Two-dimensional electronic spectroscopy of bacteriochlorophyll a with synchronized dual mode-locked lasers

How atoms and electrons in a molecule move during a chemical reaction and how rapidly energy is transferred to or from the surroundings can be studied with flashes of laser light. However, despite prolonged efforts to develop various coherent spectroscopic techniques, the lack of an all-encompassing method capable of both femtosecond time resolution and nanosecond relaxation measurement has hampered various applications of studying correlated electron dynamics and vibrational coherences in functional materials and biological systems. Here, we demonstrate that two broadband (>300 nm) synchronized mode-locked lasers enable two-dimensional electronic spectroscopy (2DES) study of chromophores such as bacteriochlorophyll a in condensed phases to measure both high-resolution coherent vibrational spectrum and nanosecond electronic relaxation. We thus anticipate that the dual mode-locked laser-based 2DES developed and demonstrated here would be of use for unveiling the correlation between the quantum coherence and exciton dynamics in light-harvesting protein complexes and semiconducting materials.

In ms. NCOMMS-20-27941-T, the experimental concept is novel and compelling. The experiment appears to be cleverly and rigorously performed. The experimental demonstration is highly convincing and presented honestly, in a balanced way. The discussion is inspiring and clearly identifies new opportunities that can be investigated with the new technique of SM-2DES. Through the multiple rounds of reviews, the authors have come up with a manuscript that is incredibly well written and well presented. It is both accessible to nonspecialists and detailed and insightful to answer the interrogations of other specialists in the field and to allow duplication of the set-up.
I also believe that it is of prime importance for spectroscopists working in chemistry and in physics that new instrumental techniques are proposed and thoroughly characterized. This is what the authors do in this manuscript with their instrument of multidimensional spectroscopy to study photon-echoes. Proof-ofprinciple experiments are necessary to evaluate and quantify the potential of a new experimental technique and to assess its novelty. Progress in the understanding of the structure and dynamic of matter relies on the ingenuity of scientists to invent instruments that measure what could not be measured before, or that significantly improve existing capabilities. From this point of view, instrumentation for spect roscopy is of broad and multidisciplinary interest. It is key to ground-breaking studies in chemistry.
I am deeply convinced that NCOMMS-20-27941-T establishes a new technique that will inspire many scientists working in chemistry and in physics. As a scientist working in the field of frequency-comb spectroscopy for many years, I also see NCOMMS-20-27941-T as an important milestone for the spread of frequency-comb spectroscopy to chemistry and biology, where the huge potential is still largely uncovered. This is excellent work and I am sure NCOMMS-20-27941-T will be a high-impact paper.
As I said above, [redacted], with the strongest enthusiasm, I have no hesitation in recommending its acceptance by Nature Communications.
Reviewer #2 (Remarks to the Author): The authors have yet again substantially improved their manuscript and clarified some issues. However, I still have some reservations regarding the appearance and analysis of the data, as I do not think that presented data and analysis provide substantial support for the conclusions written in the manuscript. That being said, I find the developed method very elegant, which most likely will find applications and will help to achieve deep understanding of photophysical processes. I would also like to remark that it is rare to come across another paper, which would match the tremendous effort invested in the present project and manuscript. In summary, as a technical demonstration of the method, which is expected to be of interest to a broad audience, I can recommend it to be published in Nature Communications.
Some minor remarks n, m, and p, which are the frequency comb mode numbers, should be defined in the text . Editorial Note: This manuscript has been previously reviewed at another journal that is not operating a transparent peer review scheme. This document only contains reviewer comments and rebuttal letters for versions considered at Nature Communications. Mentions of the other journal and prior referee reports have been redacted.

REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): lacking something.
Line 224, in general, laser spectrum effects 2D spectra on both excitation and detection frequencies, and not only on the detection frequency as stated.
Lines 284-286 The provided possible explanation for the lack of the 100 ps relaxation for the SE1 signal is inaccurate. SE1 signal originates from the Qy,0 state and not from Qy,1 state as written .
The sentence starting with "Fractions of..." on the line 459 is not complete.
Lines 496-498. I do not think that the statement "Thus, conventional 2DES techniques utilizing multiple pulses and translational stages cannot provide absolutely quantitative 2DES data even though the phase of the signal electric field could be numerically corrected by matching the projected 2DES spectra to the independently measured PP spectra" is absolutely correct. It should be possible to do so, as I believe all the necessary information for obtaining absolutely quantitative 2DES data is in principle available, especially from the pump-probe based 2D setups. This requires measuring PP with the same instrument under the same conditions, just like in the presented work.

Reviewer #1 (Remarks to the Author):
I had already reviewed manuscript NCOMMS-20-27941-T when it was submitted to In ms. NCOMMS-20-27941-T, the experimental concept is novel and compelling. The experiment appears to be cleverly and rigorously performed. The experimental demonstration is highly convincing and presented honestly, in a balanced way. The discussion is inspiring and clearly identifies new opportunities that can be investigated with the new technique of SM-2DES. Through the multiple rounds of reviews, the authors have come up with a manuscript that is incredibly well written and well presented. It is both accessible to nonspecialists and detailed and insightful to answer the interrogations of other specialists in the field and to allow duplication of the set-up.
I also believe that it is of prime importance for spectroscopists working in chemistry and in physics that new instrumental techniques are proposed and thoroughly characterized. This is what the authors do in this manuscript with their instrument of multidimensional spectroscopy to study photon-echoes. Proof-of-principle experiments are necessary to evaluate and quantify the potential of a new experimental technique and to assess its novelty. Progress in the understanding of the structure and dynamic of matter relies on the ingenuity of scientists to invent instruments that measure what could not be measured before, or that significantly improve existing capabilities. From this point of view, instrumentation for spectroscopy is of broad and multidisciplinary interest. It is key to ground-breaking studies in chemistry.
I am deeply convinced that NCOMMS-20-27941-T establishes a new technique that will inspire many scientists working in chemistry and in physics. As a scientist working in the field of frequency-comb spectroscopy for many years, I also see NCOMMS-20-27941-T as an important milestone for the spread of frequency-comb spectroscopy to chemistry and 1 2 biology, where the huge potential is still largely uncovered. This is excellent work and I am sure NCOMMS-20-27941-T will be a high-impact paper.
As I said above, [redacted], with the strongest enthusiasm, I have no hesitation in recomme nding its acceptance by Nature Communications.
Authors' Reply. We are really grateful to the reviewer #1 for her/his strong support for our work and recommendation of our manuscript for publication in Nature Communications.