Cage structure helps to improve the photoisomerization efficiency of azobenzene

Azobenzene (Azo) usually cannot achieve relatively high photoisomerization efficiency because of the overlap of the n-π* absorption bands between their isomers. In this work, three Azo units were integrated into a cage (AC) and adopt a nonplanar configuration. The distortion of the Azo units in AC results in the reduction of the n-π* band overlap, and consequently increases the full E → Z and Z → E photo-conversions to 87% and 85%, respectively, which are even higher than those of the planar parent azobenzene ADA (85% of E → Z and 76% of Z → E). Further theoretical calculations demonstrate that the n-π* excitation energy gap between the two AC isomers is larger than that of ADA, which further supports the reduction of the n-π* overlap. This AC shows the highest E ↔ Z photoisomerization efficiency among the symmetrical organic covalent cages yet reported.