LT phase. We performed a time-resolved experiment to realize suchthought by exciting the intramolecular transition in Cl 2An2- ion (~ 2.6 eV) with E || chain forpump and probe pulses in the HT phase (380 K). The photoinduced ∆R/R spectrum at 2 ps can

4be well reproduced by simulated ∆R/R spectrum in the CT energy range, suggesting a
successful observation of photoinduced reverse CT between the Fe and Cl2An ions. In addition
to the above result, we observed a threshold behavior for realizing reverse CT for the pumpexcitation fluence, i.e., Iex > 1.0 mJ/cm2, suggesting the cooperativity in reverse CT.
In summary, we demonstrate ultrafast and efficient photoinduced charge-transfer dynamics,
along with a unique lattice-structure modulation, occur within 50 fs in the donor–acceptor type
metal-organic framework, (NPr4)2[Fe2(Cl2An)3], at room temperature. Also, we successfully
demonstrated the observation of reverse charge transfer by exciting at 380 K. We believe thepresent study provides an essential first step toward realizing flexible photo-control of metalorganic frameworks and opening the way for developing magnetically active photo-switching
metal-organic frameworks in such robust structures. This discovery will also represent a novel
photoinduced phase transition material and enrich the study of optical properties of solid andfundamental physics.

5

Table of Contents
1. Research Background ........................................................................................................... 9
1.1 Photoinduced phase transition (PIPT) ................................................................................. 9
1.1.1 Overview of PIPT....................................................................................................... 9
1.1.2 History of PIPT research ........................................................................................... 13
1.1.3 Ultrafast spectroscopy in PIPT research ..................................................................... 14
1.2 Electron donor-acceptor (DA)-type systems ...................................................................... 15
1.2.1 DA-type thermally driven electron-transfer (TDET) systems ....................................... 15
1.2.2 PIPT in DA-type systems .......................................................................................... 19
1.3 An Overview of Metal-organic Frameworks (MOFs) ......................................................... 24
1.3.1 Introduction ............................................................................................................. 24
1.3.2 Classification of MOFs ............................................................................................. 25
1.3.3 Applications of MOF systems.................................................................................... 27
1.3.4 DA-type MOF systems ............................................................................................. 29
1.3.4.1 Construction of DA-type MOF systems ............................................................... 29
1.3.4.2 Properties of DA-type MOFs .............................................................................. 30
1.3.5 Short description of flexible MOFs ............................................................................ 32
1.3.6 Photoinduced dynamics in MOFs with time-resolved spectroscopy.............................. 33
1.4 Motivation of the present study ........................................................................................ 37
2. Target Material: (NPr4)2[Fe2(Cl2An)3 ] ................................................................................. 40
2.1 Sample preparation.......................................................................................................... 40
2.2 Structural overview ......................................................................................................... 41
2.3 Charge transfer (CT) type phase transition ........................................................................ 44
2.4 Research objective and expected outcomes ....................................................................... 46
3. Experimental Methods ........................................................................................................ 48
3.1 Sample selection and method of fixation ........................................................................... 48
3.2 Determination of crystal anisotropy and optic axis ............................................................. 50
3.3 Static reflectivity spectra measurement ............................................................................. 53
3.3.1 The principle and types of reflection of light............................................................... 53
3.3.2 Instruments used for static reflectivity measurements .................................................. 54

6

3.4 Raman spectra measurement ............................................................................................ 55
3.4.1 Principle of Raman spectroscopy ............................................................................... 55
3.4.2 Instrument used for Raman spectra measurement ........................................................ 56
3.5 Ultrafast time-resolved spectroscopy ................................................................................ 57
3.5.1 Background: Overview of the timescale for various molecular processes...................... 57
3.5.2 Time-resolved pump-probe spectroscopy ..................................................................... 58
3.5.2.1 Optical setup for pump-probe spectroscopy with 90 fs pulse ................................. 59
3.5.2.2 Near-infrared 6 fs pulse generation method and optical setup ................................ 64
4. Static Optical spectra of a single crystal of (NPr 4)2[Fe2(Cl2An)3] ......................................... 69
4.1 Objective and Motivation of this experiment ..................................................................... 69
4.2 Result and Discussion...................................................................................................... 70
4.2.1 Steady State Reflectivity spectra: Determination of crystal anisotropy and optic axis .... 70
4.2.2 Anisotropy of optical spectra at room temperature (RT)............................................... 72
4.2.3 Temperature-dependent optical spectra along E ||chain ................................................ 78
4.2.4 Temperature-dependent optical spectra along E ⊥ chain .............................................. 81
4.2.5 Temperature dependent Raman spectra for both E || chain and E ⊥chain directions ....... 82
4.3 Summary ........................................................................................................................ 84
5. Time-resolved spectroscopic study of (NPr 4)2[Fe2(Cl2An)3 ]: Exciting LT phase using 90 fs
pulses ........................................................................................................................................ 86
5.1 Objective and Motivation of this experiment ..................................................................... 86
5.2 Experimental condition, result, and discussion .................................................................. 87
5.2.1 Experimental condition for time-resolved experiment for (NPr4)2[Fe2(Cl2An)3] at RT.... 87
5.2.2 Photoinduced transient ∆R/R spectrum along E || chain at RT ...................................... 88
5.2.2.1 Transient ΔR/R spectrum in the intramolecular transition energy range (2.0 – 3.0 eV)...................................................................................................................................... 95
5.2.2.2 Transient ΔR/R spectrum in the CT energy range (0.5 – 1.5 eV).......................... 104
5.2.2.3 Transient ΔR/R spectrum in the mid-IR energy range (< 0.3 eV) ......................... 110
5.3 Summary ...................................................................................................................... 121
8. Conclusions ......................................................................................................................... 124
Appendices ............................................................................................................................. 128
References............................................................................................................................... 137

7

Acknowledgements ................................................................................................................. 149

8

Chapter 1

1. Research Background
This chapter first describes the overview of the photoinduced phase transition (PIPT)
phenomenon. The basic concepts of PIPT are explained by the renowned phenomenologicalmodel constructed in the early 2000s, and the historical background of PIPT research isreviewed. Subsequently, the concept of the DA-type system and its photoinduced responses issummarized. The basic overview of metal-organic framework systems is also depicted. At theend of this chapter, the objectives and the motivations for this research work are described.
1.1 Photoinduced phase transition (PIPT)
1.1.1 Overview of PIPT
A phase of matter is uniform concerning its physical and chemical properties. As defined bychemistry, thermodynamics, and other related fields, a phase transition (or phase change) is aphysical transition between one state of a medium and another. Solids, liquids, gases, andplasma are the primary phases of matter, solid H2O (ice) melting into liquid H2O (water) is an
example of a phase transition. However, free energy takes a global minimum at eachtemperature in all these phases, called an equilibrium phase.

For broad scientific interest, phase transitions accompanied by electronic, magnetic, dielectric,
and structural changes in equilibrium conditions are fascinating from the perspective ofresearch and application. These changes result from cooperative interactions between
molecules/atoms under changing external conditions/ stimuli. As an example, the equilibriumphase transitions of transition metal oxides with a perovskite structure have been widely
studied for several decades, including metal-to-insulator (M-to-I) transitions [1,2],

9

ferroelectricity [3], quantum paraelectricity [4], and superconductivity [5]. But in this research,
we are mainly interested in phase transition induced by light, which invokes the question
towards equilibrium phase transition; photo-irradiation excites materials to another potential
surface with a different structure from the ground state.

Our next topic focuses on nonequilibrium phases caused by excitations or stimulation.
Although these states usually have higher energies than the equilibrium state, depending on
how they are stimulated or excited, we can get many more varieties of states, regardless of theequilibrium phase. But surely, they also relax to the starting equilibrium phase after a while.
From a research and practical standpoint, phase transitions that emerge from cooperativeinteractions between constituent molecules/atoms under varying external conditions and are ofinterest are followed by electronic, magnetic, dielectric, and structural changes.

Fig. 1.1: Schematic conceptualizing the photoinduced phase transition (PIPT). The phasetransition from phase 1 to another phase (phase 2) of the adiabatic potential energy is induced
through the photo-excited state [6].

10Among all external stimuli, to control materials functionality in a fast, non-thermal, andcontactless way, it is essential to understand how molecular systems with strong electronelectron, electron-lattice, and magnetic correlations interact with light. Materials are excitedby photo-irradiation to a distinct potential surface with a structure from their ground state (seePhase 1 in Fig. 1.1). Phase 2 in Fig. 1.1 represents a situation where relaxation via an excitedstate potential surface could cause a material to change phases Under nonequilibrium
conditions. Generally, this type of switch is known as photoinduced phase transition (PIPT)[6,7].

In 1997, Nasu [6] added energy relaxation from the excitonic state to collective excitation,
mediated by lattice-relaxed excitons, to the theoretical framework of charge-density-wave(CDW) systems. According to Fig. 1.2, the concept of PIPT has been generalized by thisframework. Because the classical (incoherent) energy-relaxation process is a part of this familyof PIPT occurrences, it is called classical (incoherent) PIPT. Here, the vertical axis is equivalent
to the adiabatic potential energy in a microscopic viewpoint. The horizontal axis corresponds
to the configuration coordinate for structural phase transition, magnetization for magnetic
phase transition, etc., which are usually considered as order parameters. After photoexcitation,
when the material absorbs the photon energy, the ground state at the minimum free energy isvertically excited to the Frank-Condon state (according to the Franck–Condon principle). In
general, the time scale of the electronic excitation process occurs within femtoseconds (10 -15s), which is nearly the reciprocal of the excitation photon energy. Since this process is too fast,
the lattice structural configurations are considered frozen in this time scale. The Frank-Condon
state with high excess energy is unstable. Thus, it relaxes too many stable states accompanying
microscopic structural changes. Through the relaxation process, a part of the excited state
converts to the initial state, while another part goes to a new hidden macroscopic order, which

11researchers are interested in studying. The new order prevails due to the inherent cooperativeinteraction known as the domino effect, and excited domains are formed. This metastablecondition, also known as the "false ground state," is located at a local free energy minimum
and is separated from the real ground state by an energy barrier greater than the thermal energy.
As a result, the new phase remains stable (or metastable) there. The total processes of PIPT are
conducted in this manner.

Fig. 1.2: Schematic diagram of PIPT dynamics proposed by Prof. Nasu. [6]121.1.2 History of PIPT research
PIPT research began with the 1984 report on LIESST (Light Induced Excited Spin State
Trapping) in spin-crossover complexes [8]. Starting from 1990 till now, extensive studies on
PIPT have been performed. Realistic examples are reported for various materials such as chargetransfer (CT) crystals [9-13], π-conjugated polymers [14,15], insulator-to-metal transition inmetal-oxides [16-18], spin crossover complexes [19-22], Prussian-blue analogs [23], thequantum structure of diluted magnetic semiconductors [24], low-dimensional transition metalorganic complexes [25] etc. and many more.
Two pioneering studies had the greatest impact on subsequent research in this field. One isabout the PIPT of TTF-CA (tetrathiafulvalene-p-chloranil) [11,13], where the photoinduced
neutral (N) to ionic (I) phase transition served as the first example of the transitory PIPT fromthe ground state to a metastable phase. Another one is the π − conjugated polymer,
polydiacetylenes [14,15], which showed a reversible structural phase transition from ametastable phase to its ground state for the first time.
From here, the research for PIPT has been extensively performed and has shown drastic
progress. Various metastable phases have been observed in strongly correlated materials, suchas transition metal oxides, metal complexes, and low-dimensional organic conductors. For
example, insulator−metal phase transitions in organic complex (EDO-TTF)2PF6 [26, 27] or intransition metal oxides like VO2 [18, 28, 29] were reported. For polycrystalline samples of
Pr0.5Ca0.5CoO3 (PCCO), the spin-crossover effect and insulator–metal transition in PCCO andits photo-response were also studied [30]. K-TCNQ is a well-known spin Peierls transition
system with dimerization of its constituents and magnetic changes. This crystal dimerization
melt causes a PIPT from spin Peierls to non-dimerized phases.

Not only has experimental research progressed, but many theoretical studies have also been
performed. Several first principle calculations have been conducted on spin-crossover13

materials [31, 32]. PIPT was phenomenologically interpreted by Nasu in the early 2000s [6],as described in the previous section. The idea of creating diamond structures from graphiteusing PIPT was proposed in 2009 [33, 34].

In the next section, with the invention of ultrafast spectroscopy methods, how the PIPT research
provides a significant advancement in science and technology is discussed.

1.1.3 Ultrafast spectroscopy in PIPT research
Photoinduced processes are always associated with short-lived reaction intermediates in theirdynamical changes (transient or metastable states). To understand the nature of the dynamical
processes related to PIPT, the PIPT mechanism has to be understood phenomenologicallywithout considering the detailed processes connecting the true and false ground states. The
dynamical processes are usually complicated, the relaxation processes involve lots of electronelectron, electron-lattice, or collective interactions. Electron-electron scattering typically takes
place over a time scale of 10 fs (femtosecond), while electron-phonon scattering takes placeover a timescale of several 100 fs [35] as shown in Fig. 1.3. Thus, to explore the correlation
between the degree of freedom related to spin, and electronic and structural motions in solidstate materials, we need ultrafast spectroscopy based on pump-probe measurement with enoughresolution for direct visualization of their dynamics.

Fig. 1.3: Schematic of the nonequilibrium relaxation dynamics in a pump-probe experiment14

was summarized by Aoki et al. [35]. Here, the time scale is rescaled corresponding to theenergies of the phenomena. It suggests the time scale for observing electron-electron andelectron-phonon interaction lies between 10 to a few 100 fs (femtosecond) time scales.
Meanwhile, dynamic phase transition can be observed in ps (picosecond) timescale.
With the advent of ultrashort light pulses and sophisticated spectroscopic techniques, detection
and understanding of photoinduced dynamics processes have seen an impetuous growth. The
rapid development of femtosecond lasers, pulsed X-rays, and pulsed electron techniques during
the last 20 years has enabled the ultrafast classical (incoherent) PIPT research field. New
measuring techniques, such as femtosecond time-resolved photoemission spectroscopy andelectron diffraction, will also be introduced as potent new instruments in PIPT research.
1.2 Electron donor-acceptor (DA)-type systems
Photo control of the electronic state is one of the key steps for developing light-harvestingprocesses for energy conversion, photocatalysts, high-speed electronic devices, etc. For thispurpose, DA-type (D = donor and A = acceptor) systems are considered to be ideal andexpected to be suitable for research and development.
Photoinduced electron transfer is an excited state electron transfer process by which an excitedelectron is transferred from D to A.

1.2.1 DA-type thermally driven electron-transfer (TDET) systems
In recent two decades, it has become increasingly common to use molecular systems to develop
new functional materials, including organic conductors, molecular materials to develop
optoelectronics, optical data storage, fast-switching devices, molecular magnets, etc. The
possibility of inducing a reversible change in the electronic distribution of a molecular system
by varying appropriate external parameters like temperature (T), pressure (P), light (ℎ𝜈), etc.,
paves the way for a new research field to discuss at the molecular level. As a class of such15

functional materials, electron DA-type thermally driven electron-transfer (TDET) systems [3644] are an attractive target because such external stimuli can easily trigger their molecularproperties.
Quinonoid Metal Complexes
Quinonoid metal complexes [36, 45-47] are highly attractive materials for designing materials
with potential technological applications due to their peculiar red-ox active nature. Here,primarily metal-polyoxolene complexes exhibit intramolecular electron transfer processes
involving either the ligand and the metal ion or the two di-oxolene moieties of a properly
designed ligand, thus inducing electronic bistability. These systems are potentially utilized as
molecular switches because they can transition between the two metastable electronic states
when exposed to different external stimuli such as temperature, pressure, light, or pH [48,49].
As an example of such system is Co2(CTH)2(DHBQ)(PF6)3 (where CTH = dl-5,7,7,12,14,14hexamethyl-1,4,8,11-tetraazacyclotetradecane and DHBQ = deprotonated form of 2,5dihydroxy-1,4-benzoquinone) [36].

Fig. 1.4: Temperature dependence of 𝜒𝑀 T of complex [{Co(CTH)} 2(DHBQ)](PF6)3 measured
before (▲) and after (⃞) irradiation at 647 nm. The temperature dependence of the reflectivity

16

at 617 ± 5 nm is also reported in the bottom right inset. On the top left inset, a schematic viewof the cationic part of the complex is shown [36].
Here, the temperature-dependent magnetic susceptibility measurements showed that atransition between two different species (mixed valence ℎ𝑠-CoII-CoIII) occurs below 200 K (Fig.
1.4). The more interesting is that this complex was found to undergo valence tautomerism uponlight irradiation. Indeed, if light irradiation at 647.1 - 676.4 nm is applied at low temperatures,the magnetic moment immediately increases up to 𝜒𝑀 T of 1.2 cm3 K mol-1, as shown in Fig.
1.4.
DA–type neutral-ionic (N-I) phase transition systems
The neutral−ionic (N−I) phase transition in a DA pair, which involves electron transfer between
distinct neutral (N: D0A0) and ionic (I: D+A−) valence states, is an intriguing switching
phenomenon that drastically changes the fundamental physical properties of this pair, such as
spin states, electronic transport, and electrical polarization. For example, the compound[Ru2(2,3,5,6-F4PhCO2)4(DMDCNQI)].2(p-xylene)(1;2,3,5,6-F4PhCO2-=

2,3,5,6-

tetraﬂuorobenzoate; DMDCNQI = 2, 5-dimethyl-N, N'- dicyanoquinonediimine), showed adeﬁnite evidence for the stepwise N-I transitions in the temperature range between 210K ( =T2) and 270K ( = T1) with N phase at T > T1 and I phase at T < T2 [50] caused by contributions
from anisotropic interchain Coulomb interactions in a metal-complex-based covalently bonded
DA chain. As shown in Fig. 1.5(a), on DMDCNQIδ-, δ was estimated by the Kistenmacher
relationship [51], the DMDCNQI moiety has a quinonoid structure around room temperature.
In other words, the chain is neutral (N form) when T > T1. Then DMDCNQI converts to aquasi-benzenoid form, meaning the chain becomes ionic (I form) with δ ≈ 1 when T < T2. The
IM form is a 'stage of charge' with a formal δ∼0.5 between T1 and T2. The arrangements of the17N and I chain in the I phase (top), IM phase (middle), and N phase (bottom) onto the bc plane
are shown in Fig. 1.5(b), respectively.

(b)
x
(a)

Fig. 1.5: (a) The degree of charge transfer, -δ on the DMDCNQI subunit is estimated using theKistenmacher relationship. (b) Arrangements of N and I chains in the I phase below T2 (top),
IM phase between T1 and T2 (middle), and N phase above T1 (bottom) projected onto the bc
plane. Yellow and green denote the N and I chains, respectively [50].

In the category of organic DA-type systems, one of the famous examples is TTF-CA (where
TTF = tetrathiafulvalene and CA = p-chloranil), which exhibits a reversible electronic phasetransition has been discovered at which organic charge-transfer solid is transformed fromneutral to ionic phases under pressure [52 - 55]. On cooling a sublimed film of TTF-CA, thephase transition appears as a distinct, reversible change in color from yellow to red, seen near
77 K. The optical-absorption spectrum at 300 K for such a sublimed film and a powderedsample dispersed in KBr are compared in Fig. 1 with that of a film at ~ 11 K. The absorption18

of the sublimed film above and below the N-I phase transition is shown in Fig. 1.6, suggesting
the difference in spectral behaviors between the N and I phases. Optical, infrared, and Raman
measurements by Torrance et al. (1981) indicate that this is a reversible transition from anominally N solid to a nominally I salt. since this is the most prototypical compound thatshows the N - I transition not only by applying hydrostatic pressure above Pc, (equal to 11 kbar)
but by lowering the temperature below Tc.

Fig. 1.6: Absorption spectra of sublimed film above and below the N-I phase transition in TTFCA [53]. Arrows represent absorption peaks of neutral and ionized molecules in solution.

1.2.2 PIPT in DA-type systems
A study of the photo-response of TDET systems will be attractive for achieving fast, nonthermal, and contactless control of crystal functionality. The reason is that a charge-transfer(CT)-type transition—which has the lowest photoexcitation energy in such TDET systems—

19

can be an efficient mechanism for controlling electron/spin functionality. Such a DA-type
TDET system is usually a molecular material with a low-dimensional, electron-correlated
lattice, as defined by anisotropically directed electronic correlations or chemical bonds between
the constituent molecules/ions. The low dimensionality of such an electron-correlated lattice
may induce several kinds of instability associated with the electrons, lattice, and spin. These
are critical factors for achieving an efficient photo-response because of the inherently
cooperative electron–electron, electron–lattice, and magnetic interactions that are strongly
expected.
Most neutral–ionic (N–I) phase-transition systems [50, 52, 56] are one-dimensional (1D) DAtype systems, although some inter-chain Coulombic interactions have been observed [50]. In
particular, TTF–CA crystals (where TTF = tetrathiafulvalene and CA = p-chloranil) exhibit
highly efficient photoinduced N–I phase conversions [57–63]—so-called PIPTs. In 1990,
Koshihara et al. [57] found that a local charge-transfer excitation on the DA pair leads to asemi-macroscopic valence change from the quasi-ionic (D+A-) to quasineutral (D0A0) states inorganic molecular-compound TTF-CA with mixed DA stacks. They have done photoirradiation
at energies above the CT gap (at 2.0-2.5 eV) utilizing a pulse dye laser (10 ns) or a chopped
light (400 Hz) from a CW Argon laser. The anomalous photoconductivity properties can be
attributed to photogenerated N to I domain walls. Fig. 1.7 shows the reflectance spectra of
TTF-CA for the neutral and ionic phases taken on the (001) surface at 90 and 77 K, respectively.

20

Fig. 1.7: (a) reflectance spectra and (b) photo reflectance ∆𝑅/𝑅 spectra for molecular
excitations (E^a) at 77 K (ionic phase) and 90 K (neutral phase) in TTF-CA crystal. The
ordinate scale for ∆𝑅/𝑅 (b) is for the results of pulse excitation shown by open (90 K) andsolid (77 K) circles. Solid and broken lines represent the result of CW laser excitation. The
spectrum (c) is the calculated differential spectra using the respective ionic phase (RI) andneutral phase (RN) spectra shown in (a) [57].

21Fig. 1.8: (a) Polarized reflectivity spectra of TTF-CA in the I phase at 77 K (RI) and in the
N phase at 90 K (RN). (b), (c) Photoinduced ∆𝑅/𝑅 at 90 K for the IMT band (b) and the CTband (c) for the 1.55 eV excitation with E∥a [64].

Whereas in Fig. 1.7(b), photoinduced changes of reflectivity (∆𝑅/𝑅) are plotted in the neutral
and ionic phases, measured at 90 and 77 K, respectively. The photoinduced ∆𝑅/𝑅 spectra wererecorded as a relative difference (∆𝑅/𝑅) between the spectra with and without photoirradiation.
Thus, photo-induced I to N phase transition can be achieved. In contrast, N to I photo-induced
phase transition cannot be observed at that time due to the limitation of time resolution of thetime-resolved optical setup.
Then, in 2006, Iwai et al. [64] successfully reported cooperative coherent control of this lattice22phonon coupled with charge in the dynamical process of the photoinduced N-I transition in
TTF-CA, as shown in Fig 1.8. To measure the photoinduced ∆𝑅/𝑅 spectrum, they used a Ti:
sapphire regenerative ampliﬁer operating at 1 kHz as a light source. Output from the ampliﬁer
was 800 nm (1.55 eV) with a pulse width of 130 fs. In Figure 1.8, the experimental resultclearly illustrates the observation of a photoinduced N to I phase transition in TTF-CA [64].

In summary, PIPTs provide a distinctive platform for studying non-adiabatic transitions. As aresult of the technological breakthroughs provided by ultrafast lasers, they have arisen as a
thriving research topic in recent decades. The initial state that appears immediately after
photoexcitation and from which order recovers is far from equilibrium during these transitions.
Because of the discovery of this property, liquid crystals and their applications, such as opticaldevices and photo-controlled molecular motors, can now be aligned rapidly. Photo-control of
the neutral-ionic phase transition in CT complexes has been established, and various
characteristics (including ultrafast electronic and lattice dynamics) have been thoroughly
investigated. The photo-controls of charge density wave, charge ordering, and spin Peierls
systems have also been investigated. In addition, a wide range of photoinduced M-to-I phasetransitions have been studied. Here, the primary goal of my research is to demonstrate theoccurrence of the ultrafast light field (including THz, IR, and visible regions) induced phaseswitching (PIPT).
The realistic target of my research for the PIPT study is a DA-type metal-organic framework(MOF) system. Then, a short overview of MOFs and their properties and applications aredescribed in the net section.

23

1.3 An Overview of Metal-organic Frameworks (MOFs)
Here, DA - the type MOF system is chosen as our target material to study photoinduced
dynamics in molecular crystal systems. We employed time-resolved spectroscopy to examine
the ultrafast photoinduced dynamics of DA-type MOF material primarily. Therefore, it isessential to comprehend the underlying idea behind MOF materials and some of theirfundamental characteristics, such as classification, application, and photoinduced responses.
So, the key information in this section is to provide the fundamental physiochemical propertiesof MOF systems.

1.3.1 Introduction
Metal-organic frameworks (MOFs) are made when organic ligands act as linkers and metal
ions act as nodes in chemical bonds. This process results in periodic network crystalline
structures with high porosities and significant surface areas, which support their potential use
in a variety of scientific and technological fields, particularly for gas storage [65], separation[66, 67], and catalysis [68–70]. Scientists at the time were interested in Tomic's initial article
about MOFs and porous materials, published in 1965 [71]. Material importance is determinedby the role they play in human life and the impact they have on technological fields in the
future. The functionalization of MOF systems has been developed significantly during the past
two decades and has opened a new avenue in molecular materials science, especially becauseof their porous nature. The potential applications of these materials have motivated many
chemists to design new types of porous molecular materials, particularly for gas storage,
separation, and catalysis. A materializing competitor to traditional porous materials likeactivated carbons and zeolites, MOFs have a typical broad inner surface area (often 500–7000m2/g), structural flexibility, programmable porosity, changeable organic functionality, andphysical/thermal stability. Late in the 1990s, Omar Yaghi of UC Berkeley invented the rapidly

24expanding field of research known as MOFs [72]. There have been claims that there are morethan 90000 MOF structures, and the number keeps rising. MOFs are made by coordinatively
enmeshing a metal ion with organic linkers (Fig. 1.9), resulting in unfolded frameworks witha vast surface area, a big pore volume, and a central vision of limitless porosity.

Metal ions

Organic ligands

Metal-organic Frameworks (MOFs)

Fig. 1.9: Schematic representation of the formation of MOF structures by forming chemicalbonds between the metal ions as nodes and the organic molecule as a linker.

1.3.2 Classification of MOFs
"MOFs" is the abbreviation of metal-organic frameworks, which is usually used as a generalname for the group of compounds. Depending on their component units, MOFs could be
divided into various groupings. A few examples are represented below, and some are shown in
Fig. 1.10. Several review papers on MOFs have more detailed descriptions [73].
(a) Isoreticular MOFs: Isoreticular MOFs are synthesized by [Zn 4O]6+ secondary building
units and a series of aromatic carboxylates [74, 75]. These MOFs are octahedral microporous
crystalline materials. (b) Zeolitic Imidazolate Frameworks (ZIFs): They are zeolite topologicalstructured materials. ZIFs comprise ZIF-8, ZIF-90, ZIF-L, ZIF-71, ZIF-67, ZIF-7, etc. [76].
ZIFs are employed as a network to create innovative MOF composites because they have huge25pores and high chemical and thermal durability.

Fig. 1.10: Examples of different metal–organic framework structures with their corresponding
metallic clusters and organic linkers. The figure is adapted from ref. [82].

(c) Porous Coordination Networks (PCNs): PCNs are stereo-octahedron materials having ahole-cage-hole topology and a three-dimensional structure. Some of the PCNs are PCN-333,
PCN-224, PCN-222, and PCN-57. [76], (d) Materials Institute Lavoisier (MIL) MOFs: MIL
MOFs are synthesized using various elements that have valence electrons and an organic
compound containing two carboxylic functional groups. MIL MOFs contain MIL-101, MIL100, MIL-53, MIL-88, MIL-125, etc. [76, 77, 78]. (e) Porous Coordination Polymers (PCPs):
PCP materials are synthesized by carboxylic acid, pyridine, and its derivative as the primarybuilding units and transition metal ions as the secondary building units [76]. PCPs exhibit
magnificent characteristics in biomacromolecule separation and heterogeneous catalysis [79,80]. (f) University of Oslo (UiO) MOFs: A University of Oslo MOF based on dicarboxylic acidas the primary building unit and Zr6(μ3-O)4(μ3-OH) as the secondary building units was first26

synthesized by Lillerud and coworkers [81]. The abovementioned varieties are the mostpopular and common MOFs synthesized and employed in various applications.

1.3.3 Applications of MOF systems
Because of a unique combination of these three main properties—crystallinity, porosity, andthe existence of strong metal–ligand interactions—MOFs are a distinct class of materials.
MOFs are useful in a wide range of intended applications, including gas storage and delivery,
drug delivery, rechargeable batteries, super-capacitors, separation membranes, catalysis,
sensing, etc., thanks to their high surface area, small density, structural flexibility, and tunable
pore functionality. Fig. 1.11 shows the schematic or pictorial representation of a wide range ofapplications of MOFs [83]. Among them, some major fields of applications are discussed here.
For details, MOFs are widely discussed in review articles, such as ref [72, 84], whichemphasizes their real-world applications.
(a) Gas Storage: Because of the distinctive structure of MOFs, their primary function is for gas
storage due to their substantial surface area. Although zeolites and activated carbons are widelyused, they have poor CO2 absorption capacities and selective separation. Due to their highsurface area, adjustable pore sizes, and customizable and programmable frameworkarchitectures and topologies, MOFs are ideal for collecting and separating CO2. The perfectpotential of this behavior in carrying out both CO2/N2 and CO2/CH4 separations was confirmedby an actual breakthrough test [85]. In a study by Yoon et al., the reduction was carried out
using Na2SO3 as a reducing agent after Cu(I) ions were added to a mesoporous MOF using asimple technique employing CuCl 2 loading. Large C3H6 working capacity (1.3 mmoL/g) at
10e100 kPa and C3H6/C3H8 selectivity four times higher than the parent MOF were the resultsof the Cu(I) incorporation [86].

27Fig. 1.11: A schematic representation of the different applications of MOF systems, adopted
from ref. [83],(b) Catalysis: MOFs have many properties that commend them as catalysts: High catalytic
reaction rates per unit volume require high interior surface areas and active site density. (i)Possibilities for complementing catalytic group addition during and after MOF production. (ii)
Possibilities to modify MOF pore architectures to enable pores big enough for quick transit,
shape-selective catalysis, confinement effects to stabilize encapsulated catalytic species, andpositioning several catalytic groups in close proximity conditions. The application of MOFs tocatalysis is described in ref. [87 - 90] (c) Sensors: A few instances are described concerning the
beginning materials, the integration of the MOF as a sensor component, the biosensor, and thefabrication process. Ru et al. synthesized a MIL-101-NH2-SO3H as an inorganic nano-fillerthat exhibited the best sensing features by the organic-inorganic humidity sensor based on thecomposite material (SPEEK/MNS-30%) [91]. Detection of both UO2
applicationofaluminescenteuropiumorganic

framework,2+and Cu2+

with the[Eu 2(MTBC)(OH)2(DMF)3(H2 O)4].2DMF.7H2O was assessed by Liu et al. [92]. (d) Biomedical applications: Due28

to its incredible size, composition, and functionalized pore surface, MOFs are being researched
as vehicles for delivering loaded goods to targeted places. The following are the unique benefitsof MOFs as potential drug delivery materials: (i) highly adjustable characteristics and pore size,(ii) increased drug loading capacity, and (iii) manageable multifunction [93-96].

1.3.4 DA-type MOF systems
The development of MOFs is a highly promising platform for constructing DA-type systems.
Developing a new DA system in MOFs has also become a valuable method for the applicationsof MOF systems in optics, electronics, magnets, catalysis, and anti-counterfeiting research
fields [97-103]. These materials lay the foundation for charge transfer (CT) and energy transfer(ET) based properties. This fact implies a significant benefit for our research since it can beeasily tuned through photoexcitation. In the following sections, we summarize the technicaldevelopment of D-A MOFs and their design, construction, and property advantages.

1.3.4.1 Construction of DA-type MOF systems
DA MOFs can be constructed using the donor and acceptor units as part of the frame or as
guests - a typical configuration for organic DA cocrystal stacking. The DA MOFs can generally
be classified according to their structural feature. In general, DA MOFs could be classified
according to their structural features [104], such as (1) DA MOFs based on host–guest
structures (Fig. 2.4 (a)) [105,106], (2) DA MOFs based on donor–acceptor type ligands (Fig.
1.12 (b)) [107], (3) DA MOFs based on metal ion connected DA systems (Fig. 1.12 (c)) [108]and (4) DA MOFs featuring interlocked structures (Fig. 2.4 (d)) [109].

29

(a)

(b)

(c)

(d)

Fig. 1.12: (a) DA-based MOF with the host framework as the acceptor and guest as the donor.
(b) DA system in MOFs with ligands containing donor and acceptor moieties. (c) Two different
forms of DA MOF with donors and acceptors connected by metal ions. (d) DA MOF with A
as the main framework and D as an interlocked moiety [104].

1.3.4.2 Properties of DA-type MOFs
The interaction between the donor and acceptor results in CT and ET, determining materials'
optical and electrical properties. Renewal of optical and electrical properties is also essential
to developing industrial optical and electrical devices; therefore, studying the interaction
between the donor and acceptor is crucial. It will also help us progress in our research as weare interested in exploring the ultrafast CT dynamics of DA-type MOF systems.
Optical properties of DA-type MOFs: As previously mentioned, in the DA system, the precise
distance between the donor and acceptor and the proper degree of electron cloud overlap
between the donor and acceptor may promote charge transfer and energy transfer. Reviews and30

articles have discussed the energy transfer mechanism and occurrence conditions [110,111].
Energy transfer can result in luminescence enhancement or quenching [112,113], which isuseful for detection and optical switching.

Electrical properties of DA-based MOFs: Charge separation from the charge-localization effectof the donor or acceptor in the ground state and the CT interaction between the donor andacceptor moieties provide the foundation for DA-based MOFs to accomplish the electricalcharacteristics. The arrangement of the components is essential to the charge transport
properties of DA MOFs because the donor and acceptor components in the DA system canform charge transport channels under radiation. Electrons and holes flow through the channels
and ligands, which could provide the electric current path and unfold various semiconductormaterial properties. The merits of using DA MOFs to obtain electrical properties are repottedby many research groups [114,115].

In addition to the optical and electrical properties described above, DA MOFs with CT or ET
interaction may exhibit properties such as magnetism [116] and photothermal conversion [117].

31

1.3.5 Short description of flexible MOFs
The MOFs can be classified into two categories; one is rigid MOF, and the other is flexible
MOF. Rigid MOFs are just regular MOFs with porous frames and stiff backbones that arecomparably stable and strong. Whereas flexible MOFs combine cooperative structuraltransformability with the crystalline order of the underlying coordination network. Thesematerials have adjustable molecular design responses to various physical and chemical stimuli,
which is not true for any known solid-state materials (such as rigid MOFs). Here, an easy-tounderstand example of a rigid and flexible MOF—MIL-101(Cr) [rigid] and MIL-53(Cr)[flexible]—is addressed to show how these two MOFs differ when compared to one another interms of their chemical and physical characteristics [118, 119]. In this article, their absorption
behavior with clofibric acid and carbamazepine, which are contaminants in an aqueous medium,was considered. The experimental results (Powder-XRD pattern) showed that these two exhibit
distinct adsorption behaviors due to structural variations. The result suggests that flexible
MOFs are more effective than rigid MOFs in adsorption [120]. This idea tells us that the
preparation and development of flexible MOFs can be a suitable candidate for application
fields such as separation, catalyst, sensing, and biomedicine compared to rigid MOFs.

32

1.3.6 Photoinduced dynamics in MOFs with time-resolved spectroscopy
Time-resolved spectroscopy plays an essential role in fundamental and applied chemical
research. On-time scales ranging from femtoseconds to microseconds, these techniques canaccess light-initiated dynamics. As for my research topic, I am interested in observing thephotoinduced dynamics of MOF material using femtosecond time-resolved spectroscopy,
which will be discussed later. Hence, it will be necessary to know about the photoinduced
dynamics of reported MOF systems. The main idea of these studies is to illustrate how lightirradiation influences the electronic structure of MOF systems. Several primary research
related to exciton transport in the framework via energy transfer, photoinduced dynamics,
charge transfer, localization, photoconductivity, and structural dynamics are reported using
time-resolved spectroscopy [121]. Some examples for studying MOF properties using timeresolved spectroscopy are discussed here.

1) In the research field of exciton transport via energy transfer, Morris and Lin groups haveextensively studied energy transfer in Ru-chromophore doped zirconium(IV)-based
metal−organic framework (Zr-MOF), UIO-67 [122,123]. They reported that When Ru-dcbpy(bipyridine dicarboxylate) is used as a linker in UIO-67, an energy transfer pathway takes
precedence over a charge transfer pathway. Upon photoexcitation, the Ru-dcbpy structuralunits of UIO-67 interact with one another and undergo self-quenching via a resonance energy
transfer mechanism [124]. Fluorescence quenching was detected using a time-correlated single
photon counting approach in which Ru-dcbpy doped UIO-67 was suspended and agitated in
DMF before being excited selectively with 510 nm. The energy transfer was impacted by wherechromophores are positioned within the MOF. With increasing chromophore dopant
concentration, the emission lifetime and maxima size decrease, as shown in Fig. 1.13. Laser
photolysis experiments were conducted to measure the performance of Ru-dcbpy-UIO-67
MOF in DMF when excited at 355 nm. The results revealed a difference in emission lifetime

33

between low loading (20−30 μs) and high loading (100 μs), which can be attributed to energy
transfer occurring among the neighboring Ru-dcbpy chromophores within the octahedral
cavity of the MOF (Fig. 1.13). This suggests the importance of time-resolved spectroscopy inthe ﬁeld of energy transfer processes in MOFs.

Fig. 1.13: (upper panel) Tris(bipyridine)ruthenium (II) (Ru(bpy)3) incorporated into 3D MOF,
(lower panel) Emission lifetime decays (black) of Ru-dcbpy-UIO-67 samples at (a) 4 mm
(millimole of Ru-dcbpy per kilogram of MOF) and (b) 141 mm and photoexcited with a pulsed
LED at 510 nm and measured by time-correlated single photon counting. Corresponding
34

exponential fits (red) are done by a deconvolution/convolution of the instrument response
function (gray). The deviation from mono-exponential at higher Ru-dcbpy concentration isindicative of two populations of Ru-dcbpy forming. The long-lifetime component of Rudcbpy-UIO-67 at both chromophore loadings above was attributed to the Ru-dcbpy
incorporated into the backbone of the material. In contrast, the short-lifetime component athigh loading was attributed to encapsulated Ru-dcbpy chromophores [124].
2) Photocatalysis is a process in which photons serve as the source of energy, or voltage, tofacilitate a chemical reaction. Out of multiple reports, the following example illustrates acomprehensive compilation of MOFs, where time-resolved spectroscopy has contributed to aclearer understanding of their photocatalytic mechanism. Xu and colleagues studied PCN-222,
a free-base porphyrin-based MOF containing Zr nodes [125]. Their research demonstrated that
this material is effective for photocatalytic CO2 reduction in acetonitrile, using triethanolamine
as a sacrificial electron donor. The photocatalytic efficacy was investigated for PCN-222 (PCN:
porous coordination network) using time-resolved spectroscopy with the MOF suspended in
DMF (Dimethylformamide). From the time-resolved photoluminescence dynamics (Fig.
1.14(b)), the fs- transient absorption spectrum (Fig. 1.14(c)) and the kinetics of ground statebleach feature (Fig. 1.14(d) ) agree with that the observation of long-lived components with alifetime of >> 1 ns which the authors assign as a non-emissive electron trap state. The trap stateis most likely centered on the structure's Zr nodes, which are involved in photocatalytic CO 2
reduction.

35

Fig. 1.14: (a) Photocatalytic experiments with PCN-222 MOF or H2TCPP ligand in
ACN/TEOA. Controls excluded either PCN-222, TEOA, or CO2. (b) Time-resolved
photoluminescence of PCN-222 in DMF at 712 nm under 515 nm photoexcitation with
biexponential fit. (c) fs-TA spectra of PCN-222 under 500 nm excitation. (d) fs-TA kinetics ofthe ground-state bleach feature in (c) and triexponential fit [125].

3) The structure of MOF materials can be deformable or altered by thermal fluctuations andexternal pressure. Thus, another important application of time-resolved techniques is to study
the photoinduced dynamics of structural information in MOFs. One example is Nishida et al.,
who conducted the first time-resolved 2D-IR studies to investigate structural dynamics in the
Zr-based UiO-66 MOF.

36

Indeed, photoexcitation can lead to various fascinating situations, such as simultaneously
photoinduced local energy and charge transfer between D and A ions, photoconductive
phenomena, and the photothermal effect on relatively slow timescales. Regrettably, ultrafast
photocontrol of physical properties based on the cooperative interactions in MOF systems has
rarely been reported. Nevertheless, since DA-type MOFs exhibiting CT-type phase transitions
have recently become much more popular, some MOFs may be ideal for observing fast photoresponses of the electronic states. This is the main reason for starting the ultrafast dynamics
study of MOF research reported in this thesis.
1.4 Motivation of the present study
The research backgrounds and exciting concepts of PIPT in ultrafast timescales always
influenced my mind to pursue my research career in the ultrafast science of phase transition.
Ultrafast science and its development are also fascinating from an application perspective, suchas high-speed memory devices and storage systems, which create high-speed and efficient
supercomputers. Meanwhile, CT-type DA systems are usually a molecular material with a lowdimensional, electron-correlated lattice, as defined by anisotropically directed electronic
correlations or chemical bonds between the constituent molecules/ions. The low
dimensionality of such an electron-correlated lattice may induce several kinds of instability
associated with the electrons, lattice, and spin. These are key factors for achieving an efficientphoto-response because of the inherently cooperative electron–electron, electron–lattice, andmagnetic interactions that are strongly expected. In a general sense, studying the photo
response of such systems is important for the application and development of optical functional
devices or materials. Due to their inherent complexity, excited states or relaxation dynamics
are still unclear, although they provide information essential to understanding switching (andPIPT).

37

The present study aimed to perform ultrafast photoinduced dynamics for a 2D layered systemsince no detailed and extensive photophysical study has been achieved yet at ultrafast timescale.
Therefore, this study will introduce a new class of photo-switching material to PIPT scienceand enhance science and technology fundamentals.

We focused/chose a DA-type layered MOF (Metal-organic Framework) system(NPr4)2[Fe2(Cl2An)3] (where NPr4+ = tetra-n-propylammonium and Cl 2An2– = 2,5-dichloro3,6-dihydroxo-1,4-benzoquinonate) [126] to satisfy our intention. Interestingly, this compoundshows a two-step CT-type phase transition, with the steps occurring at 317 K [T1/2(1)] and 354
K [T1/2(2)]. The driving mechanism for these phase transitions has been discussed from theviewpoint of multi-stability, reflecting the balance between the valence change and the
Coulomb interaction, as in an N–I phase-transition system [113, 127] However, a robust
framework structure without appreciable lattice deformation—like the dimerization in TTF–
CA—even at each T1/2 is quite fascinating for investigating ultrafast photoinduced CT-typephase changes.

38

39

Chapter 2

2. Target Material: (NPr4)2[Fe2(Cl2An)3]

Few molecular compounds can alter their charge states multiple times in response to external
stimuli. In the present study, I primarily focused on the DA-type layered MOF(NPr4)2[Fe2(Cl2An)3] (where NPr4+ = tetra-n-propylammonium and Cl 2An2– = 2,5-dichloro3,6-dihydroxo-1,4-benzoquinonate [126], Which shows two-types of phases switching in its
own thermally driven electron transfer (TDET) process, and considered to be the first material
to exhibit such behavior. During my doctoral course research, I am interested in observing theultrafast photoinduced response from this material. In this context, the material synthesis,
crystallographic structure, and physiochemical properties will be discussed in the following
sections.
2.1 Sample preparation
The target material is synthesized in Prof. Miyasaka's Laboratory at Tohoku University. The
material is prepared from the pristine solvated compound (NPr 4)2[Fe2(Cl2An)3].2(acetone).
H2O. Prof. Miyasaka and coworkers reported the detailed preparation methods for this solvatedsample [127]. The single crystal samples of (NPr4)2[Fe2(Cl2An)3].2(acetone). H2O were madeby slowly diffusing the material in a glass tube with a narrow inner diameter (𝜙 = 8 mm).
Water (2 mL) containing FeCl 2·4H2O (7.95 mg, 0.04 mmol), NPr4Br (42.60 mg, 0.16 mmol)and CH3COOLi (10.56 mg, 0.04 mmol) was placed in the tube (bottom layer), then a mixtureof acetone (0.5 mL) and water (0.5 mL) was carefully placed on the water layer (buffer layer).
Finally, acetone (2 mL) of chloranilic acid (12.54 mg, 0.06 mmol) was placed on the top (toplayer). After 3 weeks, the black hexagonal prismatic crystals were collected from the mother40

liquid at a 37% yield. For the present sample, we synthesized crystals of solvent-free(NPr4)2[Fe2(Cl2An)3] by following the method reported in ref. [63]. However, the assynthesizedpristinesamples

containedsomecrystallization

solvents,

such

as

(NPr4)2[Fe2(Cl2An)3]·2(acetone)·H2O [127]. We removed these crystallization solvents byevacuating the crystal samples for 12 hours at room temperature to produce the solvent-freecompound (NPr4)2[Fe2(Cl2An)3].
2.2 Structural overview
Single-crystal X-ray diffraction (SC-XRD) result at 103 K revealed a 2D honeycomb-layered
structure similar to previous solvated material, (NPr4)2[Fe2(Cl2An)3].2(acetone).H2O. The
material crystallized in space group P21/n with z =2, half of the formula unit is an asymmetricunit of (1 × Fen+, 1.5 × Cl2Anm- (LA × 1, LB × 0.5), and 1 × NPr4+; as shown in Fig. 2.1 (a)with two-fold axes through the Fen+ and the midpoint of the LB Cl2Anm- ligand.

41

Fig. 2.1: Crystal structure of 1-d at 103 K. (a) Structure of the formula unit [Fe 2(Cl2An)3]. (b)Projection of hexagonal honeycomb networks along the (10-1) direction, where Fe, C, N, O,
and Cl atoms are colored in orange octahedra, grey, blue, red, and green, respectively, andhydrogen atoms are omitted for clarity. (c) Projection of layer structures along the b-axis [126].

For simplicity, here, Fig. 2.2(a) is shown, which represents a schematic illustration of thestructure of the 2D honeycomb layer of [Fe2(Cl2An)3]2–. The NPr4+ cations are placed between
the 2D honeycomb layers but omitted from this figure for clarity.

42

(a)

(b)
Fe3+ (S = 5/2)
Fe2.5+
Fe2+ (S = 2)C
Cl

Cl2An3– (S = 1/2)
Cl2An2.5–
Cl2An2– (S = 0)

Delocalized state(IMd)
T1/2(2) (354 K)
T1/2(1) (317 K)

Fe
O

Or
LT phase

HT phasea+c(|| chain)b(^ chain)

Cl2An2–

Ordered state(IMo)
IM phase

Fig. 2.2: (a) Structure of the D2A3-type honeycomb layer in (NPr4)2[Fe2(Cl2An)3] (where D =donor and A = acceptor). The portion surrounded by the thick dashed line is schematicallyillustrated in panel b. (b) Schematic charge and spin patterns in the low-temperature (LT),intermediate-temperature (IM), and high-temperature (HT) phases. Two possibilities areproposed for the IM phase, a delocalized state (IMd) and an ordered state (IMo).[126] The Feions are depicted as colored circles, and thick colored lines represent the Cl2An ions. The
arrows inside the circles and lines represent the spins [128].

43

2.3 Charge transfer (CT) type phase transition
Interestingly, this compound shows a two-step CT-type phase transition, with the steps
occurring at 317 K [T1/2(1)] and 354 K [T1/2(2)]. The phases distinguished by T1/2(1) and T1/2(2) are
named the low-temperature phase (LT phase), an intermediate phase (IM phase), and the hightemperature phase (HT phase) (Fig. 2.2 (b)). The IM phase may take the form of either a chargedisproportionate ordered state (IMo) or a delocalized state (IMd), as shown in Fig. 2.2 (b). The
charge and spin patterns in the honeycomb layer of each phase have been confirmed bymagnetic, structural, and Mössbauer spectroscopic measurements. This suggests the charge
distribution of three charge-ordered states are LT phase: [(Fe 3+)2(Cl2An2-)(Cl2An.3-)2]2-; IMphase:[(Fe2.5+)2(Cl2An2-)( Cl2An2.5-)2]2-; and HT phase: [(Fe 2+)2(Cl2An2-)3]2- that varies

according to temperature. The temperature dependence of Mm of 1-d was investigated for the
HT region of 300–400 K.
From magnetic investigation, the 𝜒m vs. T curve clearly shows two steps as the temperature isvaried, as shown in Fig. 2.3 (left panel). The 57Fe Mössbauer spectra for the respective phases

Fig. 2.3: (left figure) Temperature dependence of 𝜒𝑚 T and 𝜒𝑚for(NPr4)2[Fe2(Cl2An)3]

measured at Hdc = 1 kOe continuously between 300 K and 400 K in initial heating (red) andpost-cooling (blue) processes, where the LT, IM, and HT phases are displayed as red, green,44

and blue areas, respectively. (Right figure) Charge variations of the core unit and 57Fe
Mössbauer spectra of 1-d in LT at 300 K (a and b), IMo at 335 K (c and d), and HT at 380 K (eand f) [126].

are represented in Fig. 2.4 (right panel). These suggest the two-step phase transition with charge
and spin pattern in this material.
Despite its honeycomb layer structure, the charge alignment of the LT phase consists of quasi1D chains of Fe3+ and Cl2An3– along the a+c direction, which are connected with neighboring
chains by the diamagnetic Cl 2An2– (S = 0) ions. Since the Fe 3+ (S = 5/2) and Cl2An3– (S = 1/2)
ions are antiferromagnetically coupled along the chain, these ferrimagnetic chains aremagnetically isolated; this magnetic structure in the LT phase is confirmed to be a single-chain
magnet (SCM).[40,41] The IM phase appears upon increasing the temperature above T1/2(1), andthe HT phase appears above T1/2(2) (Fig. 2.2 (b)). The IM phase may take the form of either acharge-disproportionate ordered state (IMo) or a delocalized state (IMd), with a 1:1 ratio of Fe2+
to Fe3+ and a 1:1 ratio of Cl2An2– to Cl2An3– in the chain. In contrast, the HT phase consists
solely of a set of Fe2+ and Cl2An2– ions, i.e., it is a paramagnetic phase. The CT-type phasetransition thus corresponds to a phase transition from an SCM-related charge alignment to aparamagnetic charge alignment. The driving mechanism for these phase transitions has been
discussed from the viewpoint of multi-stability, reflecting the balance between the valencechange and the Coulomb interaction, as in an N–I phase-transition system. However, a robust
framework structure without appreciable lattice deformation—like the dimerization in TTF–
CA—even at each T1/2 is quite fascinating for investigating ultrafast photoinduced CT-typephase changes.

45

In short, the first half of this chapter represents a brief understanding of MOF systems fromstructural, physical, and application points of view. Then we discuss the essential informationfor our target material, (NPr4)2[Fe2(Cl2An)3]. The present material is the first case of TDET
materials to show multiple phases of switching in its own TDET process. The transformation
between these accessible states, composed of three distinct charge-ordered states (LT, IMo, andHT), was successfully realized. In this research, we will utilize this material to perform timeresolve measurements in fs-timescale to achieve fast and effective photo-switching.
2.4 Research objective and expected outcomes
As a primary target material for this research, (NPr4)2[Fe2(Cl2An)3], a promising material thatexhibits a two-step charge-transfer (CT) phase transition (Tc = 317 and 354 K) at which thevalence of Fe and Cl2An changes. It should be noted that there is no apparent structural change
at each Tc, probably due to the rigid bond nature between Fe and Cl 2An molecules. Therefore,
the phase transition of this system can be viewed as purely electronic. Then, an efficientphotoinduced CT phase transition can be expected without lattice structural deformation by
weak photoexcitation. To make our imagination real, we decided to perform the Photoinduced
dynamics in the charge transfer type phase transition system, (NPr 4)2[Fe2(Cl2An)3], using fstime-resolved spectroscopy. First, we decided to perform experiments at room temperature(RT), allowing us to excite the LT phase. Then, we will try to excite the system at hightemperatures over the T1/2(2): 354 K.
As an expected outcome, we can expect to observe:
1. An efficient photo-induced CT (Cl2An3- → Fe3+) can be expected when we excite LT phase.
2. Photo induced reverse CT (Fe 2+→ Cl2An2-) +) can be expected when we excite HT phase.
To study PIPT in (NPr4)2[Fe2(Cl2An)3], we began our experimental journey with these two
specific motivations.

46

47

Chapter 3

3. Experimental Methods
The experimental methods we employed to fulfill our research motivation are discussed in thischapter. Initially, we spoke about how to choose a sample for measurements. Next, wedemonstrated the main instruments and equipment for detecting static optical spectra. We also
discussed the overview of time-resolved pump-probe spectroscopy, which was used to follow
the moment-to-moment changes in electronic states upon photoexcitation. Pump-probe
spectroscopy is a technique to measure the optical properties (reflectance, transmittance, andpolarizability) of a material induced by excitation (pump) light irradiation.
3.1 Sample selection and method of fixation
A single crystal sample of (NPr4)2[Fe2(Cl2An)3], which was provided by Prof. Miyasaka'sLaboratory at Tohoku University, is used as our target material for this research. The hexagonal
shape of the crystals is the most desirable and flawless. Also, clean, shiny surface samples are
required to measure reflection from the sample surface precisely. Steady-state opticalmeasurement, especially reflectance measurement, was performed to determine electronic
structures quickly and precisely. Unfortunately, most samples are broken, tiny, and have a dullor cracked surface. To use the samples for reflectance measurement,(1) The crystal size must be sufficiently larger than the measurement light irradiation size ofthe microscope device used (at least one side is 100 μm or more).
(2) The crystal surface must be smooth and reflective without cracks or chips. The light mustn't
be scattered. In addition, since the crystal is fragile and small, it is impossible to polish it to amirror surface. So, measuring the reflectance using the crystal growth plane is necessary.

48

(3) To distinguish a good sample, it is necessary that the anisotropy of reflectance due topolarization can be observed and that the spectrum's shape is reproducible.
First, we chose the sample visually using a stereomicroscope, and the picture of some selected
samples is shown in Fig. 3.2. We fixed the samples on silver paste and used FT-IR spectroscopy
to measure their reflectivity spectrum.

Fig. 3.1: The picture of the samples we choose visually using a stereomicroscope. It shows thehexagonal shape of the single crystal.

The main idea for such experiments is to identify the anisotropy of the samples. We took many
crystals, measured them, and then, after confirming that the results were repeatable, we chosethe best samples.
The single crystal sample was fixed to a copper plate with silver paste for optical measurements.
Fig. 3.2 (right) shows a schematic diagram of how to fix the sample. To perform optical
spectrum measurement using linearly polarized light, the relationship between the optical axis
direction of the sample and the polarization of measurement light and excitation light isimportant. Because the crystals are so small, it is difficult to visually determine the orientationof the sample and align it with the direction of polarized light during measurement. Therefore,
the sample was first fixed on a small square copper plate (approximately 5 mm × 5 mm) sothat the orientation of the sample could be determined visually.

49

Fig. 3.2: (left)The picture of the selected fixed sample over a large copper plate (20 mm × 20mm). (right) schematics of the sample fixation process.

Then, the crystal, whose orientation was determined in this way, was fixed together with asmall copper plate to a large copper plate (20 mm × 20 mm) (Fig. 3.3) for attaching the sample
holder of the cryostat, taking into consideration the polarization direction during measurement.
Silver paste and copper plates are used to fix the sample to achieve sufficient thermal
equilibrium with the cryostat, whose temperature is controlled by a temperature regulator whenmeasuring temperature dependence, and to align the sample temperature with the temperatureof the sample holder. This is because high thermal conductivity is required. In particular, since
the measurements were to be conducted at temperatures higher than room temperature, wechose silver paste because it is an adhesive that can withstand high temperatures. These samples
were used for static optical spectra and time-resolved pump-probe measurements.

3.2 Determination of crystal anisotropy and optic axis
A detailed description of the actual method of determining crystal anisotropy and optic axis isgiven in this section. The typical shape of a single crystal sample of (NPr4)2[Fe2(Cl2An)3] is a50

hexagonal prism. The hexagonal portion of the sample is used for reflectance measurement.
The shape of the hexagonal prism corresponds to the microscopic crystal structure shown in
Fig. 3.3, which was obtained by X-ray crystal structure analysis. Fig. 3.3 shows the structureof the crystal plane that appears in the hexagonal part, which is a plane containing a twodimensional network of Fe2(Cl2An)3. Therefore, this plane is the most suitable for studying theelectronic structure in the two-dimensional network. The solid red line in the figure shows themacroscopic outline of the sample.
The mirror index of each plane is also shown in Fig. 3.3, and the plane used for thismeasurement has a mirror index of (10-1) or (-101). In other words, this surface contains theb-axis and the direction of the sum of the unit lattice vectors of the a-axis and b-axis. Theintersection direction between this plane and the (0-10) (010) plane, which is the side of thehexagonal prism, is the chain direction of the one-dimensional chain magnet that appears in
the low-temperature phase described above. As the crystal orientation is shown in the figure,the direction of this one-dimensional chain magnet is orthogonal to the b-axis. Therefore, thesuitable direction for measuring the anisotropy of the optical spectrum is the direction of the
one-dimensional chain magnet and the b-axis are orthogonal to the one-dimensional chain
magnet.

Fig. 3.3: (NPr4)2[Fe2(Cl2An)3] Microstructure outline and Miller index of the crystal growth51

plane used for the reflectance measurement of crystal. The macroscopic crystal outline reflects
this microstructure and is shown by the red line in the figure [126].

The measurement plane is hexagonal. The four crystal structures (1-21), (-12-1), (121), and (1-2-1) planes, and the two planes (0-10) and (010) are equivalent to each other. On the other
hand, the (10-1) plane, the hexagonal plane, has three different edges in three different
directions. Therefore, when linearly polarized light is applied parallel to the direction of thesides of the hexagonal plane and the spectrum is measured in three directions, opticalanisotropy should be observed in the two directions of intersection between the (1-21), (-121), (121), and (-1-2-1) planes and in one direction of intersection between the (0-10) and (010)
planes.
It gives the idea that the axis that will show a different reflectance spectrum from the other twoaxes is the chain direction of the one-dimensional chain magnet. In Fig. 3.3, if we use thedirection of an electric field, E, and denote it as 𝐸 ∥ chain or same as 𝐸 ⊥ b.
Here, it should be noted that when the solvent remains in the crystal of this sample, the phasetransition temperature decreases significantly, and a low-temperature phase does not appear atroom temperature but an intermediate phase in which the one-dimensional magnetic chainsdisappear [9]. Therefore, before all measurements were performed in this study, we firstmeasured and evaluated the infrared spectra of the crystals used for confirmation. Crystals withsmall spectral anisotropy were found to be due to poor crystallinity of the sample or thepresence of solvents. Since it remained in the sample, it was not used because it did not have a
low-temperature phase at room temperature, and only crystals with remarkable anisotropy wereselected and used for optical measurements.

52

3.3 Static reflectivity spectra measurement
3.3.1 The principle and types of reflection of light

For solid-state material, a sample's reflectance is determined by shining light on it andmeasuring the light reflected. There are two types of reflected light: specular reflected lightand diffuse reflected light, referred to as the total reflected light when combined (specularreflected light plus diffuse reflected light). A diagram of all types of light is shown in Fig. 3.4.
An incident light shines on a sample, and an angle formed between it and the sample is anincidence angle (θ). The incident angle for our reflectance spectra measurements is assumed to
be close to zero as shown in Fig. 3.4.
Relative or absolute reflectance is measured in reflectance measurements. Relative reflectancemeasurements determine how much light is reflected from a sample surface concerning how
much light is reflected from a reference plate, such as a mirror or barium sulfate. We assume
the reference plate has a 100% reflectance to determine the relative reflectance.

Amount of Light Reflected from the sample

Relative reflectance (%) = Amount of Light Reflected from the reference plate × 100

Instead of utilizing a reference plate-like barium sulfate or a mirror, absolute reflectancemeasurements determine the proportionate quantity of reflected light compared to the amountof light measured directly from a light source.

Absolute reflectance (%) =Amount of Light Reflected from the sample
Amount of Light used

× 100

53

Fig. 3.4: The illustration of different types of reflection phenomena from sample.
Here, we measured the relative reflectance of our target material using a gold (Au) mirror as areference. The sample preparation and experimental procedure are discussed below.

3.3.2 Instruments used for static reflectivity measurements
We measured static optical-reflectivity spectra of our sample, (NPr 4)2[Fe2(Cl2An)3], using acommercial Fourier-type infrared (FT-IR) spectrometer (Thermo 6700) with a Cassegrain
microscope for the mid-IR range (0.08–1.0 eV). We measured the reflectivity spectra in thenear-IR to the visible range (0.9–3.0 eV) using a homemade microscope-spectroscopy systemwith a diffraction-type monochromator (Bunkokeiki M25) and adequate photodiode detectors.
We measured the temperature-dependent static reflectivity spectra using a commercial cryostat
with BaF2 or quartz windows, Microstat He (Oxford Instrument).

54

3.4 Raman spectra measurement
3.4.1 Principle of Raman spectroscopy
The Raman Spectroscopy technique is a non-destructive method for analyzing the chemical
structure of a sample that provides detailed information about the sample's phase andpolymorphism, crystallinity, and molecular interactions. An interaction between light andchemical bonds within a material is the basis of this process. Raman spectra show Raman
scattered light's intensity and wavelength position through a number of peaks. It shows thevibration of molecular bonds and their various modes, such as C-C, C=C, N-O, and C-H, andthe breathing mode of the benzene ring (for example), polymer chain vibrations, or lattice
vibrations.

Fig. 3.5: Scheme and Jablonski diagram illustrating Raman scattering and Rayleigh scattering[129].

55

The inelastic component of light scattering produced when incoming light interacts withmolecules is known as Raman spectroscopy. It can be separated from Rayleigh-type elastic
scattering as shown in Fig. 3.5. The energy intake or give-off produced by the photon's contact
with molecules is represented by the changing frequency between incident (ω) and scatteredphotons (ωυ) in Raman scattering. The molecules' internal energy is modified due to theseinteractions (usually due to vibrational transitions). When it comes to photon energy loss during
the scattering process, the red-shifted frequency (ω – ωυ) is described as the Stokes shift,whereas the blue-shifted frequency (ω + ωυ) is characterized as the anti-Stokes shift andcorrelates to a photon energy gain. Because most molecules at ambient temperature are in the
ground electronic and vibrational states, the Stokes shift often dominates Raman scattering.

3.4.2 Instrument used for Raman spectra measurement
We measured the Raman spectra using a JASCO NRS-4500. To perform the experiment, thesingle-crystal sample, (NPr4)2[Fe2(Cl2An)3], was placed on a copper plate, and a film heater
under it controlled its temperature. The sample was irradiated with a 532 nm Raman excitation
laser. We measured the Raman spectra with the polarization directions of the analyzer oriented
in the x- and y-directions of the incident and scattered light.

56

3.5 Ultrafast time-resolved spectroscopy
3.5.1 Background: Overview of the timescale for various molecular processes
To fully understand any physical or chemical process during phase transition, it is essential that
we observe not only the static states but also the evolution of intermediate structures along
reaction coordinates. However, due to the generic timescale of molecular processes, it isdifficult to trap and identify the dynamical processes during phase transitions (for example,
PIPT). Fig. 3.6 shows an overview of the timescales for various molecular processes. Now, theprimary focus of our study is on the lattice-phono interaction, charge transfer, structuralmodulations, atomic configuration changes that underlie bond creation and breaking, etc., allof which take place between 10-9 (nanosecond) and 10-15 s (femtosecond) time scales. Thus,
capturing those dynamic processes seems extremely difficult and even impossible in normal
circumstances. The advent of femtosecond laser systems and ultrafast optical spectroscopy has
made it easier to capture these dynamical processes.

Fig. 3.6: Typical time scales for structural and electronic processes in solids [130].

57

3.5.2 Time-resolved pump-probe spectroscopy
Pump-probe spectroscopy is the most useful and widely used experimental method for researchand development in ultrafast science. This method involves splitting an ultrashort laser pulse
into two beams: a weaker beam (probe) is used to track changes in the sample's opticalconstants (like transmission or reflectivity) caused by the pump, and a more robust beam(pump) is used to excite the material and create a non-equilibrium state. Information regarding
the relaxation of electronic states in the sample may be obtained by measuring the changes in
the optical constants as a function of the time delay (∆𝝉) between the arrival of the pump andprobe pulses. Fig. 3.7 shows the schematic representation of pump-probe spectroscopy.

Fig. 3.7: Schematic diagram of reflection pump-probe spectroscopy.
This study performed time-resolved spectroscopy reflectivity measurements using thefollowing two types of light sources.

58i) Wavelength-tunable 90 fs pulse in the wide energy range (0.1 – 3.0 eV)ii) A near-infrared 6 fs ultrashort pulse with a central wavelength of 1.7 μm that lasts only 1.3
cycles of electric field oscillation.

3.5.2.1 Optical setup for pump-probe spectroscopy with 90 fs pulse
Optical path for intramolecular transition and CT energy range: We measured thetransient reflectivity change (∆R/R) using a pump–probe technique. We used a Ti: sapphire
regenerative amplifier system (center wavelength, 792 nm; pulse width, 90 fs; repetition rate,
1 kHz; Spectra-Physics Solstice Ace) as the light source. We divided the optical-pulse output
from the amplifier system into pump and probe pulses using a beam-splitter plate. The pumppulse delivered from the optical parametric amplifier (OPA: Light conversion, TOPAS) was
either 0.73 or 0.8 eV, and we tuned the repetition rate to half of the fundamental frequency(500 Hz) using an optical chopper (New focus 3501). Using frequency-mixing processes, weconverted the probe pulse into the 0.4–3.0 eV energy range to cover intramolecular transition
and CT energy range of our target material, (NPr4)2[Fe2(Cl2An)3]. Here, we used the OPA
either in the range 0.5–0.7 eV (idler light) or the range 0.8–1.0 eV (signal light). We employedsecond-harmonic generation of the idler light in the range 1.1–1.5 eV, signal light in the range
1.6–2.1 eV, and fourth-harmonic generation of the idler light in the range 2.2–3.0 eV. The
average widths of the pump and probe pulses depend on their energies; a typical probe-pulse
width was 150 fs.

The schematic representation of the almost realistic pump-probe setup which we used in ourlaboratory to measure photoinduced reflectivity change (∆R/R) is shown in Fig. 3.8. Here, theoutput of the source pulsed laser is divided into two parts and enters into the OPA 1 and OPA
2. For our case, we use the OPA 2 as a pump (a fixed photon energies of 0.7 and 0.8 eV) that59

passes through a chopper, to tune the repetition rate to half of the fundamental frequency,
followed by a delay stage to hit the sample. At the same time, the probe light comes out from
OPA 1, for several probe photon energies within the range from 0.4 – 3.0 eV, hits the sample.
Then, the reflected light (pump-probe response) was detected by detectors. By using thetranslational stage, we can measure time–resolved data. The results for this measurement are
discussed in detail in chapter 5.

Fig. 3.8: Optical setup for reflection/transmission pump-probe spectroscopy using 90 fs pulses.
Experimental and optical setup for mid-IR energy range

The measurement in mid-IR range has been performed in two different ways:

1. Low energy resolved spectra (rough scan): For measurement in the low energy resolved
spectra (photoinduced ∆R/R spectra) in mid-IR range (0.1- 0.4 eV) we employed the same
optical set up as like in CT and intramolecular transition energy range. During thismeasurement the wavelength resolution is about 150 cm –1. The results for this measurement
are discussed in chapter 5.

60

2. High energy resolved spectra (precise scan): Here, we measured the ∆R/R spectra much
more precisely to obtain better insight into the spectral nature of the photoinduced state inthis range. The optical setup we used for the measurement in the mid-IR range is bit
different from the measurement during measuring low energy resolved spectra. The
schematic optical set is shown in Fig. 3.9. To measure time-resolved mid-IR ∆R/R (1200–
2000 cm−1), we detected the probe light reflected from the sample using the MCT detectors
after passing it through a monochromator (Bunkokeiki M10, the only difference from ourpreviously described setup) to obtain high-energy-resolution spectra with a wavelength
resolution of ~6 cm−1, so that the intramolecular vibration modes can be distinguished, evenat the expense of a temporal resolution degraded to the order of 1 ps. For the measurement
We employed differential frequency generation (DFG) between the signal and idler lightin the range 0.12–0.4 eV, which comes out from OPA 1 and used as a probe and the OPA
2 (specific photon energy of 0.8 eV) as a pump that passes through a chopper followed bya delay stage to hit the sample. Now, to measure a wide energy range of mid-IR ∆R/R
(1200–2000 cm−1) spectra, we first select a specific probe energy in DFG energy range (0.1
– 0.4 eV) and measure the broad band pump-probe response from the sample using themonochromator. Then we repeated the same method as discussed above by only changing
probe energies to cover such a wide energy range of photoinduced ∆R/R spectra.

Laser output

monochromator

detector

Fig. 3.9: Optical setup for reflection pump-probe measurement in mid-IR energy range.
61

Detection of pump-probe signal: We measured ∆R/R using MCT (HgCdTe) (0.1–0.5 eV),
InGaAs (0.51–1.1 eV), Si (1.1–2.5 eV), and GaP (2.5–3.0 eV) detectors in the photon-energy
ranges of the probe light. We collected the signals from the detectors using a gated integrator(Stanford research, SR250) and an analog-to-digital converter (NI, NI-6143), which we
synchronized with the frequencies of the optical chopper and the Ti: sapphire regenerative
amplifier. The samples were maintained at RT, with both pump and probe pulses entering thesample at near-normal incidence. To measure time-resolved mid-IR ∆R/R (1200–2000 cm−1),
we detected the probe light reflected from the sample using the MCT detectors after passing it
through a monochromator (Bunkokeiki M10). The energy resolution of this system wasapproximately 6 cm−1.

The raw signal detected is the reflection/transmission intensity of the probe light. The
conversion from the detected signal to reflectance change (ΔR/R) is performed as follows. Let
𝑅 and 𝑅′ denote the reflectance without and with excitation light, respectively. Then,

𝑅=

𝐼𝑅
𝐼0

𝑅′ = 𝑅 + ∆𝑅 =𝐼′𝑅
𝐼0

where 𝐼0 represents the intensity of the incoming probe light, and 𝐼𝑅 and 𝐼’𝑅 are the reflected
intensity of the probe light without and with excitation light, respectively. The change inreflectance Δ𝑅 due to photoexcitation is expressed by the following equation.

∆𝑅 = 𝑅′ − 𝑅 =𝐼′𝑅 − 𝐼𝑅 ∆𝐼𝑅=
𝐼0
𝐼0

Hence, as a whole from the above equations, we can write the value of relative reflectivity
change (∆R/R) as,

62

∆𝐼𝑅
∆𝑅
∆𝐼𝑅
𝐼
= 0 =
𝐼𝑅
𝑅
𝐼𝑅
𝐼0
The excitation light is irradiated to the sample at 500 Hz and thinned by a chopper synchronizedwith the trigger signal from the regenerative amplifier's Pockels driver. Since the probe light isirradiated to the sample at 1 kHz, the detected intensity of the probe light is 𝐼′𝑅 , 𝐼𝑅, 𝐼′𝑅 , 𝐼𝑅 ⋯.
From here, the operation (𝐼′𝑅 - 𝐼𝑅) ⁄𝐼𝑅 is performed in PC and converted to ΔR/R. By recording
this ΔR/R value together with 𝜏d obtained from the position information of the automatictranslation stage controlled by the PC, ΔR/R(𝜏d ) is obtained. Here, the determination of whether
the detected probe light is 𝐼𝑅 or 𝐼′𝑅 is made by detecting a portion of the excitation light pulse
after it is thinned by the chopper. The schematic representations for detecting and measuring
relative change in reflectance are shown in Fig. 3.10.

Fig. 3.10: Schematic diagram of detection signal processing in pump-probe spectroscopy using
90 fs pulse.

633.5.2.2 Near-infrared 6 fs pulse generation method and optical setup
6 fs pulse generation method
The time waveform and spectrum of an optical pulse are related to each other by a Fouriertransform, and an uncertainty relation (Δ𝑡 ⋅ Δ𝜔 ≥ 𝐾, where K is a constant that depends on thepulse shape) holds for pulse width (Δ𝑡) and spectrum width (Δ𝜔 𝜔). Therefore, it is essential(a necessary condition) that the spectrum be broadband (Δ𝜔 is large) to generate extremely
short pulses on the time axis (Δ𝑡 is small). The generation of near-infrared 6 fs ultrashort pulses
is performed by the following procedure [126].
(a) Generation of a broadband pulse in the near-infrared region (broadening of thespectrum)(b) Compression of the broadband pulse (correction of the group delay time)
Figure 3.11 shows the optical system for generating the near-infrared 6 fs pulse. In (a),surrounded by the blue dotted line, the spectrum is broadband; in (b), surrounded by the green
dotted line, the spectrum is compressed.

(a)

(b)

Fig. 3.11: Schematic and photograph of the optical system for 6 fs pulse generation: (a)broadening of the spectrum, (b) pulse compression technique [131, 132].
64

The details of the 6-fs pulse generation technique and method are reported in Ref [131]Measurement of pulse width by autocorrelation method
Figure 3.11 shows the autocorrelation (AC) waveforms of (a) the idler light immediately after
OPA emission and (b) the light pulse after pulse compression. The autocorrelation waveform
was measured by splitting the optical pulse into two, adding a delay time 𝜏 to one of them, andobserving the second harmonic generated by superposition on a β-BBO crystal (thickness of
10 μm). The intensity waveform of the optical pulse, 𝐼, was measured with the full width at
half maximum (pulse width) as Δ𝑡:
𝐼(𝑡) = 𝐼0 exp [− 4 𝑙𝑛 2 (

𝑡
)]
∆𝑡

Assume a Gaussian function like. The autocorrelation waveform 𝐺2 is expressed as a functionof delay time 𝜏∞

𝐺2 (𝜏) = ∫ 𝐼 (𝑡) 𝐼(𝑡 − 𝜏)𝑑𝑡−∞

The full width at half maximum Δ𝜏 of 𝐺2(𝜏) is √2 times the pulse width Δ𝑡 of the original
optical pulse. From this relationship, the pulse width Δ𝜏 of the optical pulse can be estimated65Fig. 3.12: Autocorrelation waveforms of (a) idler light immediately after OPA output and (b)optical pulse after pulse compression.

by measuring the autocorrelation waveform. The autocorrelation waveform Fig. 3.12(a) of theidler light immediately after OPA emission was Δ𝜏 ~ 60 fs, but the autocorrelation waveform
after pulse compression was Δ𝜏 = 9 fs (Fig. 3.12(b)). From this, it can be confirmed that thepulse width Δ𝑡 of the optical pulse after compression is 6 fs.

Optical setup of pump-probe spectroscopy with 6 fs pulse
Fig. 3.13 shows the optics of reflective pump-probe spectroscopy using a 6-fs pulse. After
afterpulse compression using a chirped mirror and a shape-variable mirror as described in theprevious section, the pulse is divided into excitation (red line) and probe (blue line) beams
using a 2-μm-thick pellicle beam splitter (BP145B3 from Thorlabs) to avoid pulse stretching
due to transmission through a dispersive medium. The excitation light is modulated by achopper operating at 500 Hz and then given a delay time by a translation auto stage (KST(GS)50X from Sigma Koki). The light intensity and polarization are then controlled by a wire-gridpolarizer pair (see below for details), and the light is focused onto the sample in the cryostat
by a 90° off-axis parabolic mirror (f = 200 mm). The window material of the cryostat is a 0.4-66

10 μm broadband, transparent BaF2 crystal (3 mm thick). As with the excitation light, the probe
light is focused onto the sample by a 90° off-axis parabolic mirror (f = 200 mm) after
controlling the light intensity and polarization with a wire grid polarizer pair. The light reflectedfrom the sample is collimated by a lens and detected by an InGaAs detector (New Focus Model2034) through a monochromator (Spectrometer M10). The detected signal is integrated by a
Boxcar integrator (Stanford Research Systems Model SR250) and collected by a PC via an AD
converter (National Instruments PCI-6036).

Cryostat

BoxcarIntegrator

SampleDetector

Splitter

Pulse compression
Wire grid
Polarizer

SystemPellicle BS

Parabolic Mirror

lens

Chopper
Delay
stage

Excitation light Monitor

Fig. 3.13: Optical setup for reflection pump-probe spectroscopy with 6 fs pulse

67

68

Chapter 4

4. Static Optical spectra of a single crystal of (NPr 4)2[Fe2(Cl2An)3]In this chapter, we described the results of static optical spectra measurements for our target
material, (NPr4)2[Fe2(Cl2An)3]. The topics of this chapter include the results of anisotropic
reflectivity spectra measurement. Also, we reported the results of temperature-dependent
reflectivity, optical conductivity, and Raman spectra measurements for both E ∥ chain and 𝐸 ⊥chain directions.
4.1 Objective and Motivation of this experiment
In this research topic, our main target is to observe the photo-response from our target material
DA-type MOF system, (NPr4)2[Fe2(Cl2An)3]. For DA-type systems, a charge-transfer (CT)type transition—which has the lowest energy of photoexcitation in the electron transfer
process—can be an efficient mechanism for controlling electron/spin functionality. Hence,
before performing the time-resolved experiment, we need to understand/know the energy gapof CT transition between D and A in our system, the nature of intramolecular vibration peaks(which are considered sensitive towards charge transfer), intramolecular transitions, etc. Here,the purpose of this work is to find an answer to the above requirements.
Hence, we decided to measure static reflectivity and Raman spectra. In Chapter 3, we discuss
the details of the experimental methods and the necessary instruments used for theseexperiments. The results will be discussed in this chapter.

69

4.2 Result and Discussion
4.2.1 Steady State Reflectivity spectra: Determination of crystal anisotropy and optic
axisFig. 4.1(a) shows the picture of the sample crystal used for reflectivity measurement (forexample, we selected visually using a stereomicroscope). Here, we can observe three visible
planes in the hexagonal single crystal. We performed reflectivity measurements in three
directions of our chosen hexagonal crystal to determine the proper direction of a onedimensional chain magnet.
Here, we measured the sample's reflectivity using linearly polarized light in the mid-IR range(0.08–1.0 eV) parallel to the A, B, and C axes, as shown in the image for the sample at RT. The
result of the measurement in the range (1000 – 2000 cm-1) suggests that the spectra for 𝐸 ∥A and C (electric field direction is denoted as E) are similar results, whereas the spectrum along
𝐸 ∥ B is different, as shown in Fig 4.1 (b).
From our expectation, as discussed in Chapter 3 (section 3.2), we indeed observed a clear anddistinguishable reflectivity spectrum for the different directions of the single crystal for the
chosen sample. Hence, we can say that 𝐸 ∥ A and C must be 𝐸 ⊥ chain or 𝐸 ∥ b axis direction.
Whereas 𝐸 ∥ B must be along the E ∥ chain or 𝐸 ⊥ b axis of the crystal. A large anisotropic
result can be observed. It suggests we can successfully determine the optic axis and the chain
direction of the one-dimensional chain magnet by reflectivity measurements. In other word
words, we can be able to distinguish the direction of the E ∥ chain and 𝐸 ⊥ chain in the single
crystal by optical reflectivity measurement.
Fig. 4.1 (c) and (d) show the reflectivity spectra for the E ∥ chain and 𝐸 ⊥ chain in the single
crystal in the range of 1000 – 2000 cm-1 and 1000 – 9000 cm-1, respectively.
To successfully control the material properties of (NPr4)2[Fe2(Cl2An)3] by light, we must selecta sample with large anisotropy like the demonstrated sample.

70

(a)

A
B
C
Sample

(b)

(c)

(d)

Fig. 4.1: Crystal with pronounced spectral anisotropy (a) picture of the single crystal of theselected sample. The silver area is the silver paste used to fix the sample. The bright square in
the center indicates the measurement range of infrared micro-spectroscopy. (b) IR reflectivity
spectra measured with polarizations parallel to the A, B, and C axes of (a). (c) and (d)Anisotropy spectra between E ∥ chain and 𝐸 ⊥ chain direction in the range of 1000 – 2000
cm-1 and 1000 – 9000 cm-1, respectively.

71

4.2.2 Anisotropy of optical spectra at room temperature (RT)
Anisotropy of reflectance spectra at RT
We obtained anisotropic reflectivity spectra of a single crystal of (NPr 4)2[Fe2(Cl2An)3] at RT(LT phase) using linearly polarized light in a wide energy range (0.1 - 3.0 eV), which are plottedin Fig. 4.2(a). The spectra reflect the anisotropic electronic structure of this crystal. We
observed a large difference between the spectrum with the polarization parallel to the 1D chain(the a+c axis, blue line) and perpendicular to the chain (the b axis, orange line).

(a)

(b)

Cl2An3Fe3+
LT phasea+c(|| chain)b(^ chain)

Fig. 4.2: (a) Steady-state reflectivity (R) spectra of a single crystal of (NPr4)2[Fe2(Cl2An)3]
polarized with E || chain (blue line) and E ⟘ chain (orange line) at room temperature (300 K:
RT). (b) Schematic charge and spin patterns in the low-temperature (LT) phase.

Anisotropy of optical conductivity spectra at RT
Reflectance spectrum measurement is suitable for evaluating the physical properties of singlecrystal samples because it is non-contact and requires relatively little sample preparation if thecrystal surface has a mirror surface. However, obtaining physical property parameters directlyfrom the spectra is difficult. The most commonly used analysis methods are either fitting
analysis using Lorentz oscillator or calculation of complex reflectance spectrum using Kramers

72Kronig transform, converted into intuitively easy-to-understand spectrum such as opticalconductivity spectrum representing light absorption. Here, we examine the latter method.
The details of the Kramers-Kronig transform method are written in Appendix A. However, it
is essential to note that the actual calculation requires reflectance spectra for the entire energy
range from 0 to infinity (∞). As a practical matter, it is impossible to prepare such a spectrum
experimentally, so a simple function extrapolation is used to replace the spectrum outside themeasurement range. Since the material is an insulator in the entire temperature range according
to the results of electrical resistivity measurements, etc., extrapolation on the low-energy side
was performed by extrapolating the lowest energy value of the measured reflectance spectrum
to 0 eV as a constant. In contrast, extrapolation at high energies is not straightforward. Usually,
extrapolation is done as a power of photon energy (𝜔-𝑛). Here, the power value 𝑛 is usually anarbitrary integer less than or equal to 4. This is because, according to the Lorentz oscillator
model often used in reflectance spectral analysis, the functional form of reflectance at energies
sufficiently higher than the resonance frequency is proportional to 𝜔-4.
Fig. 4.3 shows the optical-conductivity [()] spectra obtained from a Kramers–Kronig (KK)transformation of the reflectivity. We observed two absorption peaks at around 0.8 and 2.6 eV.
Assignment for these two optical transitions is discussed based on Quantum-chemical
calculations in the next section.

73

Fig. 4.3: Optical conductivity [()] spectra of the single crystal with E || chain (blue line) andE ⟘ chain (orange line) at RT.

The calculation of absorption spectra using time-dependent density functional theory(TD-DFT)
We performed the DFT calculations using Gaussian16 [133] with the B3LYP functional andemploying the LANL2DZ basis set for the Fe atoms and the 6-31G(d) basis set for the C, Cl,
and O atoms.
For the calculation of absorption spectra using time-dependent density functional theory (TDDFT), we utilized the structure of the D2A3-type honeycomb layer, Fe 2(Cl2An)3, as shown in
Fig. 4.4(a). The portion surrounded by the tick dashed line is magnified in Fig. 4.4(b) and used
to obtain the absorption spectra by quantum chemical calculation. The XYZ coordinates of theatoms in the cluster are from the crystallographic information file (CIF) for the target
compound (NPr4)2[Fe2(Cl2An)3] at 300K in Ref. [126] (Table in Appendix B). The time-74

dependent density functional theory (TD-DFT) calculation with B3LYP functional and
LANL2DZ for Fe atom, and 6-31G(d), for C, Cl, and O atoms, basis sets have been performed
using Gaussian 16 [133] without optimization of the structure of the cluster. Peak positions arewell reproduced when comparing the experimental absorption spectrum shown in Fig. 4.4(c)and the calculated spectrum in Fig. 4.4(d). It should be noted that the ratio between the numberof Fe and Cl2An ions in this cluster is 2:5, which is different from 2:3 in the target material.
Then, the ratio of the oscillator strength among each transition must be different from the actual
result. Only the peak energy should be discussed to assign the absorption peaks. The schematic
representation for the assignment of absorption peaks is shown in Fig.4.5(d) and (e). XYZ
coordinates of the cluster in Fig. 4.4(b) for TD-DFT calculation are provided in Appendix B.
Regrettably, the absorption spectrum for the HT phase could not be reproduced with the samelevel of theoretical calculation. Therefore, to understand the change of spectral weight of theintra-molecular transition by valence changes at T1/2(1) and T1/2(2), the absorption spectra forsingle molecules Cl2An3– and Cl2An2– (with B3LYP functional and 6-31G basis set) has been
calculated. The peak positions of the Cl2An3– and the Cl2An2– ions are almost the same as thosein the target material, as shown in Fig. 4.4(e). The spectral weight of the intra-molecularabsorption peak of the single ions shows the same tendency to the valence change of the Cl 2An
ions in the spectral weight in the experimental absorption spectra of the target material.

75

Fig. 4.4: (a) Structure of D2A3-type honeycomb-layer, Fe2(Cl2An)3. (b) The molecular cluster
is used for density functional theory (TD-DFT) calculation. (c) and (d) The comparison
between the LT phase's theoretical and experimental absorption spectra. (e) The theoreticalabsorption spectra for the isolated Cl2An3– and Cl2An2– ions.

Suppose we closely observe the electronic transition nature of ~ 0.8 eV peak (molecular orbital(MO) representation). In that case, the absorption peak assigns the CT transition between D
and A, as shown in Fig. 4.5(a), (b), and (c). Those three transitions are the major contributions
to the CT transition. However, among all the major transitions in the 0.8 eV peak, the transition
for HOMO – 1 (highest occupied MO) to LUMO + 1 (lowest unoccupied MO) (Fig. 4.5(b)) is
the primary one. The transition is not purely CT type; rather, we consider this transition as a
combination of ligand to metal (LMCT), band-like, or d-d type. Overall, the peak at around 0.876eV has the character of a CT transition between the Fe and Cl2An ions (Fig. 4.5(d)), while thepeak at around 2.6 eV is attributed to an intramolecular transition in the Cl 2An ions (Fig. 4.5(e)).

(a)

(c)

LUMO+3

LUMO+5

HOMO-3

HOMO-3(b)(d)
LUMO+1(e))

HOMO-1

2.6 eV: Intramolecular transition

Cl2An3–

Cl2An2–

Fig. 4.5: (a), (b), and (c) are the MO diagrams of major electronic transitions for ~ 0.8 eV peak77

in the optical spectrum along the E || chain in molecular clusters. (d) The schematic
representation of CT transition between D and A. (e) The intramolecular transition between
Cl2An ions.

4.2.3 Temperature-dependent optical spectra along E ||chain
The reflectance spectra of E || chain polarization were measured from 296 to 380 K over theentire measurable energy range. The results are shown in Fig. 4.6. Significant temperature
variations were observed.
We reported the () spectra with E || chain at 296, 340, and 380 K depicted in Fig. 4.7(a) are
typical of the LT, IM, and HT phases, respectively. We integrated the () spectra with E ||chain at each temperature in the photon-energy range 0.7–0.9 eV and 2.5–2.7 eV to determine
the spectral weights of the CT and intramolecular transitions, as shown in Fig.4.7(b) and Fig.
4.7(c), respectively. The spectral weight of each peak shows a steep drop at around each T1/2
due to the LT–IM and IM–HT phase transitions [the vertical dashed lines in panels (b) and (c)].
The variations of the spectral weights are consistent with the expected valence changes of the
Fe and Cl2An ions that accompany the two-step phase transition [126]. These integrated
intensities can, therefore, be utilized as suitable probes for detecting the degree of CT between
the Fe and Cl2An ions.

78Fig. 4.6: Temperature dependence of the reflectivity spectra at 296 (blue line), 340 (orangeline), and 380 K (green line) with E || chain.

79

(a)

(b)

(c)

Fig. 4.7: (a) Temperature dependence of the () spectra with E || chain. (b) Temperaturedependence of the integrated () between 0.7 and 0.9 eV. (c) Temperature Dependence of theintegrated () between 2.5 and 2.7 eV. The dashed lines represent the transition temperatures
T1/2(1) (317 K) and T1/2(2) (354 K) of the charge-transfer (CT)-type phase transitions based on
previous magnetic and structural studies [126].

80

4.2.4 Temperature-dependent optical spectra along E ⊥ chain
The reflectance spectra of E ⊥ chain polarization were measured from RT to 380 K within theenergy range of 0.08 – 1.4 eV. The results are shown in Figure 4 8. The difference changes
along the E⊥ chain are very small, but explaining the ultrafast photo-induced responses along
the E ⊥ chain is useful, which will be discussed later.

(a)

(b)

Fig. 4.8: Temperature dependence of (a) the reflectivity spectra and (b) () spectra along theE ⊥ chain.

81

4.2.5 Temperature dependent Raman spectra for both E || chain and E ⊥chain directions
Raman spectra of (NPr4)2[Fe2(Cl2An)3] were collected over the temperature range 293 – 380
K for both E || chain and E ⊥ chain directions. The result of Raman spectra without using ananalyzer of polarization is shown in Fig. 4.9 (a) and (b).

(a)

(b)

Fig. 4.9: Temperature-dependent Raman spectra along (a) E || chain and (b) E ⊥ chain
directions.

82

Also, we measured the Raman spectra over the temperature range 293 – 380 K using theanalyzer of polarization, in which 'xx' and 'xy' mean that the polarization directions of theanalyzer are respectively parallel and perpendicular to the polarization of the input light (E ||chain or E ⊥ chain). The Raman spectra at 293 K (LT phase), 340 K (IM phase), and 380 K(HT phase) are shown in Fig. 4.10.

(a)

(b)

cry

Fig. 4.10: Raman spectra at 293 K, 340 K, and 380 K with 'xx' and 'xy' polarization geometries
along (a) E || chain and (b) E ⊥ chain directions.

834.3 Summary
The results and observations from the static optical spectroscopy measurement are summarized
in this chapter. The CT transition between Fe and Cl 2An ions and intramolecular transitions
are well clarified using static optical conductivity spectrum along the E || chain, located at 0.8
eV and 2.6 eV, respectively. On the other hand, the LT–IM and IM–HT phase transitions causea sharp decline in the temperature-dependent optical conductivity spectrum throughout the E ||chain at about every T1/2 for the temperature ranges from RT to 380 K. This suggests that static
optical spectroscopic measurement can be an excellent probe for observing CT-type phasetransition in our sample, (NPr4)2[Fe2(Cl2An)3]. The temperature-dependent Raman spectrum
we recorded here will also assist in explaining the intriguing results of ultrafast time-resolved
spectroscopy (pulse width, 90 fs, and 6 fs), which will be covered in the upcoming chapters.

84

85

Chapter 5
5. Time-resolved spectroscopic study of (NPr 4)2[Fe2(Cl2An)3]: Exciting LT phase using
90 fs pulses
In this chapter, we discussed the ultrafast photoinduced dynamics in a DA-type layered MOF
that exhibits a charge-transfer (CT)-type phase transition: (NPr4)2[Fe2(Cl2An)3] at room
temperature (RT: 300K). We measured photoinduced transient spectra in intramoleculartransition, CT, and mid-IR energy range. Moreover, the ultrafast formation of new spectral
structures in the CT transition and the mid-IR energy ranges demonstrates, for the first time,
the appearance of a new, transient electronic state in the present MOF system, with a unique
lattice structure that is different from the one observed in thermal equilibrium, i.e., aphotoinduced hidden state.
5.1 Objective and Motivation of this experiment
From the published article in 2020 by Prof. Miyasaka's group, it is evident that our selected
sample, (NPr4)2[Fe2(Cl2An)3], exhibits a two-step CT type phase transition, Tc at around 317
and 354 K. The primary goal of this research is to control crystal functionality− based on theelectronic structural change using external stimuli, such as light. Commonly, after
photoexcitation processes, such as a charge transfer, chemical reaction, or photovoltaic, it often
exhibits subsequent dynamics within the attosecond (10 -18) to picosecond (10-12) timescale.

Here, our motivation is to observe the dynamics related to CT-type phase transition using
ultrafast femtosecond pump-probe spectroscopy with a time resolution of 90 fs. The detailed
procedure of measurement and experimental setup is discussed in Chapter 3 (section 3.5.2.1).
The result, analysis method, and discussion will be presented in this chapter.

86

5.2 Experimental condition, result, and discussion
5.2.1 Experimental condition for time-resolved experiment for (NPr 4)2[Fe2(Cl2An)3] at
RTWe performed time-resolved reflectivity measurements using an optical pump–probe
technique to confirm the occurrence of a photoinduced CT-type phase transition in this MOF
system. We used a pump light with a photon energy of either 0.73 or 0.8 eV (E || chain)(considering CT transition peak excitation) and a 90 fs pulse width. We monitored the relativereflectivity change (∆R/R) induced by photoexcitation of the CT band over a wide range ofprobe-photon energies (0.1–3.0 eV) at RT, as shown in Fig. 5.1, i.e., in the LT phase. We
maintained the pump fluence of 1.0 mJ/cm 2 during all the measurements.

Fig. 5.1: Optical conductivity [()] spectra of the single crystal with E || chain at RT. Here,
we excited CT transition and monitored photoinduced ∆R/R over a wide range of probe-photon
energies (0.1–3.0 eV).

87

Result and discussion
5.2.2 Photoinduced transient ∆R/R spectrum along E || chain at RT
The photoexcitation conditions must be the same during the measurement to obtain the ∆R/R
spectra of the photoinduced transient state. However, we had to choose two different photon
energies, 0.73 and 0.8 eV, as the pump light to make the ∆R/R spectra around the CT peak. The
reason for two different photon energies is the lack of suitable optical filters to cut the pumplight scattered from the crystal surface effectively. The energies of the pump lights are denoted
as red (0.73 eV) and black (0.8 eV) dashed lines in Fig. 5.2(a) with the initial state ()spectrum, at 296 K using E || chain. In Fig. 5.2(b), the ∆R/R spectrum plotted by red circles ismeasured using 0.73 eV excitation at 0 ps, and that plotted by black circles is measured using
0.8 eV excitation at 0 ps. The red and black circles seem to be connected smoothly. Fig. 5.2(c)shows the temporal profiles of ∆R/R for probe energy of 1.46 eV, using 0.73 (red circles) and
0.8 (black circles) eV excitations. The similarity of the amplitude and the temporal dependenceof ∆R/R assures the observation of the same photoinduced dynamics. Based on these results,
the validity of using two different photon energies to make the ∆R/R spectrum by CT excitationin the wide energy range must be approved.
Compare photoinduced and thermal-induced ∆R/R spectra
Fig. 5.3(a) shows the isothermal static reflectivity spectra measured at the temperatures 296,
340, and 380 K—corresponding to the LT, IM, and HT phases, respectively—and with E ||chain. The broad photon-energy range covers the range of intramolecular transitions (2.0–3.0eV), the CT transition (0.5–1.5 eV), and molecular vibrations (mid-IR, 0.15–0.25 eV). These
three photon-energy ranges are labeled as green-, brown-, and yellow-shaded regions,
respectively. The differences (∆R/R) in the isothermal reflectivity spectra between 296 K and
340 K and between 296 K and 380 K are shown as orange and green lines, respectively, in Fig.
5.3(b).

88

a

b

c

Fig. 5.2: (a) The () spectra with E ∥ chain at 296 K. The dashed red (0.73 eV) and black(0.8 eV) lines are the positions of the photon energies of the pump lights. (b) The transientreflectivity change (𝛥R/R) spectra at 0 ps. Red and black circles were 𝛥R/R with the excitationof 0.73 and 0.8 eV, respectively. (c) The typical temporal profiles of 𝛥R/R at a probe energy of
1.46 eV and pump energies of 0.73 (red circles) and 0.8 eV (black circles) [128].

89

They correspond to the ∆R/R spectra expected in thermally induced phase changes from the LT
to the IM and HT phases. The ∆R/R spectra observed just after photoexcitation (0 ps, open
black circles) and at 10 ps (open pink circles) with a pump fluence of 1.0 mJ/cm 2 are alsoplotted in Fig. 5.3(b). We observed considerable differences between the spectral shapes of thephotoinduced ∆R/R (open black and red circles) and the thermally induced values (the orange
and green lines) both in the region of the CT transition (the brown-shaded area) and in the midIR region (the yellow-shaded area). For simplicity, we next consider the transient reflectivity
spectra in these three energy ranges separately and hereafter discuss in detail what we observedin each energy range to obtain further insight into the effect of photoexcitation in the present
MOF.

(a)

(b)

Fig. 5.3: (a) Temperature-dependent R spectra of a single crystal of (NPr4)2[Fe2(Cl2An)3] with
E || chain. (b) Transient reflectivity (∆R/R) spectra with E || chain for pump and probe pulses at∆t = 0 (black circles) and 10 ps (pink circles) at RT (LT phase). The fluency of the pump pulse
is Iex = 1.0 mJ/cm2. The ∆R/R spectrum with the E || chain between 296 K (the LT phase) and90

380 K (the HT phase) is shown as a green line, and that between 296 K and 340 K (the IMphase) is shown as an orange line. The energy ranges corresponding to intramoleculartransitions (2.0–3.0 eV), the CT transition (0.5–1.5 eV), and molecular vibrations (mid-IR,
0.15–0.25 eV) are designated by green-, brown-, and yellow-shaded regions, respectively [128].
Time evolution of reflectance change (ΔR/R)
Fig. 5.4 shows a few examples of the time profile of the reflectance change after the time origin
correction. Excitation was performed with E || chain polarized light at 0.73 or 0.8 eV, and theenergy in the legend represents the photon energy of the Probe light (E || chain). The time
evolution of ∆R/R in the mid-IR, CT-transition, and intramolecular transition energy range areshown from left to right. The reflectance change is significant immediately after lightirradiation at all wavelengths, followed by more than one relaxation process. From preliminary
observation, the relaxation process has three components: (i) a fast relaxation component
immediately after excitation (red arrow), (ii) a slow relaxation component (green arrow), and(ii) a long-lived constant component (blue arrow). Next, we attempted to understand thephenomena associated with photoexcitation by fitting analysis of the temporal profiles in a few
probe energies.

91Fig. 5.4: A few examples of the time profile of the reflectance change (∆R/R) spectra fordifferent probe energy ranges (Mid-IR, CT, and intramolecular transition).

92

Excitation intensity dependence of ΔR/R

Fig. 5.5 shows the time evolution of the reflectance change at each excitation intensity(measurement temperature RT, probe energy 0.65 (Fig. 5.5(a)) and 2.74 eV (Fig. 5.5(b))) in
the time region from up to 20 ps. We measured time evolution for the excitation intensity of
0.2 - 1.0 mJ/cm2. For both the probe energies, the shape of the graph does not depend on the
excitation intensity and is the same for all excitation intensities. Fig. 5.5(a) and (b), lower panel,shows the plot of ∆R/R concerning the excitation intensity. The result shows the linear
dependence of ∆R/R on Iex. Hence, it follows the second law of photochemistry, the StarkEinstein law, which states that for each photon of light absorbed by each molecule, only one
molecule is activated for subsequent photoexcitation.

(a)

(b)

Fig. 5.5: The time evolution of ∆R/R for each excitation intensity of pump light and the plot of∆R/R with respect to the excitation intensity for (a) 0.65 and (b) 2.74 eV probe energies.

93

Fitting of time evolution of ΔR/R
Fig. 5.6(a), Fig. 5.6(b), and Fig. 5.6(c) show typical temporal profiles of 𝛥R/R. The fitting
analysis has been performed using equation (5.1) with the convolution of the Gauss function,
which represents the time resolution determined by the pulse width of the pump and the probe
lights. Due to the convolution of the Gauss function, the full width at half maximum (FWHM)
of the Gauss function is added as a fitting parameter.
𝑡

𝑡

1

2

(5.1)

𝑓(𝑡) = 𝑎1 exp (− 𝜏 ) + 𝑎2 exp (− 𝜏 ) + 𝑎3 .

The experimental curves are well-fitted, as shown in Fig. 5.6(a), Fig. 5.6(b), and Fig. 5.6(c).
Then, the photoinduced dynamics by CT excitation have at least three kinds of relaxation
processes. The obtained parameters are summarized in Table S1.

(c)

(b)
(a)

Fig. 5.6: The typical temporal profiles of ∆R/R with fitting results at a probe energy of (a) 0.65
eV, (b) 0.99 eV, and (c) 2.74 eV.
Table 5.1. The values of fitting parameters for fitting analysis of temporal profiles of ∆R/R at
the probe energies of 0.65, 0.99, and 2.74 eV.
0.65 eV

0.99 eV

2.74 eV

FWHM

123 ± 6 fs

110 ± 3 fs

162 ± 4 fs

𝜏1

160 ± 9 fs

160 ± 8 fs

150 ± 10 fs

𝜏2

3.1 ± 0.26 ps

3.17 ± 0.25ps

3.3 ± 0.2 ps

94

The spectral shape of the photoinduced transient state just after the photoexcitation at whichthe ∆R/R shows the maximum amplitude in the temporal profile curve is discussed in this work.
The obtained fitting result for the probe photon energies suggests that the time resolution isaround 110 fs, as shown in Table 5.1. Thus, the generation speed of the photoinduced state(just after photoexcitation) is within the time scale of 110 fs.
5.2.2.1 Transient ΔR/R spectrum in the intramolecular transition energy range (2.0 –3.0 eV)

Fig. 5.7(a) shows the isothermal static reflectivity spectra measured at the temperatures 296,
340, and 380 K with E || chain, and the differences (∆R/R) in the isothermal reflectivity spectra
between 296 K and 380 K are shown as the green line in Fig. 5.7(b) in the energy range of 2.0–
3.0 eV. The open black, orange, and pink circles show the ∆R/R spectrum with a time delay of0, 1, and 10 ps at RT, respectively. The photoinduced ΔR/R spectra are very similar to thethermal induced ΔR/R spectrum. It suggests photoinduced conversion of the LT phase to the
HT phase. A quantitative evolution is necessary to comprehensively grasp photo-conversion
efficiency, which will be discussed in the following sections.

95

(a)

(b)

Fig. 5.7: (a) Temperature-dependent R spectra of a single crystal of (NPr4)2[Fe2(Cl2An)3] with
E || chain. (b) Transient reflectivity (∆R/R) spectra with E || chain for pump and probe pulses at∆t = 0 (black circles), ∆t = 1 (orange circles), and 10 ps (pink circles) at RT (LT phase). The
fluency of the pump pulse is Iex = 1.0 mJ/cm2. The ∆R/R spectrum with E || chain between 296
K (the LT phase) and 380 K (the HT phase) is shown as a green line within the 2.0 -3.0 eVenergy range.

Quantitative analysis of transient ΔR/R spectrum in the intramolecular transition energy
range (method and discussion)
To evaluate the photoinduced ∆R/R spectra quantitatively, it is necessary to consider the spatial
distribution of the density of photoinduced states because the intensity of the pump lightweakens as it penetrates deeper into the crystal from the light-irradiated sample surface.
Therefore, we adopted the multilayer method of analysis described in refs. [61] and [134].

96

According to the multilayer method, we consider Just after the photoexcitation, the distributionof the photoinduced state 𝑟(𝑧) can be assumed as exponential-type, as shown in Fig. 5.8(a) and5.8(b), when the volume of the photoinduced state is proportional to the absorbed photon
number.

Fig. 5.8: (a) Schematic illustration of the distribution of the absorbed photon density. (b) The
distribution of the excited state against the depth from the sample surface, which we assumed
in the analysis, is exponential-type one.

Then, we constructed the dielectric function [𝜀(𝜔)] at depth 𝑧 as a uniform mixture of thedielectric functions of the initial [𝜀 𝑖𝑛𝑖 (𝜔)] and photoinduced [𝜀 𝑃𝐼 (𝜔)] states. Given theseassumptions, 𝜀(𝜔) at depth, 𝑧 is given by𝑧𝑧𝜀(𝜔, 𝑧) = 𝜀 𝑃𝐼 (𝜔)𝑟0 exp(− 𝑑 ) + 𝜀 𝑖𝑛𝑖 (𝜔){1 − 𝑟0 exp(− 𝑑 )},(5.2)

where 𝜔 is the frequency of the light and 𝜀 𝑖𝑛𝑖 (𝜔) is 𝜀(𝜔) at 296 K. For discussing theintramolecular transition range 2.0–3.0 eV, we assumed 𝜀 𝑃𝐼 (𝜔) to be 𝜀(𝜔) for the HT phaseat 380 K [𝜀 𝐻𝑇 (𝜔)], since it is reasonable to assume that the photoinduced CT excitation
modulates the valence of the Cl 2An3– ions (in the initial LT phase) into that of the Cl2An2– ions97

(in the HT phase). The quantity 𝑟0 (0 < 𝑟0 < 1) represents the yield of the observed
photoinduced state at the surface. The value of d is the specific length of the exponential-decay
type distribution of photoinduced states, which we assume to be equal to the penetration depthof the pump light, about 162 nm. We determined this value from the KK analysis using thestatic reflectivity at 296 K.

𝑑=

𝑑=

1
𝐴𝑏𝑠0.81= 162 𝑛𝑚
61756 cm−1

The absorbance spectrum at 296 K (LT phase) is shown in Fig. 5.9 The dashed line shows the
position of the pump photon energy 0.8 eV.

Fig. 5.9: The absorbance spectrum at 296 K. The dashed line shows the position of the pump
photon energy 0.8 eV.
The reflectivity spectrum of the transient state can be calculated by using the transfer-matrix

98method [135] for the accumulation of thin films with uniform 𝜀(𝜔, 𝑧). The simulated ∆R/R
spectrum has been calculated by the multilayer method in the energy range of 2.0–3.0 eV at alate delay time. As an assumption, the d value is fixed at 162 nm, the penetration depth ataround CT transition energy, for all the delay time for simplicity. Only 𝑟0 is the variable
parameter in equation (1) (see the main text). Then 𝑟0 has been evaluated by utilizing the leastsquare method.
The thick purple line in Fig. 5.10(a) indicates that the calculated ∆R/R spectrum reproduces theexperimentally observed spectrum well. We determined the value of 𝑟0 to be 0.48 ± 0.002 froma least-squares fit to the 0 ps spectra. The result suggests the successful observation ofphotoinduced phase transition resembles the thermal-induced one.

(a)(b) LT Phase

HT Phase

Cl2An3–

Cl2An2–

Fig. 5.10: (a) ∆R/R in the intramolecular transition region (~ 2.6 eV).

The black circles

represent the photoinduced values of ∆R/R at 0 ps and RT. The purple line shows the values of∆R/R at 0 ps calculated by considering the distribution of the photoinduced state (see text). In
this calculation, the photoinduced state is assumed to be the HT phase (380 K). The parameters
are d = 162 nm and 𝑟0 = 0.48. (b) The schematic representation of photoinduced phasetransition (Cl2An3- (LT phase) → Cl2An2- (HT phase))

99

The transient ∆R/R spectrum for other delay times (for example, 1 or 10 ps) can also be wellfitted using the optimal 𝑟0 values are drawn with the experimental ∆R/R spectra in Fig. 5.11,

Fig. 5.11: The experimental and simulated ∆R/R spectra of 0 ps (black circles and solid blackline), 1 ps (orange circles and solid orange line), and 10 ps (pink circles and solid pink line).

Since 𝑟0 can be evaluated at any delay time using the same method of analysis, we obtained
the time dependence of 𝑟0 shown in Fig. 5.12, where we performed the fitting analysis using
equation (5.1):

This result revealed a combination of two types of relaxation: a very fast relaxation on atimescale of 140 ± 32 fs (𝜏1 ) and a slow relaxation on a timescale of 3.40 ± 0.24 ps (𝜏2 ). In thefollowing, we focus our discussion solely on the photoinduced state observed just after
photoexcitation because the spectral shapes of ∆R/R for the photoinduced state in the observed100energy range did not show any apparent change during ~15 ps, and only the density change
can explain the delay-time dependence.

Fig. 5.12: The black circles show the values of 𝑟0 at each delay time, along with a fit to thedata (the solid red line). The temporal dependences of the three terms in equation (1) are shown
as dashed lines [128].
Estimation of photo-conversion efficiency
For estimating the photo-conversion efficiency by the CT excitation, the number of theabsorbed photon (Nph) and the number of the excited states, Fe 2+ and Cl2An2– pairs (Nex), areevaluated as follows. Nph and Nex are evaluated in the cylinder of 𝑆 × 𝑑, as illustrated in Fig.
5.13. Here, 𝑆 = 50.24 × 10−4 cm2 and 𝑑 = 162 nm are the spot size and the penetration depthof the pump light, respectively.

101

Fig. 5.13: Schematic illustration for the distribution of absorbed photon density as a functionof the distance from the surface (𝑥).

Considering the reflectivity (R) around the CT transition energy and the pump fluence (Ipump),
Nph can be expressed as the following formula.
𝑁ph = (1 − 𝑅)𝐼pump𝑆(1 − 𝑒 −1 )On the other hand, Nex can be expressed as follows, considering our assumption of the
distribution of the excited state as shown in Fig. 4.12b.
𝑑𝑁ex = ∫ 𝑑𝑥 (
0

2𝑍
𝑥
2𝑍
) 𝑆𝑟0 exp (− ) = ( ) 𝑟0 𝑆𝑑(1 − 𝑒 −1 )𝑉0𝑑𝑉0Here, 𝑍 and 𝑉0 are the number of formula units and the volume of the primitive cell,respectively. 𝑟0 is the yield of the observed photoinduced state defined in the main text.
Then, the conversion efficiency is,
𝑁ex⁄𝑁ph = (

2𝑍
𝑟0 𝑑
.
)
𝑉0 (1 − 𝑅)𝐼pump

Here, (Z/𝑉0) is 7.34 × 1020 /cm3, R is 0.2, and Ipump is 7.7 × 1015 photons/cm2 at 0 ps.
Therefore,102

𝑁ex⁄𝑁ph =2×7.34×1020 ×0.48×162
0.8×7.7×1015= 1.85.

Hence, photo-conversion efficiency just after photoexcitation (𝑁ex /𝑁ph) is around 1.85 DA
pair/photon.
Hence, we found that it exhibits an efficient photoinduced CT conversion between Fe andCl2An ions at RT by virtue of the cooperative valence instability.
As discussed in the previous sections, the observed spectral change at ultrafast speed in theintramolecular-transition range can be explained by an efficient photoinduced valence change
from the LT phase (Cl 2An3–) to the HT phase (Cl2An2–). This raises the naïve question ofwhether the photoinduced state resembles a pure HT phase. To answer this question adequately,
it is essential to evaluate quantitatively the photoinduced ∆R/R spectra in the CT transition andmid-IR regions, as discussed in the following sections.

1035.2.2.2 Transient ΔR/R spectrum in the CT energy range (0.5 – 1.5 eV)
We first want to emphasize that, as shown in Fig. 5.3(b) and Fig. 5.14(b), the shape of thephotoinduced ∆R/R transient spectrum in the CT-transition energy range at RT is different fromthat of the thermally induced spectrum. There is a clear visible redshift at the zero-crossing
point in the photoinduced ΔR/R spectrum just after photoexcitation (0 Ps) with respect to thethermal ΔR/R spectrum. A similar difference can also be observed in the transient spectrum ofother delay times (1, 5, and 10 ps.)

(a)

(b)

Fig. 5.14: (a) Temperature-dependent R spectra of a single crystal of (NPr4)2[Fe2(Cl2An)3] with
E || chain. (b) Transient reflectivity (∆R/R) spectra with E || chain for pump and probe pulses at∆t = 0 (black circles), ∆t = 1 (orange circles), and 10 ps (pink circles) at RT (LT phase). The

104

fluency of the pump pulse is Iex = 1.0 mJ/cm2. The ∆R/R spectrum with E || chain between 296
K (the LT phase) and 380 K (the HT phase) is shown as a green line within the energy range of
0.5 -1.5 eV.

In Fig. 5.15(a) and (b), we plotted the photoinduced R and optical conductivity spectrum overthe photon energy range of 0.5–1.5 eV, corresponding to the CT transition energy. This spectral
discrepancy is due to the appearance of a shoulder located on the higher-energy side of the R
and optical conductivity spectrum, as shown in Fig. 5.15(a) and (b), which can be attributed tothe photoinduced appearance of a new structure (a shoulder peak).

(a)

(b)

Fig. 5.15: photoinduced (a) reflectivity and (b) optical conductivity spectrum just after
photoexcitation (0 ps), a clear shoulder peak is observed in the transient spectrum, which makes
the photoinduced state distinct from the thermally induced one.

In the next section, we will discuss quantitatively the origin of the shoulder peak in the higher
energy side of the CT band.

105Quantitative analysis of transient ΔR/R spectrum in the CT energy range (0.5 – 1.5 eV)(method and discussion)
This section describes a simulation result for the transient ΔR/R spectrum in the CT energy
range based on equation (5.2). Note that a simulation of the R variation using equation (2) andassuming that the photoinduced state is the HT phase (the purple line in Fig. 5.16) fails to
reproduce the experimental data, even though utilizing the same parameter values for 𝑟0 and dthat obtained from the analysis for the intramolecular transition energy range (0 ps). This
spectral discrepancy is due to the appearance of a shoulder located on the higher-energy sideof the reflectivity spectrum, between 0.8 and 1.2 eV.

Fig. 5.16: Reflectance (R) spectrum of the photoinduced state of (NPr 4)2[Fe2(Cl2An)3],
obtained using a pump with E || chain and Iex = 1.0 mJ/cm2, at ∆t = 0 and RT around the CTtransition range (0.5–1.5 eV). The observed spectrum is plotted as black circles. Calculated R
spectra at 0 ps are also shown; they consider the photoinduced state to be 380 K (HT phase;106the purple line). The blue solid line and green dashed line represent the R spectra of the LT(296 K) and HT (380 K) phases.

To reproduce this unexpected spectral feature, we added one Lorentzian oscillator (The detailabout the Lorentzian oscillator model is described in Appendix C) 𝜀 𝑙 (𝜔) to 𝜀 𝑃𝐼 (𝜔) in equation(5.2) to represent another absorption feature, i.e., 𝜀 𝑃𝐼 (𝜔) = 𝜀 𝐻𝑇 (𝜔) + 𝜀 𝑙 (𝜔), with𝜀 𝑙 (𝜔) =𝑆0,2
𝜔0 − 𝜔2 + 𝑖𝛾0 𝜔(5.3)

where 𝜔0 , 𝑆0 , and 𝛾0 are the resonance frequency, oscillator strength, and damping constant,respectively, of the Lorentzian oscillator. We determined the optimal values of these parametersfrom a least-squares fit to the data at 0 ps. The resulting calculated photoinduced reflectivity
spectrum (the thick red line in Fig. 5.17(a)) matches the experimental data (open black circles)
well. The simulated () spectrum of the photoinduced state based on this analysis is shownas the thick red curve in Fig. 5.17(b). This feature looks like a superposition of the spectrum of
the HT phase (the dashed green line) and the spectrum of the new absorption feature (the dashedorange line). The peak position of the new absorption is around 1.02 ± 0.003 eV—with 𝛾0 =0.22 ± 0.001 eV and 𝑆0 = 0.20 ± 0.001 eV2—which is significantly higher than the energy of
the CT transition for either the LT or the HT phases. It is, therefore, reasonable to assume thatthe new absorption band can be assigned to a CT transition between D and A, i.e., to a doublepeaked structure produced by the splitting of the CT transition.

107

a

(a)

b

(b)

Fig. 5.17: (a) Reflectance (R) spectrum of the photoinduced state of (NPr 4)2[Fe2(Cl2An)3],
obtained using a pump with E || chain and Iex = 1.0 mJ/cm2, at ∆t = 0 and RT around the CTtransition range (0.5–1.5 eV). The observed spectrum is plotted as black circles. Calculated R
spectra at 0 ps are also shown; they consider the photoinduced state either to be the 380 K (HTphase; the purple line) or to be the sum of the 380 K (HT phase) and a Lorentzian oscillator(the thick red line). Details of the calculation are presented in the text. The solid pink fill shows108the difference between the purple and red lines. The dashed lines represent the R spectra of theLT (296 K) and HT (380 K) phases. (b) Calculated () spectra of the photoinduced state (thick
red line), 296 K (dashed blue line), and 380 K (dashed green line). The dashed orange line
shows the new photoinduced absorption peak. Purple arrows indicate the resonance frequencyof the Lorentzian oscillator.
Simulated ∆R/R around CT range (0.5–1.5 eV) for other delay times
In the CT energy range (from 0.5–1.5 eV), the photoinduced ∆R/R spectrum has been
evaluated using equation (5.3). Here, the simulated spectra at every delay time are calculatedwith the same parameters set for 0 ps except 𝑟0 . Using the obtained 𝑟0 values from the intramolecular transition energy range analysis, the experimental data have been reproduced very
well, as shown in Fig. 5.18 (for 1 and 10 ps).

Fig. 5.18: Upper and lower panels are the ∆R/R spectra of the photoinduced state. The
experimental spectra are plotted as open orange and pink circles for 1 and 10 ps, respectively.

109

The calculated spectra are drawn as purple [without 𝜀 𝑙 (𝜔)], orange (1 ps), and pink(10 ps)[with 𝜀 𝑙 (𝜔)] lines.

From the experimental result after photoexcitation, unexpected spectral behavior can beobserved in the CT energy range. A new absorption band can be observed at the higher energy
side of the CT transition. The emergence of the new peaks, which can only be observed in thephotoinduced transient state, implies generating a photoinduced, thermally hidden state. Thisphenomenon is distinct from what we see in the thermally induced CT states. Later, we discuss
the possible origin of this splitting in connection with the photoinduced spectral change in the
mid-IR region, i.e., in the molecular vibration region, which will be discussed in the next part.
5.2.2.3 Transient ΔR/R spectrum in the mid-IR energy range (< 0.3 eV)Experimental and optical setup
To measure time-resolved mid-IR ∆R/R (1200–2000 cm−1), we detected the probe lightreflected from the sample using the MCT detectors after passing it through a monochromator(Bunkokeiki M10, the only difference from our previously described setup). The energy
resolution of this system was approximately 4 cm −1. The schematic optical set is shown in Fig.
5.19, a monochromator is the main difference between this setup and the previous one.
Laser output

monochromator

detector

Fig. 5.19: Optical setup for reflection pump-probe measurement in mid-IR energy range.

110The photoinduced transient spectra in the mid-IR energy range cannot be explained simply byphoto-conversion from the LT phase to the HT phase, as shown in Fig. 5.3(b) and Fig. 5.20.
After obtaining the results we sought, we should be able to come up with some interestingscientific findings. The upper panel of Fig. 5.20 High-resolution, temperature-dependent R
spectra of (NPr4)2[Fe2(Cl2An)3] in the mid-IR range (1200–2000 cm−1) with E || chain (296 K:
blue line, 340 K: orange line, and 380 K: green line). The middle panel of Fig. 5.20 shows the∆R/R with E || chain (black line) and E ⟘ chain (brown line) probes for the pump with E || chain
and Iex = 1.0 mJ/cm2 at ∆t = 0 ps and RT. These spectra imply that the photoinduced spectralchange is strongly anisotropic.

Fig. 5.20: (upper) Temperature-dependent R spectra of a single crystal of (NPr4)2[Fe2(Cl2An)3]
with E || chain. (middle) ∆R/R with E || chain (black line) and E ⟘ chain (brown line) probes

111for the pump with E || chain and Iex = 1.0 mJ/cm2 at ∆t = 0 ps and RT. The ∆R/R spectrum with
E || chain between 296 K (the LT phase) and 380 K (the HT phase) is shown as a green dashedline. (lower) Photoinduced) ∆R/R with E || chain (black line) at ∆t = 0 ps and the calculated∆R/R spectra at 0 ps using d = 162 nm and 𝑟0 = 0.48, considering the photoinduced state to be
the 380 K (HT).

The spectrum obtained with the E || chain produced a broad dispersion-type spectral shapeoverlapped with spiky structures due to the intramolecular vibrational modes, while thespectrum obtained with the E ⟘ chain showed no change. The differences (∆R/R) in theisothermal reflectivity spectra between 296 and 380 K are shown as green dashed lines. Thissuggests the clear difference between photoinduced and thermal induced ΔR/R spectrum. In thelater section, by using a similar method of analysis as discussed previously during the CT
energy range analysis, we will try to understand the nature of the spectral behavior and possibly
the origin of this discrepancy.
The amplitude of the photoinduced ∆R/R is smaller than that of the thermally induced ∆R/R inthis energy range. Since there are many spiky structures due to intramolecular vibrationalmodes, the energy resolution of the data in Fig. 5.3(b), where the wavelength resolution is about
150 cm–1, is insufficient for this analysis. Therefore, we measured the ∆R/R spectra much more
precisely to obtain better insight into the spectral nature of the photoinduced state in this range.
In the setup used for this experiment, we sent the light reflected from the sample directly intoa monochromator to obtain high-energy-resolution spectra with a wavelength resolution of ~6
cm−1, so that the intramolecular vibration modes can be distinguished, even at the expense of
a temporal resolution degraded to the order of 1 ps. The spectral width of the probe pulse in
the pump-probe measurement determines the energy resolution of the photoinduced ∆R/R
spectra. The spectral width of the ultrashort pulse must be wide due to the Fourier relation112

between the spectral width and the temporal width. As written in the main text, the spectral
width of the intramolecular vibrational structure is about 4–10 cm-1. Still, the spectral width of
our probe pulse is approximately 150 cm -1 in the mid-IR range. It is necessary to improve theenergy resolution to distinguish the spiky structure. Therefore, we put the reflected light fromthe sample into a monochromator. This allows us to improve the energy resolution to roughly
6 cm-1 or less. However, the time resolution drops to about 1 ps in contrast to the improvementof the energy resolution.
Simulation of photoinduced transient spectrum in the mid-IR range
As a first step in analyzing these results, we calculated the photoinduced ∆R/R spectrum with
E || chain using the 𝑟0 and d values obtained in the previous analyses and assuming thephotoexcited phase to be the HT phase. This result is represented by the thin-dashed purple
curve in Fig. 5.21(b). There is an apparent discrepancy between this simulated curve and theexperimental spectrum (the black line in Fig. 4.25(b)). This result supports the hypothesis thatthe photoinduced state differs from the thermally induced state. To test this hypothesis, we next
included the additional Lorentzian oscillator absorption not observed in the HT phase as asecond step in the simulation. We performed this spectral simulation successfully using
equations (5.2) and (5.3), and the calculated ∆R/R spectrum that includes the additional
oscillator (the thick red line in Fig. 5.21(b)) reproduces the curve of the experimental dataconsiderably better, except for the spiky structures due to intramolecular vibrations. Using theleast-squares fit, we determined the optimal values of the parameters for the added newabsorption in the transient state to be 𝜔0 = 1570 ± 9.06 cm–1, 𝑆0 = 72.60 ± 4.34 cm–2, and 𝛾0 =403.28 ± 5.06 cm–1. The difference in the spiky structure due to the intramolecular vibration
modes—especially those found below 1500 cm–1—may reflect a difference in the molecular
structure between the HT phase and the photoinduced state. Fig. 5.21(c) shows the simulated𝜎(𝜔) spectrum for the photoinduced state (the thick red line), including the additional new

113

absorption (the dashed orange curve). The peak position of the new absorption is higher thanthose of any of the peaks assigned to intramolecular vibrational modes, which are observed as
spiky peaked structures in the spectra at 296 and 380 K (the thin blue and green dashed lines).
Clarifying the parameters' delay time dependence must also be important in the mid-IR range.
However, measuring the high energy-resolved spectra (precise scan) at several delay times isdifficult because it takes too much time. Then, the low energy-resolved spectra (rough scan)were measured in the same energy range. The ∆R/R spectra of the precise and the rough scan
at 0 ps are shown by a black line and closed circles, respectively, in Fig. 5.21(c). The roughscan's spectral shape is accurate enough to evaluate the overall structure discussed in the maintext. The rough scan spectra have been measured at several delay times and compared with thesimulated spectra in Fig. 5.21(d).

(a)

(c)

(d)
(b)

Fig. 5.21: (a) High-resolution, temperature-dependent R spectra of (NPr4)2[Fe2(Cl2An)3] in the
mid-IR range (1200–2000 cm−1) with E || chain. (b) ∆R/R with E || chain (black line) and E ⟘114chain (brown line) probes for the pump with E || chain and Iex = 1.0 mJ/cm2 at ∆t = 0 ps and
RT. The R spectra at 0 ps, calculated using d = 162 nm and 𝑟0 = 0.48, consider the photoinduced
state to be either the 380 K (HT) phase (the purple dashed line) or the sum of the 380 K (HT)phase and a Lorentzian oscillator (the thick red line). (c) The ∆R/R spectra at 0 ps (black line:
precise scan, and black circles: rough scan spectra of photoinduced ∆R/R). (d) The calculated
and experimental photoinduced ∆R/R spectra at the delay time of 0, 1, and 15 ps are represented
by black, orange, and pink dashed lines and black, orange, and pink circles, respectively [128].

The experimental photoinduced ∆R/R results can be well reproduced by calculation [equation(5.3)] using fixed d value (162 nm), 𝜔0 = 1570 cm –1, 𝛾0 = 403.28 cm –1, and 𝑆0 = 72.60 cm –2 and only changing the 𝑟

0 values obtained from the analysis of the intra-molecular transitionenergy range.
According to the results shown in Fig. 5.11 and 5.18, the photoinduced dynamics induced by
the CT excitation can be explained only using the 𝑟0 the value which reflects the degree ofvalence change.

To explore the origin of the new absorption in the mid-IR energy range, we compared thetransient 𝜎(𝜔) spectrum with Raman spectra (Fig. 5.22(b) and 5.22(c)), in which 'xx' and 'xy'
mean that the polarization directions of the analyzer are respectively parallel and perpendicular
to the polarization of the input light (E || chain). Fig. 5.22(b) shows these spectra for the HTphase (380 K), and Fig. 5.22(c) shows the 'xx' alignment spectra measured at temperatures 293,
340, and 380 K. The Raman spectra in the HT phase exhibit a characteristic mode at 1596 cm −1,
which is assigned to a C–O stretching mode [136] with Ag symmetry, as judged from theintensity ratio between 'xx' and 'xy'; [137] the inset of Fig. 5.22(b) depicts a schematic
illustration of the atomic motions in this mode (the red arrows) [138]. Interestingly, we did notobserve this C–O stretching mode at 1596 cm −1 in the LT phase (Fig. 5.22(c)), indicating that115

this mode is related to the Cl 2An2– ions. The peak position of this mode is very close to that of
the new photoinduced absorption. This suggests that the Raman mode became IR-active in thephotoinduced state. Considering the strongly anisotropic nature of the broad structure of ∆R/R
observed and the Ag symmetry of the Raman mode at 1596 cm −1, we conclude that the newly
observed absorption band at around 1600 cm −1 in the photoinduced state can be attributed toan electron–molecular-vibration (e–MV) coupled mode. The e–MV coupled modes in DA
complexes have been intensively studied in TTF–CA [139]. The CT energy between the D and
A molecules becomes asymmetric because of the disappearance of the inversion centers inthese molecules due to the dimerization-type lattice distortion. In other words, a totallysymmetric Ag mode can become an IR-active mode in this situation. A characteristic of the eMV coupled mode is its strong oscillator strength borrowed from the CT transition.[50] The Ag
mode inherent to a single Cl 2An2– ion can thus become the origin of the observed broad
photoinduced IR band based on the same mechanism. That is, local inversion-symmetry
breaking due to lattice modulation in the photoinduced state may induce the emergence of a
new IR absorption band in the present system. Two possible structural modulations in thephotoinduced state are shown schematically in Fig. 5.23(c) and 5.23(d). Model 1 is a DA-type
dimer modulation similar to that in the ferroelectric I phase of TTF–CA, and model 2 represents
an ADA-type trimer modulation. In model 1, the Fe-related e-MV coupled mode may be active.
Therefore, observations of the Fe-related e-MV coupled modes associated with the symmetryaround Fe ions are key for determining which mechanism is the actual origin of this state.
Regrettably, we have not yet been able to carry out such experiments because the Fe-related eMV coupled modes are characterized by wavenumbers—e.g., around 600 cm−1—lower thanthose in our measurable range.

However, the lattice-modulation scenario can also be explained by another piece of evidence
that is due to the appearance of the new absorption band in the CT transition region described116

above: the double-peaked CT transitions tentatively associated with the disappearance of theinversion center from the Cl 2An ion in the smallest DAD model (see Appendix D). Indeed,similar observations of double-peaked structures have been reported in segregated [140] andmixed [64] stacked DA complexes with dimerization. In refs. [141] and [64], an asymmetric
CT energy explained a double-peaked structure due to the disappearance of the inversion
centers on the constituent molecules.

117

In addition to CT, lattice modulation in the 1D chain self-consistently explains the observed
photoinduced spectral change in this system over the wide range of photon energies extending
from the region related to intramolecular transitions to the mid-IR region. The remaining issue
in this discussion concerns the speed of the lattice deformation.

a

b

c

Fig. 5.22: (a) The calculated () spectrum just after photoexcitation is shown as a thick redline. The photoinduced absorption in the model is also shown as an orange dashed line. The
blue and green dashed lines are the observed () spectra of the LT (296 K) and HT (380 K)

118phases, respectively. (b) Polarized Raman spectra with 'xx' and 'xy' polarization geometries (seemain text) at 380 K. The vertical dashed lines represent the oscillator's center wavenumber andthe Raman spectral peak. (c) Raman spectra at 293 K (blue line), 340 K (orange line), and 380
K (green line) with 'xx' polarization geometry [128].
From the temporal dependence of ∆R/R observed in the CT-transition energy range, thestructural change occurred immediately after photoexcitation, i.e., within the ~ 110 fs time
resolution of our study (see Table 5.1). This timescale is much shorter than that for a typicalchange in the lattice structure, which, for example, was found to be ~600 fs in TTF–CA [62].
The TTF–CA system is constructed from – stacking between D and A; i.e., it can be
described approximately as a "soft bonding" between D and A. In contrast, the present MOF
system is composed of relatively "rigid bonds" between D and A so that the structural changes
may be related primarily to the C–O stretching mode (around 1600 cm −1) and the Fe–O
stretching mode (about 600 cm −1) [142], which have vibrational periods of about 20 or 60 fs,
respectively. Thus, an ultrafast structural change within our time resolution (110 fs) may bereasonable.

Finally, the effect of the NPr4+ cations should be discussed because we neglected them during
the above discussions. The initial photoexcitation occurs inside the 2D honeycomb layer, andthen the successive dynamics are induced by energy transfer from the locally excited state. The
time scale of the energy transfer should be evaluated by the inverse of the energy scale of theinteraction. Then, we believe the dynamics related to the NPr 4+ cation should be slower thanthe dynamics within the layer and can be neglected in the time scale we discussed here.

119

a

Fe3+ (S = ) (D)b

Fe2+ (S = 2) (D)Cl2An3– (S = ) (A)��

1D chain…A D A D A D A D A D A …

Cl2An2– (S = 0) (A)< 110 fsc

Model 1

1D chain… A DA DA DA D A D A …

2D layered systemdModel 2

… A D ADA D A D A D A …

Fig. 5.23: (a) Schematic representation of the charge and spin patterns in the LT phase of the
2D-layered system [Fe2(Cl2An)3]2−. The donor units, Fe3+ and Fe2+, are represented as pink andbrown circles, respectively, whereas the acceptor units, Cl2An3– and Cl2An2–, are represented
as blue and green cylinders, respectively. (b) Schematic 1D chain with DA units before
photoexcitation. (c) and (d) Candidates for the photoinduced state at 0 ps. (c) DA-type dimer
modulation between DA units are similar to that in the ferroelectric TTF–CA. (d) ADA-type
trimer modulation [128].
Ultrafast and efficient photoinduced charge-transfer dynamics, along with a unique latticestructure modulation, occur within 110 fs in the donor–acceptor type metal–organic framework(NPr4)2[Fe2(Cl2An)3] at room temperature (300 K). The present study provides an essential firststep toward realizing flexible photo-controlled MOFs, opening the way for developing
magnetically active photo-switching in such robust structures.

120

��
n
chai
< 110

fs

ice
latt ngetshaafa
Ultr ctural c
stru

5.3 Summary
We have performed ultrafast time-resolved photoexcitation experiments using a wellshaped crystal sample of the DA-type layered MOF (NPr4)2[Fe2(Cl2An)3]. We found that itexhibits an efficient photoinduced CT conversion between Fe and Cl 2An ions at RT by virtueof the cooperative valence instability. Two absorption bands emerged immediately after
photoexcitation at the main CT band's higher-energy side (~ 1.02 eV) and at around 1570 cm−1in the mid-IR energy range. The emergence of these two new peaks, which can only beobserved in the photoinduced transient state, implies generating a photoinduced, thermallyhidden state. This phenomenon is distinct from what we see in the thermally induced CT states.
The hidden state may be produced by unexpected lattice modulations along the chain direction,with accompanying symmetry breaking of the local inversion centers on the Cl 2An ions. Twotypes of lattice modulations are expected, which may demonstrate a change from the SCM state
to a paramagnetic and ferroelectric state similar to that of TTF–CA (model 1) or to a121paramagnetic state (model 2). It remains to be confirmed in the future whether or not thisstructural change involves long-range lattice and charge ordering. This will require latticestructure measurements and magneto-optical measurements resolved on fs timescales.
Ultrafast photocontrol of the charge, spin, and lattice structures in MOF systems may be quite
challenging, but they will provide opportunities to develop ultrafast phase-switching systems.
We also mention that the present study can be considered as a first step toward the developmentof high-speed photo-responsive MOFs with magnetic natures.

Note: This work is published as:

Samiran Banu, Mone Kato, Kou Takubo, Yoichi Okimoto, Shinya Koshihara, Kaoru Iwano,
Wataru Kosaka, Hitoshi Miyasaka, and Tadahiko Ishikawa; Appearance of a photoinduced
hidden

state

in

the

electrondonor–acceptor

type

metal–organic

framework(NPr4)2[Fe2(Cl2An)3], Adv. Optical Mater., 2023, 2301554.

122

123

Chapter 8
8. Conclusions

In this thesis, we investigated the ultrafast photoinduced dynamics of donor-acceptor-typelayered MOF system, (NPr4)2[Fe2(Cl2An)3].

In Chapter 4, the static optical spectra of (NPr4)2[Fe2(Cl2An)3] for both E || chain and E ⊥ chain
have been reported. The reflectivity spectra reflect the anisotropic electronic structure of thiscrystal. The optical conductivity spectra led us to understand the optical transition peaks in ourmaterial. Herein, we observed two absorption peaks at around 0.8 and 2.6 eV. The peak ataround 0.8 eV is attributed to the CT transition between the Fe and Cl 2An ions, while the peakat around 2.6 eV is attributed to an intramolecular transition in the Cl 2An ions. The spectral
weight of each peak along the E || chain shows a steep drop at around each T1/2 due to the LT–
IM and IM–HT phase transitions. Therefore, optical spectra measurement can be utilized as
suitable probes for detecting the degree of CT between the Fe and Cl 2An ions. We alsomeasured the Raman spectra for both the E || chain and E ⊥ chain. These results are very
helpful in performing the ultrafast photoinduced study in our system.

Here, we performed the ultrafast photoinduced dynamics in (NPr4)2[Fe2(Cl2An)3] for both the
LT and HT phase excitation. In chapters 5 and 6, we reported the results of LT phase excitation.
In Chapter 5, we performed ultrafast time-resolved photoexcitation experiments using a wellshaped crystal sample of the DA-type layered MOF (NPr4)2[Fe2(Cl2An)3] at RT (300K). We
found that it exhibits an efficient photoinduced CT conversion between Fe and Cl 2An ions at
RT by the cooperative valence instability. Two absorption bands emerged immediately after

124photoexcitation at the higher-energy side (~ 1.02 eV) of the main CT band and at around 1570
cm−1 in the mid-IR energy range. The emergence of these two new peaks, which can only beobserved in the photoinduced transient state, implies generating a photoinduced, thermallyhidden state. This phenomenon is distinct from what we see in the thermally induced CT states.
The hidden state may be produced by unexpected lattice modulations along the chain direction,with symmetry breaking of the local inversion centers on the Cl 2An ions.

In Chapter 6, using a 6 fs ultra-short pulse, we can successfully observe the early-stage
photoinduced dynamics of (NPr4)2[Fe2(Cl2An)3]. A fast spectral change within 50 fs can beobserved for both E || chain and E ⊥ chain probe direction for E || chain CT excitation. The
experimental results suggest the formation time scale for the photoinduced state, which we
assigned as a structural modulation with local inversion symmetry breaking within thetimescale of ~50 fs. The oscillator analysis shows that in the photoinduced state, a Raman
inactive and IR active mode ~494 cm -1 becomes Raman active, suggesting breaking ofsymmetry inside D and A units leading to structural modulation.
In Chapter 7, we reported the result of photoinduced dynamics for HT phase excitation. We
discussed the successful observation of revere CT behavior between D and A (Fe2+ + Cl2An2→ Fe3+ + Cl2An3-) by exciting the high temperature (HT) using fs-time resolved spectroscopy(pulse width of 90 - fs). Also, in this measurement, we observed the threshold behavior for
realizing reverse CT, i.e., Iex > 1.0 mJ/cm2, suggesting the photoinduced cooperativity effect.

In conclusion, we can mention that the present study can be considered a first step toward
developing high-speed photo-responsive MOFs with magnetic nature.

125Outcomes and Prospective of this research
In this research, primarily we employed the ultrafast time-resolved spectroscopic method toobserve/detect the photoinduced responses from a well-shaped hexagonal DA-type MOF
system, (NPr4)2[Fe2(Cl2An)3]. As a major outcome from these experiments; some importantfeatures are discussed as follows:
1. Ultrafast control of electronic structure in our MOF system upon light irradiation can bewell established.
2. In the process of photoinduced dynamical relaxation; within the time scale of 50 fs a unique
lattice modulation (‘Photoinduced hidden state’) is observed. Which suggests photoinduced
softening of rigid bond, which can never be achieved by thermal excitation.
3. As we observed a local inversion symmetry breaking in this system; can we expect apossibility to get a photo-ferroelectric MOF material in near future (maybe some structuralmodification or some other experiment is necessary to prove such comment, this is an open
question to all future researcher in this field).
4. Ultrafast photoinduced bidirectional switching of valances between Fe and Cl2An units canbe observed.
As we can control the electronic structure of this material in an ultrafast time scale by photo
irradiation and also make a system soft in its transient state, hence it can be easy to bemodulated and apply it in various application fields. For example, it can be used in manypotential application fields like Ultrafast electron storage devices, Gas sensor, fast energy
transfer devices etc.

126

However, I believe we need to take on a challenging part in order to use such materials in realworld applications. The photo-induced research for MOF systems needs to be performed or
expanded in order to make progress. Finally, it will provide the best material in terms of its
application in a broader sense of how it can be applied. As a young scientist and researcher, myresearch represents the first step toward realizing this goal.
In addition, as a short comment for future dream, a combined excitation by 800 nm pulse andTerahertz (THz) pulse may expand the local inversion symmetry breaking. It will help us toexpand the research towards realizing photo-ferroelectricity in MOF system.

127

Appendices
Appendix A: KRAMERS-KRONIG TRANSFORM

The Kramers - Kronig (KK) [143, 144] transformation, known in the field of reflectance spectrometry also as
Robinson-Price [3, 4] analysis, is a powerful tool enabling a calculation of absorption spectra in cases where onlyreflectance spectra can be measured. The complex reflectance spectrum is mathematically decomposed into two
separate spectra - extinction coefficient and refractive index spectrum. These are also called K and N spectra. The
extinction coefficient spectrum can then be used to calculate the absorption spectrum [145].
The real (n = refractive index) and imaginary (k = extinction) parts of the complex index of refraction arecalculated from the reflectance spectrum using the following formulas:𝑛(𝜈) =𝑘(𝜈) =

1 − 𝑅(𝜈)1 + 𝑅(𝜈) − 2√𝑅(𝜈)𝑐𝑜𝑠(𝜃(𝜈))− 2√𝑅(𝜈)𝑐𝑜𝑠(𝜃(𝜈))1 + 𝑅(𝜈) − 2√𝑅(𝜈)𝑐𝑜𝑠(𝜃(𝜈))

(1)

(2)

Where R is the reflectance, n - wavenumber, θ – phase shift angle of the sample.
For a given wavenumber, the phase shift is calculated using the equation (3):

𝜃(𝜈𝑚 ) =

2 𝜈𝑚
𝜋

∞

∫
0

𝑙𝑛√𝑅(𝜈) 𝑑𝜈
𝜈 2 − 𝜈𝑚 2

(3)

The KK Transform algorithm assumes that the reflectance spectra are measured at incidence angles close to zero.
Calculating the integral presents two problems. First, in practice, the spectrum is obtained over a finite range, so
approximations are required at either end. Second, there is a pole at ν = 𝜈𝑚 , which also requires an approximation.
To overcome these problems, a Fourier transform is applied to solve the integral. [146, 147]

128

Appendix B: XYZ cluster coordinates in Fig. 4.4(b) for DFT calculation
AtomicCoordinates (Angstroms)

Number

X

Y

Z

Fe

–1.776

14.3916.226

Fe–0.739

6.537

5.75

Cl

–0.928

6.814

0.766

Cl

–5.76717.278

5.222

Cl

–4.65

3.617

4.741

Cl

–2.04414.081

1.247

Cl–4.40910.052

5.828

Cl

1.894

10.8766.148

O–0.34715.7236.598

O–2.168

5.205

5.378

O

–4.52715.6690.61

O–1.85414.391

8.23

O

–0.66

6.537

3.746

O

–3.341

5.09

0.119

O

0.838

5.374

6.107

O

–3.35315.554

5.869

O

–4.93

3.919

1.762

O

–0.751

6.545

7.75

O

–1.76414.3844.226

O–2.857

12.611

6.2

O

0.342

8.317

5.776

O

–2.244

7.989

5.745

O

–0.27112.939

6.231

C

0.005

15.759

7.832

C

–2.52

5.169

4.144

C

–4.17515.6341.844

C–0.84915.0048.764

C–1.666

5.925

3.212

C

–5.02816.389

2.776

C

–1.94

5.935

1.86

C

–4.75416.399

4.128

129

C

–3.024

5.203

1.356

C

1.156

5.261

7.344

C

–3.671

15.667

4.632

C–3.912

4.493

2.299

C

0.267

5.971

8.287

C

–2.78214.958

3.689

C

1.11

16.4398.322

C–3.625

4.489

3.654

C

–3.06914.9532.334

C–2.181

11.5556.073

C–0.334

9.374

5.903

C

–2.69

10.286

5.911

C

0.175

10.642

6.065

C–1.856

9.181

5.848

C

–0.65811.747

6.128

C–0.57514.99310.116O–6.03417.001

2.242

C

2.24

4.529

7.848

C

0.555

5.975

9.642

C

1.398

16.435

9.677

Cl

2.136

17.3127.235

Cl–1.587

14.115

11.21

C

0.509

15.726

10.62

Cl

3.252

3.651

6.754

C

2.513

4.539

9.2

Cl

–0.471

6.848

10.729

C

1.66

5.295

10.132O

2.416

17.00910.214O

0.827

15.83911.857

O
O

3.519
2.012

3.927
5.259

9.734
11.366130

Appendix C: Lorentzian oscillator model
Lorentz oscillator models describe bound charges' optical response. Lorentz's model is named after the Dutch
physicist Hendrik Antoon Lorentz. It is a classical, phenomenological model for materials with characteristic
resonance frequencies (or other characteristic energy scales) for optical absorption, e.g., vibrations in ions andmolecules, interband transitions (semiconductors), phonons, and collective excitations. [149]As a spring-mass-damper system, an electron orbits a massive,
stationary nucleus [150]. A hypothetical spring connects the electron to
the nucleus, and a hypothetical damper dampens its motion. At its
resonance frequency, the oscillator's response is finite due to thedamping force. Newton's second law can be applied to the electron to
obtain its motion and expression for the dipole moment for a timeharmonic driving force generated by the electric field.

Equation of motion for electron oscillator:
𝐹𝑛𝑒𝑡 = 𝐹𝑑𝑎𝑚𝑝𝑖𝑛𝑔 + 𝐹𝑠𝑝𝑟𝑖𝑛𝑔 + 𝐹𝑑𝑟𝑖𝑣𝑖𝑛𝑔 = 𝑚

−𝑚 𝑑𝑟
𝜏 𝑑𝑡

𝑑 2𝑟
𝑑𝑡 2

− 𝑘𝑟 − 𝑒𝐸(𝑡) = 𝑚

𝑑 2𝑟
𝑑𝑡 2

𝑑 2𝑟
1 𝑑𝑟
−𝑒
+
+ 𝜔02 𝑟 =
𝐸(𝑡)
2
𝑑𝑡
𝜏 𝑑𝑡
𝑚
Where,⚫

r is the displacement of charge from the rest position,⚫

t is time,⚫

𝜏 is the relaxation time/scattering time,⚫

k is a constant factor characteristic of the spring,⚫

m is the effective mass of the electron,⚫

𝜔0 = √𝑘/𝑚 is the resonance frequency of the oscillator,⚫

e is the elementary charge,⚫

E(t) is the electric field.

131

For the time-harmonic field:
𝐸(𝑡) = 𝐸0 𝑒 −𝑖𝜔𝑡𝑟(𝑡) = 𝑟0 𝑒 −𝑖𝜔𝑡
The stationary solution of this equation of motion is:𝑟(𝜔) =−𝑒𝑚
𝐸(𝜔)𝜔02 − 𝜔2 − 𝑖𝜔/𝜏Because the given solution is complex, a time delay (phase shift) exists between the driving electric field and the
response of the electron's motion.
Now, in dielectric function:
Complex permittivity function using the Lorentz oscillator model can be written as:

𝑆𝑗,2𝜔𝑗 − 𝜔2 + 𝑖𝛾𝑗 𝜔𝑗𝜀(𝜔) = 𝜀∞ + ∑Here, 𝜀∞ is the permittivity in the high-energy region. 𝑆𝑗 represents the vibration strength, 𝜔𝑗 represents theresonance frequency and 𝛾𝑗 represents the relaxation constant, respectively. j represents the number of oscillators
to be considered, and in this analysis, its values are 𝑗 = 1, 2, 3, 4……etc.

132

Appendix D: Optical transitions in donor-acceptor-donor (DAD) model

In the main text, the double peak structure in the absorption spectrum of the photoinduced state has been discussed
and assigned the higher energy peak to the CT transition absorption due to the lattice modulation around Cl 2An2–
ions. The other works have already discussed the simplified picture of the CT excitation in the DA complex[139,141]. In addition, the existence of the higher energy peak has been mentioned in refs. [140] and [64].
However, there is a significant difference between the charge filling of previous works [140, 64, 151, 152] andour system. Then, the CT transition in the trimer model for our target material was evaluated here.

For simplicity, only the HOMO of D and the LUMO of A are considered for calculation. In the HT phase, thevalences in the trimer unit can be written as D2+−A2–−D2+. This trimer can be mapped to the two electrons in thetrimer unit when only the electron number that concerns the CT transition is considered. In addition, the energy
level of HOMO of D (ED) is lower than that of LUMO of A (EA), as shown in Fig. A1.

Fig. A1: The schematic representation of the energy levels of molecular orbitals for the DAD unit (HT phase).

When the double-occupied states can be neglected, considering large on-site Coulomb repulsion energy, the basisof the two-electron state can be written as follows.

|𝑎⟩ ≡ |110⟩, |𝑏⟩ ≡ |101⟩, |𝑐⟩ ≡ |011⟩

Here, the numbers in the ket represent the electron number in the DAD site.

133

The Hamiltonian for this system is, ℋ = ∑𝑖 𝐸𝑖 𝑛𝑖 − ∑𝑖,𝑗 𝑡𝑖𝑗 𝑐𝑗† 𝑐𝑖 . After the simple calculation, the matrix
representation of the Hamiltonian for the above three bases is:
𝐸𝐷 + 𝐸𝐴ℋ = ( −𝑡23
0

−𝑡32
2𝐸𝐷
−𝑡12

0
−𝑡21 )𝐸𝐷 + 𝐸𝐴

Diagonalizing the matrix, the solutions of the Schrodinger equation are as follows.
The eigenvalues:
𝐸0 − 𝐸1 , 𝐸+ , 𝐸0 + 𝐸1
Corresponding eigenvectors are:
𝑡32
𝑡32
−𝑡21
1
1
𝛹0 ≡ ( 𝐸− + 𝐸1 ) , 𝛹1 ≡ ( 0 ) , 𝛹2 ≡ ( 𝐸− − 𝐸1 )
2
2
𝑡23
𝑡12
𝑡12

1
1
where 𝐸± ≡ 𝐸𝐴 ± 𝐸𝐷 , 𝐸0 ≡ 𝐸+ − 𝐸− , 𝑊 ≡ 𝑡12 𝑡21 + 𝑡23 𝑡32, and 𝐸1 ≡ √𝐸− 2 + 4𝑊.
2

2

1
1
Noting that 𝐸− > 0 and 𝐸1 = √𝐸− 2 + 4𝑊 > 𝐸+ − 𝐸0 = 𝐸− , the three states' energy levels can be22

schematically drawn in Fig. A2a.

(a) 𝐸0 + 𝐸1𝐸+𝛹2(b) 𝐸0 + 𝐸1

𝐸0 − 𝐸1𝐸+𝛹1′

𝐸0 − 𝐸1

𝛹0′

𝛹1

𝐸0

𝛹2′

𝐸0
𝛹0

Fig. A2: Schematic energy diagram of the trimer model with two electrons. a. With lattice modulation. b. Withoutlattice modulation.

The ground state of the trimer model with two electrons is 𝛹0 , while the excited states are 𝛹1 and 𝛹2 . Therefore,
two CT transitions can be considered, 𝛹0 to 𝛹1 and 𝛹0 to 𝛹2 .

134

If we consider the no lattice modulation case, the values of 𝑡12 and 𝑡23 become equal, 𝑡 ≡ 𝑡12 = 𝑡23 , then the
eigenvectors can be expressed as:
11−1
𝐸− + 2𝐸1
𝐸− − 2𝐸1′′
𝛹0 ≡ () , 𝛹1 ≡ ( 0 ) , 𝛹2 ≡ (
)
2𝑡
2𝑡
1
1
1
′

𝛹0 ′ and 𝛹2 ′ have even parity for the inversion operation, while 𝛹1 ′ has odd parity. Then, the CT transition from𝛹0 ′ to 𝛹2 ′ is optically forbidden (Fig. A2b) in the no lattice modulation case, then only one CT transition can beconsidered.
From the above discussion, it is concluded that the lattice modulation around Cl 2An2– can induce the newabsorption at higher energy than the original CT transition without lattice modulation.

135

136

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Acknowledgements
This study has been carried out at Koshihara - Okimoto Laboratory in the School of Science,
Department of Chemistry, Tokyo Institute of Technology.
I would like to express my special thanks to the supervisor Prof. Shinya Koshihara for his
constant support and much insightful advice. I have no words to express my gratitude to mysupervisor for his invaluable patience and feedback. Additionally, this endeavour would nothave been possible without the generous support from Asst. Prof. Tadahiko Ishikawa. A special
thanks to him. I am grateful to Prof. Y. Okimoto and Asst. Prof. K. Takubo for their fruitful
discussion to complete my research work. The constant support of Mrs. H. Sakurai (our lab.
secretory) in solving academic and technical problems is also greatly appreciated.
I am greatful to Prof. H. Miyasaka (IMR, Tohoku University) and Assoc. Prof W. Kosaka for
their provision of intriguing samples and trustful cooperation. I also thank Prof. S. Iwai, Dr. T.
Amano, Assoc. Prof Y. Kawakami, Asst. Prof. H. Itoh in Tohoku University, Dept. of physics,
for their support in performing the 6 – fs experiment and fruitful discussions. I would also liketo thank the late Prof. K. Iwano (Institute of Materials Structure Science, KEK) for his support
in theoretical work.
I would also like to thank all the colleagues in our group for their benign association: Ms. M.
Kato, Mr. H. Yu, Mr. A. Sugisawa, Mr. S. Shimanuki, Mr. S. Ueno, Mr. X. Wang, Mr. K.
Kawasaki, Mr. Y. Saburi, Mr. K. Aoyagi, and Mr. T. Koike.
Furthermore, I am deeply grateful to my father (Late Sunil Kumar Banu), my mother (RitaBanu), and my family for their generous and encouraging support. Last but not least, I
acknowledge my friends (especially S. Panja), without whom I would have completed thisthesis way sooner.
For allowing me to conduct this research, I would like to thank the Tokyo Institute ofTechnology. My sincere gratitude goes out to all the funding and scholarships (mainly JST
SPRING, Grant Number JPMJSP2106) that have made my research life in Japan possible
without any difficulties.
Finally, I am grateful to my dissertation committee: Prof. Y. Okimoto, Prof. Y. Ohshima, Prof.
M. Kawano, and Prof. K. Taniguchi.

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Samiran Banu
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