Use of Aggregation-Induced Emission for Selective Detection of Phase Transformation during Evaporative Crystallization of Hexaphenylsilole.

Crystallization of organic molecules is quite complicated because the crystallization process is governed by weak intermolecular interactions. By exploiting aggregation-induced emission (AIE), we attempted to realize the selective detection of phase transformation during the evaporative crystallization of hexaphenylsilole (HPS), which shows different fluorescent colors in the amorphous and crystalline phases. No fluorescence emission was observed in the HPS solution immediately after dropping on the glass substrate due to the non-radiative deactivation induced by intramolecular rotational or vibrational motion, suggesting that HPS exists as a monomer in solution. As time elapsed after dropping, green emission first appeared, which changed to blue after solvent evaporation, because of phase transformation from the amorphous state to the crystalline state. This phenomenon supports not only the two-step nucleation model involving an intermediate such as a liquid-like cluster prior to nucleation but also the real-time detection of Ostwald's rule of stages during evaporative crystallization.