Polymorphism-Dependent and Switchable Emission ofButterfly-Like Bis(diarylmethylene)dihydroanthracenes.

Organic fluorophores with reversibleemission switching behaviorare promising materials for applications in sensors, optical recording,security inks, and optoelectronics. A variety of aggregation-inducedemission (AIE) luminogens with mechanochromic luminescence has beenprepared, and the transformation of efficient bluer-emitting crystalsto amorphous powders with redder and weaker emission is proposed tobe the cause for such behavior. However, detailed mechanistic understandingfrom experimental to theoretical is lacking. In this work, we presentthe design and synthesis of a group of bis(diarylmethylene)dihydroanthraceneswith butterfly-like shapes. These molecules exhibit aggregation-inducedemission characteristics due to the restriction of intramolecularmotion in the aggregated state. They show mechanochromism, becauseof the transformation between crystal and amorphous states with differentcolors and efficiencies aided by grinding/heating or solvent fumingprocesses. By investigation of their single-crystal structures andtheoretical calculations, the loose molecular packing with noncovalentintermolecular interactions, the extent of conformational twisting,and the packing density of the luminogens, as well as freedom of intermolecularmotion in the excited state, are stemmed for their reversible polymorphism-dependentemission behaviors. [ABSTRACT FROM AUTHOR]