Torsional Disorder in Tetraphenyl [3]-Cumulenes: Insight into Excited State Quenching

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This investigation, undertaken by Musser et al., delves into the mechanisms governing the quenching of excited states in tetraphenyl [3]-cumulenes. The authors substantiate their assertions predominantly through femtosecond transient absorption experiments, demonstrating meticulous control over environmental conditions to modulate the torsional degrees of freedom inherent in phenyl moieties. Preceding this inquiry, the specific quenching mechanism in derivatives of tetraphenyl [3]-cumulene remained largely nebulous, especially in elucidating the predominant deactivation pathway between phenyl or cumulene rotation. The authors have effectively addressed this ambiguity by employing two distinct molecules, namely the reference TPC and the controlled variant TPCb. In these instances, phenyl rotation is either constrained or facilitated contingent upon the presence or absence of a covalent bond between two phenyl groups. Furthermore, the researchers implemented sophisticated methodologies such as viscosity manipulation and pump wavelength adjustment to provide additional clarity on the underlying mechanism. The manuscript exhibits exceptional craftsmanship, presenting experimental data of high caliber. Consequently, I recommend its unaltered publication in the journal Photochem.

 

Author Response

We thank Reviewer 1 for their positive feedback. Reviewers 2 and 3 provided additional feedback that led us to make changes to the manuscript as addressed in our further responses.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors perform a detailed transient absorption spectroscopy analysis on a set of model [3]-cumulene derivatives in a wide range of environmental conditions, e.g., by changing the solvent polarity or the viscosity. 

They demonstrate how the interplay between phenyl group torsion and central cumulene bond rotation results in excited state quenching. They ultimately attribute quenching of the excited state to planarization of the phenyl groups, which proceeds through either a conical intersection or other vibronic mechanism back to the ground state. Planarization of phenyl groups therefore quenches the excited state, by increasing the lifetime up to a few nanoseconds.   The manuscript is well written and I believe the investigation and the experimental data are very much interesting and solid.   I have few points I would like the authors address before considering the manuscript suitable for publication.  

• The authors often use the terms “order/disorder": I think this is somehow an improper use of the terms. I would specify conformational order/disorder. Depending on the scientific communities, order/disorder might have very different meanings.

 

• Is it really a single exponential decay (for TCP) a suitable model to fit the data? What about the early relaxation times (<1ps)?

 

• How the authors justify the following sentence: “Regardless of the initial geometry, rapid geometrical relaxation in solution means that the same configuration for ultrafast relaxation can always be accessed”?

 

• How can TPCb leaves the cumulene bond rotation unhindered? Such torsional variation can always happen, especially in the excited states where the bond order changes due to a different electron density.

 

• How can the authors justify the following sentence: “the decay dynamics of TPCb are nearly identical in solution and matrix, where we selectively eliminate cumulene bond rotation as a decay channel”. I would not be so sure about the elimination of the bond rotation. To my understanding this is rather a (quite probable) speculation, however no real proofs are provided supporting such hindering.

 

• How can the authors justify the following sentence: “We find that the planar structure of TPCb must be very similar to the ideal quenching geometry accessed dynamically in TPC."

 

• What about the - probable - different cumulene bond rotation dynamics between TPC and TPCb. What can the authors say about this point?

 

• I would ask to report the PIA redshift in solution: TPC vs. TPCB, as well as the PIA redshift in matrix: TPC vs. TPCb. From the data it seems that in matrix, for both species, no shifts occur, while in solution the shifts are quite different/scattered between TCP and TCPb.

 

• What is "the level crossing geometry that enables rapid relaxation to a hot ground state”? I found also this explanation quite speculative.

 

• What is the mechanism explaining the internal decay between S1 and S0, namely the conical intersection? 

 

• What about the calculation of the torsional disorder (i.e., via CREST) for TPC in S0?

 

• What about the calculation of the central cumulene torsional barrier in S0 and in S1 for both TPC and TPCb?

  I think the authors should address the above points before reconsidering the manuscript.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Cumulenes are a class of chemicals with consecutive double bonds that are under investigation for their potential as electrocatalysts and other optoelectronic applications. Of particular interest are cumulenes functionalized by phenyl groups such as TPC. These structures have been shown to rapidly quench by internal conversion, with proposed explanations that include twisting around the cumulene bond, or by the rotation of attached phenyl rings. This work by Bain et al. seeks to uncover the connection between these two modes of molecular motion and excited state quenching, primarily through the use of transient absorption.

Viscous surroundings decrease the ability of molecules to undergo confirmational changes. The hypothesis put forth by Bain et al. is straightforward, if the rotational motion of the phenyl groups is constrained, then any change to the TA lifetime as the viscosity of the matrix is increased is due to restriction of the cumulene bond twisting motion. This simple hypothesis was complicated by the unexpected rapid quenching of TCPB in both high and low viscosity. The authors reasonably interpret this as the phenyl rings being confined to a position that favors rapid internal conversion. This is supported by computational work largely contained to the S.I.

The authors conclude that excited state quenching is controlled primarily by the planarization of the phenyl groups. The data supports the conclusion that the rotation of the phenyl rings is the dominant factor.  However, it is not conclusively shown that it is explicitly planarization. (a small technicality) In addition, this paper does not adequately address the extent of the contribution of cumulene twisting. The discussion of cumulene bond twisting is limited to an assignment to a small redshift in the TA signal. Based on previous studies on similar structures the authors interpret this as a sign that both modes are active in low viscosity matrix. While the authors have shown that quenching due to phenyl group rotation has a rate on the order of a few ps^-1 it is unclear how it compares to the nonradiative rate due to cumulene twisting. Most interestingly from the TA data given in Figure 4, it appears that the nonradiative rate of TPCB slows by a significant fraction as the viscosity is increased. This is partially obscured by the large squares/diamonds used for plotting.

The insights that the authors have gathered from this study leads them to propose a series of new investigations into steric and structural variations of this system that have a high probability of increasing the QY of phenyl substituted cumulene species.

 

Recommended Revisions

A plot of the decay rates vs mol fraction of viscous material or similar. The decay rate of TPCB shows a small variance with viscosity that would logically be caused by the changes in cumulene bond twisting. This allows for a rough calculation of both rates and has large implications for whether or not phenyl group rotation is the only factor leading to internal conversion or just the fastest by some margin.

Recommend plotting the corresponding normalized absorbance and PL next to the TA data to allow for easy comparison.

Figure s9 shows that computational modeling of TPC has good agreement with the experimental data. However, the modeling is a very poor match to the experimental data for TPCB. Why? A brief discussion could be included in the S.I.

The data in their S8 hints at an even less optimal phenyl ring position at 90 degrees. Computational modeling was not extended to this region. While the 1/12 ps^-1 decay rate of TCPB is in relatively close agreement with the 1/3 ps^-1 decay of TCP, a less favorable angle may account for the difference. Could the calculations be extended to 90 degrees?

 

It would benefit the SI to include page numbers.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised the manuscript and replied to the reviewer's comment appropriately. I believe the manuscript is complete and the data/discussion are robust, nevertheless I still think that some aspects of the decay mechanisms characterising such molecules  are not completely unveiled. As the authors mentioned, it would be interesting to unlock the initial stages of the decays, something that now is uncomplete.