摘要/Abstract

依据B=N键与C=C键的电子结构相似性，以Feringa型二苯乙烯型光驱动分子马达（CC-stilbene）为母体，设计了含极性旋转轴的模型马达BN-stilbene.CASPT2//CASSCF计算结果表明，优化所得的BN-stilbene分子的基态存在四个与CC-stilbene马达结构相似、相对能量一致的螺旋异构体；B=N极性共价双键对BN-stilbene的基态和激发态电子结构有显著影响.对BN-stilbene模型马达的工作机理研究表明，极性旋转轴的引入使得BN-stilbene中S₁/S₀-CI与激发态中间体构型更加相似且能量更低，同时可增加旋转的驱动力，起到改进分子马达光异构化过程的单向性的目的.
关键词: 光异构化, 分子马达, 圆锥交叉, 无辐射跃迁, 极性旋转轴
Light-driven molecular motors have attracted overwhelming attention due to their potential applications in a wide range of fields. Despite of the great successes obtained in alkene-based light-driven molecular motors and switches, scientists pursuing high-efficient alternatives with superior working mechanisms have never suspended. In this report, a promising model of light-driven rotary motor, namely BN-stilbene motor, constructed by replacing the central C=C axis of a CC-stilbene rotary motor with a polar B=N bond, was rationally designed. Multireference Complete Active Space Self-Consistent Field (CASSCF) method and Time-Dependent Density Functional (TDDFT) theory were applied to study the mechanism of BN-stilbene, along with the Complete Active Space Second-Order Perturbation Theory (CASPT2) energy corrections. Our calculations show that the B=N axis well preserves the conjugation of between the rotor and stator, leading to four ground-state helical conformers (i.e., cis-stable, trans-unstable, trans-stable and cis-unstable), whose geometries and energies are in line with their counterparts in CC-stilbene motor; in addition, BN-stilbene has similar absorption spectra and more slopped excited-state potential energy curves at Franck-Condon region, which can fascinate a spontaneous rotary motion around B=N axis, thus generates directional photo-induced isomerization from cis-stable to trans-unstable (or from trans-stable to cis-unstable). Moreover, the barriers for helical inversions (trans-unstable → trans-stable or cis-unstable → cis-stable) are found to be lower than those of the reversed thermal rotations (i.e., cis-stable → trans-unstable and trans-stable → cis-unstable), which further insures the unidirectionality of rotation. These features sufficiently allow BN-stilbene to serve as a candidate for light-driven molecular rotary motor. Finally and most importantly, as compared with that of CC-stilbene, the photoisomerization mechanism of BN-stilbene motor shows advantages in nonadiabatic transition:Due to the introducing of polar B=N axis, the S₁/S₀ conical intersections of BN system are both geometrically and energetically closer to the excited-state intermediate, which is thus expected to improve the nonadiabatic transition probabilities and the unidirectionality of the rotation. Therefore, the BN-stilbene motor is expected to perform a unidirectional, repetitive 360° rotation upon sequential applying of photo and thermal inputs. The findings suggest BN-hetero stilbene as a promising type of light-driven rotary motor and may inspire the design and synthesis of novel molecular motors.
Key words: photoisomerization, molecular motor, conical intersection, nonradiative transition, polar axis of rotation


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