Molecular species investigated as possible candidates for molecular photoswitches often toggle between two (low and high conductance) conformations implying skeletal rearrangement, bond breaking, and substantial changes of molecular length. All these represent shortcomings that impede the switching speed and straightforward incorporation in nanodevices. In the present paper we propose a mechanism wherein the photoinduced switching is from a nonplanar conformation to a planar conformation, and involves neither skeletal rearrangement nor bond breaking or significant molecular length changes. Specifically, by choosing typical floppy molecules consisting of two benzene or benzene-like rings that can easily rotate relative to each other, we present results of both ab initio and DFT quantum chemical calculations demonstrating that the lowest electronic excitation corresponds to a planar molecular conformation (φ = 0), in contrast to the nonplanar ground state characterized by φ ≠ 0. Because the low bias conductance scales as G ∝ cos² φ, the planar conformation has a higher conductance than the non-planar conformation, acting therefore as ON and OFF states of the molecular switch, respectively. We analyze recent experimental data on illuminated single-molecule junctions (E.-D. Fung et al., Nano Lett., 2017, 17, 1255) and show that the measured photoinduced conductance enhancement is consistent with the presently proposed mechanism. Furthermore, based on recent results demonstrating the substantial impact of the SAM coverage on the twisting angle (I. Bâldea, Faraday Discuss., 2017, 204, 35) we show that a photoinduced conductance enhancement can be much stronger than the rather modest enhancement obtained in the aforementioned experiment.

中文翻译：

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软盘分子是实现光电分子器件而无骨架重排或键断裂的候选

作为分子光开关的可能候选物而研究的分子种类通常在两种（低电导率和高电导率）构象之间切换，这意味着骨架重排，键断裂和分子长度的显着变化。所有这些都代表了阻碍开关速度和直接集成到纳米器件中的缺点。在本文中，我们提出了一种机制，其中光诱导的转换是从非平面构象到平面构象，并且既不涉及骨架重排，也不涉及键断裂或显着的分子长度变化。具体来说，通过选择由两个易于相对旋转的苯或类苯环组成的典型软盘分子，我们可以得出两种从头算的结果和DFT量子化学计算表明最低电子激发对应于平面分子构象（φ = 0），而相比之下，其特征在于，该非平面基态φ ≠0。因为低偏压电导秤作为g ^ αCOS ²  φ，则平面构象比非平面构象具有更高的电导率，因此分别充当分子开关的ON和OFF状态。我们分析关于照射的单分子结最近的实验数据（E.-D.丰等人，纳米快报。，2017年，17（1255），并显示所测得的光诱导电导增强与目前提出的机制是一致的。此外，根据最近的研究结果证实了SAM覆盖的扭转角的实质性影响（一Bâldea，法拉第讨论。，2017年，204，35），我们表明，光诱导电导增强可以比获得相当温和增强强得多在上述实验中。