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

Many aggregation-induced emission
(AIE) materials are featured
by the diphenylethene (DPE) moiety which exhibits rich photophysical
and photochemical activities. The understanding of these activities
behind AIE 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 (ΦF) up
to 99.8%. Along with the AIE 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 AIE characteristics.