Fluorescence visualization of crystal formation and transformation processes of organic luminogens with crystallization-induced emission characteristics

An understanding of crystal formation and transformation is crucial in the pharmaceutical, food, chemical, and optoelectronic industries. However, direct observation of such processes under realistic conditions in real time remains a challenge because of the lack of advanced techniques to discriminate the phase boundaries and capture the intermediate states. In this work, (Z)-1-phenyl-2-(3-phenylquinoxalin-2(1H)-ylidene)ethanone (PPQE) with crystallization-induced emission properties was reported. Three polymorphs of PPQE with various luminescence behaviors were obtained with good reproducibility under controlled conditions and crystal transformations between two pairs of polymorphs were observed. With the crystallization-induced emission characteristics and polymorph-dependent luminescence of PPQE, we have successfully realized a real-time, on-site, nondestructive fluorescence imaging technique to monitor the crystal transformation processes, as well as crystal formation from amorphous state and dilute solution, respectively. This work provides a useful and convenient fluorescence tool for in situ crystal analysis, from which detailed experimental evidence and mechanistic insights into crystal formation and transformation can be obtained through direct fluorescence visualization.