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.%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/So-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.
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