Control of Chiroptical Properties on Chiral Phosphangulenes Having Azobenzene Moieties by Photoirradiation Based on Synergistic Exciton Coupling

C₃-symmetric phosphangulene derivatives feature planar chirality with high inversion barrier due to the phosphorus atom. Chiral phosphangulenes have been extensively studied in recent years and have been reported to show exceptionally characteristic chiroptical properties, which are extraordinarily large values of anisotropy factor. In this study, phosphangulenes having three azobenzene moieties were designed and synthesized and their chiroptical properties were controlled upon photoisomerization of azobenzenes. The phosphangulenes having three azobenzene moieties exhibit the significant decrease of the Cotton effect upon trans-to-cis photoisomerization. This decrease was not merely due to a reduction in absorbance; calculation of the anisotropy factor revealed that the intrinsic chiral properties themselves had changed. TD-DFT calculations indicated that the observed circular dichroism originates from exciton coupling among the three azobenzene moieties. In addition, the effect of exciton coupling was found to decrease markedly upon isomerization of azobenzene to the cis form. In particular, exciton coupling implies a synergistic interaction among multiple azobenzene units, suggesting that the chiroptical properties change nonlinearly as the number of cis isomers increases due to this synergistic effect.