Restriction of Intramolecular Motion(RIM): Investigating AIE Mechanism from Experimental and Theoretical Studies

Mechanistic studies promote scientific development from phenomena to theories. Aggregation-induced emission(AIE), as an unusual photophysical phenomenon, builds the bridge between molecular science and aggregate mesoscience. With the twenty-year development of AIE, restriction of intramolecular motion(RIM) has been verified as the working mechanism of AIE effect. In this review, these mechanistic works about RIM are summarized from experimental and theoretical perspectives. Thereinto, the experimental studies are introduced from three parts: external rigidification, structural modification and structural characterization. In the theoretical part, calculations on the low-frequency motion of AIEgens have been performed to prove the RIM mechanism. By virtue of the theoretical calculations, some new mechanisms are proposed to supplement the RIM, such as restriction of access to conical intersection, suppression of Kasha transition, restriction of access to dark state, etc. It is foreseeable that the RIM mechanism will unify the photophysical theories for both molecules and aggregates, and inspire more progress in aggregate science.