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Review of 'Indandiazocines: Unidirectional molecular switches'

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5
Sets the stage for calculations and experiments on molecular machines exploiting this chromophore
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    Rated 4.5 of 5.
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    Rated 5 of 5.
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    Rated 5 of 5.
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    Rated 4 of 5.
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Indandiazocines: Unidirectional molecular switches

 Bernd Hartke (corresponding) ,  Tim Raeker (2015)
We report theoretical investigations on azobenzene-based indandiazocines, novel chiral systems that perform unidirectional cis↔trans isomerizations upon photo-excitation. For three different systems of this kind, we have simulated excited-state surface-hopping trajectories for both isomerization directions, using a configuration-interaction treatment based on system-specifically reparametrized semiempirical AM1 theory. Our results are also compared to experimental and theoretical results for the parent system diazocine. We show that, as intended by design, the trans→cis bending of the azo unit in these indandiazocines can only happen in one of the two possible directions due to sterical constraints, which is a new feature for photoswitches and a necessary prerequisite for directional action at the nanoscale.
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    Review information

    10.14293/S2199-1006.1.SOR-CHEM.ARDTLN.v1.RKHAVH

    This work has been published open access under Creative Commons Attribution License CC BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com.

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    Review text

    The publication presents simulations of the smallest currently known chiral photoswitches, including extensive comparisons to non-chiral parent compounds, to which some of the methods have been previously parametrized and validated. The text is well written and can usually be understood without reading the previous work, although reference to that previous work is made rather frequently.  

    There are a few places where the self-contained character could be improved. In particular, Table 5 could make more explicit that the new results are all computational (repeat the level in the caption) and gas phase, because not all of the table entries are. The gas phase character of the simulations could also be made more explicit early on in the publication (earlier than page 6).

    This also touches one of the topics where the manuscript is hardest to digest, the influence of the solvent. In the parent compound there was a large solvent effect in the computations and only the solvent calculation came close to experiment. In the new compounds, where experimental values and solvent computations are unknown, the computed gas phase quantum yields do deviate from those of the parent compound and move in the direction of its solvent effect. This might be coincidental, but in the conclusions it is speculated that it may also be due to a solvent-like influence of the added ring frames. However, solvent-including simulations supporting this hypothesis are not presented for the new compounds.

    In the comparison of MP2 and AM1 calculations, some readers may be curious to learn about the computational speed-up involved in the AM1 approach. Also, the effect of quantum decoherence correction could be exemplified for one of the systems, as this is speculated to be one of the (smaller) reasons for deviations in the reference system.

    Overall, the manuscript nicely sets the stage for future calculations of molecular machines exploiting this chromophore (ref. 10), and of course also for future photoswitching experiments using these chiral molecules. It definitely confirms the design of these photoswitches to be unidirectional and is therefore important.

    Comments

    wrote:

    We would like to thank M. Suhm for the review of our contribution.
    By the way, this is a good example for several of the beneficial aspects of open peer review: Even some time after the original publication, reviews of it are still possible, in contrast to the traditional review process. Additionally, open peer review is more like a discussion which helps to clarify things also for other readers, instead of an examination for manuscript acceptance behind closed doors.

    In particular, Table 5 could make more explicit that the new results are all computational (repeat the level in the caption) and gas phase, because not all of the table entries are. 

    All table entries which we have not computed ourselves were explained further by a short description in the subcaption; this includes the level of theory and if they were obtained theoretically or experimentally. Nevertheless, we agree that mentioning the presence of an experimental value in this table would help to increase the readers' awareness.

     The gas phase character of the simulations could also be made more explicit early on in the publication (earlier than page 6).


    This paper follows the traditional custom of theoretical chemistry papers: If the solvent is included into the calculations, this is mentioned explicitly. However, if nothing about any solvent treatment is mentioned, then these are gas-phase calculations. We agree that it would be better to explicitly state the presence or absence of solvent in every case.

    [...] However, solvent-including simulations supporting this hypothesis are not presented for the new compounds. 

    Yes, we agree that simulations including solvent are important and should be done. Nevertheless, we decided to start the theoretical examination of these new molecules with gas-phase simulations only, for two reasons:
    1) Dynamic simulations with explicit solvent molecules require substantially greater effort, for the computations themselves but in particular also in their setup and analysis, as can been seen in Ref.8 (also from our work group). Therefore, we decided to wait for experimental values, to decide if simulations with explicit solvent are really necessary. If there are deviations between the experimental and theoretical values, the calculations will definitely be re-done, to check if these deviations arise from the missing solvent or from inherent errors in the semi-empirical CI treatment.
    2) There are indeed similarities between solvent effects observed in Ref.8 and effects seen in the present gas-phase study. Some of the prominent solvent effects found in Ref.8 for the parent system were these: For the Z isomer, one solvent positions itself between the two phenyl rings, causing an opening and thus a pre-orientation of the molecular frame towards the E isomer. For the E isomer, efficient solvent cooling after isomerization was found, i.e., transfer of vibrational energy from the solute molecule into the solvent. For the present molecular systems, we see these effects already without a solvent: The Z isomers already have an opened molecular frame; therefore, a solvent molecule between the phenyl rings would not cause a much bigger opening/pre-orientation.  For the E isomers, the additional internal degrees of freedom cool down the molecules more easily, by dissipation of kinetic energy away from the reactive centre, i.e., from the N=N bond.
    We agree that a more thorough explanation of this analogy in the original text would have made things clearer. 

    In the comparison of MP2 and AM1 calculations, some readers may be curious to learn about the computational speed-up involved in the AM1 approach. 

    The speed-up for FOCI-AM1 vs. MP2 is about 2-3 orders of magnitude, e.g. for Z-ID the calculations had a walltime of 22h17min with MP2/cc-pVTZ in contrast to 92s with FOCI-AM1.

     Also, the effect of quantum decoherence correction could be exemplified for one of the systems, as this is speculated to be one of the (smaller) reasons for deviations in the reference system.

    This is part of ongoing work, also involving other derivatives of these molecular switches. 

    2016-02-26 14:41 UTC
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