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Review of 'Indandiazocines: Unidirectional molecular switches'

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Computational study of the photoisomerization of two indiandiazocines with surface hopping.
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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.RGRFDI

    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 potential energy surfaces are obtained on the fly
    using a semiempirical reparameterized AM1 Hamiltonian, with the FOMO-CI
    approach. A serious effort is made by the author to validate the
    semiempirical data (energies and optimized geometries) with respect to
    ab initio and experimental values, so that the reader may have an idea
    of the accuracy of the results.

    The nonadiabatic molecular dynamics results are compared with that of
    the parent compound brAB; interestingly, the authors found that the
    increasing steric effect from Z-ID to Z-DIDmeso to Z-DIDrac, plays a
    similar (but reversed) role with respect to solvent in brAB.

    I only have some minor remark:
    1) Why, for the dynamics starting from the E isomers, the early surface hops to S0 and the subsequent revival are claimed to be an artifact of the surface hopping approach, in particular connected with quantum decoherence effects? In fact, as the early hops to S0 and the backward ones happen really early (within the first ~20 fs), the quantum decoherence should play very little or no role. This behavior was also noted in the parent compound brAB, and attributed to the presence of a plateau in the S0 PES: if this is the case for ID
    and DID too, then the revival in the S1 population is due to a single
    passage through an extended strong interaction region (rater than to a
    recrossing of a localized strong interaction region) and then, again,
    the decoherence should play no role.

    2) The semiempirical S0->S1 transition energies for the E isomers appear
    to be too low. While I do agree with the authors that this most probably
    do not change the qualitative pattern of the dynamics, something can be
    said on the influence that this may have from the quantitative point of
    view. In particular, both the S1 lifetime of the E isomers and the E->Z
    photoisomerization quantum yields are probably underestimated.

    3) It would be of help to the reader if the authors could add PES cuts
    along the CNNC coordinate for the S1 and S0 states of the three isomers.

    Comments

    We would like to thank G. Granucci for the review of our article. We largely agree with his remarks. Nevertheless, we provide brief answers to his issues 1-3 here, for further illustration: 1) We agree that the quantum decoherence correction developed by Granucci et al. is more important in situations where the gradients of the S1 and S0 potential energy hypersurfaces differ strongly (which causes the corresponding parts of the nuclear wavepacket to propagate away from each other, reducing their coupling), and that this is _presumably_ not the case at such short times after the system has left the Franck-Condon region. However, the original intention of the present study was to provide initial and qualitative support to our experimental partners, rather than to establish a quantitative and global view of the dynamics and of the S0/S1 surfaces. As noted in our answer to issue 3, PES scans are less straightforward for these systems than for the parent compound AB. Therefore, at this point in our investigation of these systems, we do not yet have sufficient information on S0/S1 surface features in all regions of interest. Nevertheless we tried to explain some features in our data, trying to indicate that these statements are speculative hypotheses. Of course we acknowledge that G.Granucci has more experience with quantum decoherence corrections to trajectory surface hopping and may hence judge its effects differently. 2) We are aware of this problem, but the intent of this article was the introduction of indandiazocines to the community and their general features and advantages compared to e.g. brAB, so qualitative trends between the lifetimes and quantum yields are more than sufficient. We agree with G.Granucci's estimate but would like to defer further discussion to the near future when experimental data for these properties hopefully become available. 3) One has to be careful in attempts to explain multidimensional dynamics via one-dimensional cuts, but ultimately we do agree also on this point. Nevertheless, we also want to point out the difficulty of such a scan of the CNNC coordinate: Since no free rotation around the NN double bond is possible (in constrast to azobenzene), for such bridged systems an automatic scan is challenging, because the dihedral angles of the ethylenic bridge have to be allowed to take on reasonable values at the same time. However, this should indeed be less problematic for the indandiazocines than for brAB. Therefore, we appreciate the suggestion and will make use of it in future work on these and similar systems.

    2016-02-26 21:27 UTC
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