Polymorphism-Dependent and Switchable Emission of Butterfly-Like Bis(diarylmethylene)dihydroanthracenes

Organic fluorophores with reversible emission switching behavior are promising materials for applications in sensors, optical recording, security inks, and optoelectronics. A variety of aggregation-induced emission (AIE) luminogens with mechanochromic luminescence has been prepared, and the transformation of efficient bluer-emitting crystals to amorphous powders with redder and weaker emission is proposed to be the cause for such behavior. However, detailed mechanistic understanding from experimental to theoretical is lacking. In this work, we present the design and synthesis of a group of bis(diarylmethylene)dihydroanthracenes with butterfly-like shapes. These molecules exhibit aggregation-induced emission characteristics due to the restriction of intramolecular motion in the aggregated state. They show mechanochromism, because of the transformation between crystal and amorphous states with different colors and efficiencies aided by grinding/heating or solvent fuming processes. By investigation of their single-crystal structures and theoretical calculations, the loose molecular packing with noncovalent intermolecular interactions, the extent of conformational twisting, and the packing density of the luminogens, as well as freedom of intermolecular motion in the excited state, are stemmed for their reversible polymorphism-dependent emission behaviors.