Reversible Thermochromism of Aggregation-Induced Emission-Active Benzophenone Azine Based on Polymorph-Dependent Excited-State Intramolecular Proton Transfer Fluorescence

In this work, (2-hydroxy-4-methoxyphenyl)(phenyl)methanone azine (1) was found to exhibit aggregation-induced emission (AIE) and tunable solid fluorescence upon alternate annealing/melting treatments. According to the characterizations by X-ray crystallography, X-ray powder diffraction, and differential scanning calorimetry, the switching between the two different polymorphs was responsible for the tunable solid fluorescence as a consequence of polymorph-dependent excited-state intramolecular proton transfer (ESIPT) fluorescence, while the thermochromism was contributed by the conformational flexibility of rotary phenyl rings. The change in the tightness of packing upon annealing or melting thermal treatments resulted in the emission at different wavelengths. Therefore, polymorph-dependent ESIPT fluorescence could be utilized as a new strategy to develop efficient AIE-active materials in response to external stimuli.