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.