Spiro-Functionalized Diphenylethenes: Suppression of a Reversible Photocyclization Contributes to the Aggregation-Induced Emission Effect

Many aggregation-induced emission (ME) materials are featured by the diphenylethene (DPE) moiety which exhibits rich photophysical and photochemical activities. The understanding of these activities behind ME is essential to guide the design of fluorescent materials with improved performance. Herein by fusing a flexible DPE with a rigid Spiro scaffold, we report a class of novel deep-blue material with solid-state fluorescent quantum yield (Phi(F)) up to 99.8%. Along with the ME phenomenon, we identified a reversible photocyclization (PC) on DPE with visible chromism, which is, on the contrary, popularized in solutions but blocked by aggregation. We studied the steric and electronic effects of structural perturbation and concluded that the PC is a key process behind the RIMs (restriction of intramolecular motions) mechanism for these materials. Mitigation of the PC leads to enhanced fluorescence in solutions and loss of the ME characteristics.