Peer review of: Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X- (2020)

Reviewers' comments: 

 

Reviewer #1 (Remarks to the Author): 

 

The authors investigated the photoinduced electron transfer dynamics of [Fe(bmip)2]2+ in 
acetonitrile solution using time-resolved X-ray emission spectroscopy (TR-XES) and time-resolved 
wide angle X-ray scattering (TR-WAXS). The results from two complementary tools unveiled 1) the 
3MC wave packet motion with the time constant of 278 fs and 2) the MLCT to 3MC electron transfer 
with the time constant of 110 fs. 

The overall data reduction procedure and the level of simulation seem suitable for their final 
arguments. I can hardly find any serious logical jump throughout the reasoning of both TR-WAXS and 
TR-XES. Besides, these two tools show clear accordance in conclusion. Finally, the target itself is 
worth being studied considering its potential as a photosensitizer. Therefore, I strongly recommend 
the publication of this manuscript if the authors can clarify some ambiguities regarding the detailed 
analysis. 

1. The term “wide-angle X-ray scattering (WAXS)” can confuse readers. WAXS was coined to 
distinquish itself from SAXS (small-angle X-ray scattering) for macromolecules. In other words, WAXS 
has a relevant meaning with respect to macromolecules. Therefore, instead of TR-WAXS, I suggest 
time-resolved X-ray solution scattering (TRXSS) or time-resolved X-ray liquidography (TRXL). 

2. In page 10 line 320, the authors have assigned the 500 fs decay to the vibrational cooling, which is 
faster than usual values (~10 ps) from similar studies. Also, it does not coincide with the values that 
can be approximated from the classical diffusion theory of Landau and Lifshitz. Further qualitative 
reasoning for the conclusion should be provided. 

3. In Fig. 5C, the simulations and the experimental data look quite different from each other. While 
slight differences in low-q regions (up to 1.5~2.0 Å-1) are relatively understandable, the peak 
positions in high-q range that encrypts the structural information must be taken into account with 
certain accuracy. I suggest, for example, releasing more degree of freedom in molecular structure to 
fully make use of the strengths of the X-ray solution scattering data. 

4. Among the biexponential dynamics toward the ground state, the faster one with the time 
authors cited [38], this kinetics was not visible not only in the excited state absorption but also in the 
ground state bleaching, in contrary to the fact that the ground state population must be changed 
even if it is assumed that the authors’ assignment is correct. I suggest giving a plausible explanation 
for such disagreement between the two studies. 

5. In the supplementary information, Fig. S28 is missing.
6. The authors argue that in the SVD, only the highest 3 components are above the noise level. Then, 
I wonder whether it is mathematically reasonable to extract 6 time-dependent components from the 
global analysis using the same data set. Additionally, I am satisfied by the fit quality shown in Fig. S29 
considering the clear discrepancies in Fig. 5(C). 

7. The reported relaxation kinetics of the optical Kerr effect (OKE) in acetonitrile last far longer than 
the IRFs, and therefore should not be fit to a simple Gaussian function. 

8. In section 15 of the SI, the authors mentioned that the solvent heat signal can be described by two 
rise constants. I wonder if the authors just phenomenologically exclude those for the analysis by 
fitting the time axis and the authors have ever thought of a physical origin for each of them. I 
suggest that the authors can add a relevant discussion. 

9. In page 4 line 108, I recommend to cite previous studies (on [Fe(bimp)2]2+ complex) in “contrary 
to prior ultrafast XES measurements, the Kα XES signal shows the signature of vibrational 
wavepacket dynamics”. 

10. It is required to state standard deviations for the structural parameters fitted in the overall TR-WAXS
analysis.

11. In page10 line 340, the subtitle “Origin of the XES oscillations” seems to exaggerate the contents 
of the paragraph as no comments were given regarding the oscillatory motion that might exist in the 
3MLCT state. 

12. The detailed explanations for each subfigure lack in the caption of Fig. S31. 

13. It is better to use equal offsets for each scattering curve. For example, the top two curves of Fig. 
S30 may distract readers even though the MLCT* and the OKE signal do not have any real physical 
relationship. 

14. The citation for previous time-resolved X-ray solution scattering is too much biased toward their 
own work (more than 10 self citation versus 1 citation of Ihee group as an example). This should be 
improved by citing the works from other groups in proportion to the quantities of published works. 

 
 

Reviewer #2 (Remarks to the Author): 
Report on manuscript “Origin of vibrational wavepacket dynamics…” by Kuunus et al. 


This manuscript contains a fs WAXS/XES experiment on solvated Fe(bmip)2 photosensitizer ions, 
where both tools show a 278 fs period oscillation, identified as the Fe-ligand stretching mode on a 
3MC excited state. This is rationalized by a branching of the initially photoexcited reactant in the 
MLCT manifold to the (vibrationally active) MC states, while a larger fraction undergoes internal 
conversion into the lowest excited 3MLCT manifold, which is not vibrationally active. 

Experimentally, this is certainly groundbreaking work, employing both WAXS and XES in a 
femtosecond experiment, and utilizing each tool for the analysis and interpretation, which nicely 
merges into one picture. The analysis utilizes all aspects of data analysis and theoretical modeling, 
including structural models for WAXS, and RASSCF calculations for the XES, and the observed 
wavepacket dynamics are just beautiful. As such it warrants publication in this journal. 

For a better understanding by the reader I would suggest the authors to go over their assessments 
once more, and I outline a few key points I noticed: the reactant Fe(bmip)2 has an optical absorption 
strength of about 1000 /(M cm) in Fig. 1 of the main text. Therefore I would estimate for the OD 
shown in the SI 

OD(50 um, 20.3 mM, 400nm) = 0.005cm x 0.0203mM x 1000/(M cm) = 0.1015, and not 0.59 

This will become relevant when estimating the photoexcitation yield, which – in the analysis (and 
further presented in the SI) – was determined to lie in the 74-94 % range. I would suggest to clarify 
these issues (or possibly correct), also my idea that 45 mJ/cm2 corresponds to 4 uJ pulse energy. In 
order to achieve such a high excitation yield I would expect a laser fluence, which corresponds to 
several photons pointing to a single absorption cross section, and clarification of these fluence issues 
would make the presentation more clear 

The XES analysis is entirely governed by the suitability of the correction applied to the falsely aligned 
4th XES crystal yielding a distorted GS spectrum and subsequent transient spectra, as demonstrated 
in the SI. This is an important correction, and should therefore be mentioned in the main text (while 
the treatment itself can remain in the SI). I have to admit that I cannot really imagine how 
overlapping energy regions in the pumped spectra can be so reliably “resorted” in the analysis, but 
this may be explainable. 

Apart from the need to clarify the experimental conditions, I applaud the authors to this truly 
pioneering study of the excited state pathways in unusual transition metal complexes. 

 

Reviewer #3 (Remarks to the Author): 

This work presents an experimental investigation of the ultrafast dynamics following the 
photoexcitation of a Fe3+ complex. The dynamics of the initially produced MLCT(1) state is followed 
at three different levels simultaneously in the experimental setup, namely K\alpha and K\beta XES 
and WAXS. 

On the basis of the combined analysis of the three signals and computational modeling of the x-ray 
spectra, the authors put forward a kinetic model that explains the short-time dynamics and 
subsequent relaxation, as well as the origin of the sensitivity of the XES signal to the wavepacket 
dynamics in the valence states. 


The latter is very illustrative. Following the conclusions of this work, it becomes a simple matter to 
pre-compute, for a given system, how sensitive a TR-XES spectrum will be to structural wavepacket 
dynamics. 


The paper is clearly and concisely written. The steps of the analysis leading to the conclusions are 
explained in much detail in the main paper and SI. Although I am not an experimentalist, I have the 
impression that this level of detail renders the various steps reproducible. Also, the theoretical 
results are explained with a sufficient level of detail and are performed at a high level of theory. 

 

The paper will likely promote the further application of XES as a sensitive tool to follow structural 
dynamics in the fs time-scale and potentially influence future work. 

 

I have a few questions mostly related to mechanistic aspects of the considered process. The paper 
should, in my opinion, be published, provided these can bee convincingly addressed. 

 

Questions: 

(1) Maybe I missed the point but I wonder if the authors considered the direct decay MLCT(3) -> GS 
pathway as a possibility in their kinetic models. In Fig. 7 the MLCT(3) -> MC(3) channel is marked 
with a 9ps time-constant, but it could equally, at least in part, go down to GS directly with a good FC 
overlap. 


(2) The MC(3) state decays with a time-constant of 1.5ps to the GS. According to the scheme of 
energy levels in Fig. 7, though, it would have a sharp crossing with the GS at a quite elongated 
geometry, such that the intersystem crossing through this channel might display some of the 278fs 
oscillatory dynamics as well. Is this something that could be maybe buried in the WAXS signal or 
somewhere else?
 

(3) The paper alludes to Jahn-Teller effects in the MC(3) state after photoexcitation, which is the 
consequence of the high symmetry of the complex. Yet the scheme in Fig. 7 refers only to one single 
(effective?) Fe-L bond length. 

 

How is this considered in the analysis? For example, are the XES spectra calculated for geometries 
with equal elongation of all Fe-L bonds, or along a symmetry coordinate? The computational details 
mention that one of the geometries correspond to the MC(3) minimum. Are the Fe-L distances all 
equal or different in this geometry? 


Does this make a difference for the vibronic coupling responsible for the XES sensitivity? 


(4) Following up on a similar token, I presume there must be energy values along the corresponding 
valence and core-excited PES for the various states involved. These could be used in conjunction 
with Fig. 7 or in the SI to further support the kinetic model. How do these PES look like?