Chemically Optimizing Operational Efficiency of Molecular Rotary Motors
Chemically Optimizing Operational Efficiency of Molecular Rotary Motors
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DOI：
10.1021/ja5041368
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发表时间：
2014-07-09
影响因子：
15
通讯作者：
Meech, Stephen R.
中科院分区：
文献类型：
--
作者：
Conyard, Jamie;Cnossen, Arjen;Meech, Stephen R.
利用光诱导顺反（E-Z）异构化的单向分子旋转电机是将光能转换为纳米级分子机器中的机械运动的有前途的工具。在优化基态旋转频率方面已经取得了相当大的进展，但对激发态过程的关注较少。这里研究了分子马达的激发态动力学，该分子马达具有电子供体和受体取代基，其位置可以修改激发态反应坐标，而不改变其立体化学。取代基显示出可以改变异构化的光化学产率而不改变电机频率。通过将 50 fs 分辨率的时间分辨荧光与超快瞬态吸收光谱相结合，可以表征潜在的激发态动力学。弗兰克-康登激发态在几百飞秒内弛豫，通过利用体积守恒结构变化的途径填充较低能量的暗态。这归因于异构桥双键的碳原子处的金字塔化。由此达到的暗态的结构和能量是取代基的函数，吸电子基团产生较低能量且寿命较长的暗态。暗态与弗兰克康登态耦合，并通过对溶剂摩擦敏感的坐标（例如绕桥键的旋转）在皮秒时间尺度上衰减。亚皮秒和皮秒动力学对溶剂极性都不敏感，这表明分子内电荷转移和溶剂化不是反应速率的关键驱动力。相反，空间因素和介质摩擦决定了反应途径，空间远程取代主要影响能量。因此，这些数据表明了一种化学方法，可以优化这些分子马达的运行效率，而不改变它们的整体旋转频率。
Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited state processes. Here the excited state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency.