Investigation on CD Inversion at Visible Region Caused by a Tilt of the π‑Conjugated Substituent: Theoretical and Experimental Approaches by Using an Asymmetric Framework of Diarylethene Annulated Isomer

A substituent effect of asymmetric diarylethene annulated isomer on their chiroptical properties was investigated by means of theoretical and experimental approaches. The absolute configuration of the annulated isomer was determined by X-ray structural analysis and DFT calculation. The TD–DFT calculation successfully reproduced not only the sign but also the shape and magnitude of experimental CD spectrum by considering the Boltzmann-weighted average of four atrop-isomers. A fragment decomposition (FD) analysis of rotatory strength clearly revealed a noteworthy effect; the tilting motion concomitant with the rotating motion of the substituent affects the sign and magnitude of CD signals. It was found that even when the absolute structure of the chiral core moiety does not change, the slight motion of the substituent can trigger the inversion of the CD signal.本研究采用理论与实验相结合的手段，探究了不对称二芳基乙烯稠合异构体的取代基效应对其手性光学性质的影响。该稠合异构体的绝对构型通过X射线结构分析与密度泛函理论（Density Functional Theory, DFT）计算得以确定。含时密度泛函理论（Time-Dependent Density Functional Theory, TD-DFT）计算通过对四种阻转异构体的玻尔兹曼加权平均进行考量，不仅成功复现了实验圆二色（Circular Dichroism, CD）光谱的信号符号，还还原了其谱形与强度。旋转强度的片段分解（Fragment Decomposition, FD）分析清晰揭示了一项值得关注的效应：与取代基旋转运动伴随的倾斜运动，会对圆二色信号的符号与强度产生影响。研究发现，即便手性核心片段的绝对构型未发生改变，取代基的微小运动也可触发圆二色信号的反转。