Cooperative and Hierarchal Photoresponses of Molecular Assembling Processes Probed by Organic Fluorescent Molecules

Crystal formation in solution starts from nucleation in the saturated state. The quality of the product and the number, size, and structure of crystals are determined in the initial stage of the nucleation process. Understanding nucleation is the key to control crystal properties including polymorph formation. In this chapter, the fluorescence visualization of the crystal formation process probed by organic fluorescent molecules exhibiting cooperative and hierarchal photoresponses is summarized. Fluorescence observations of evaporative crystallization revealed a two-step nucleation model for both nuclei and polymorph formation. A Dibenzoylmethanatoboron difluoride complex exhibiting mechanofluorochromism and a dipyrrolyldiketone difluoroboron complex derivative displaying polymorphism-dependent fluorescence were studied. The fluorescence from the droplets showed dramatic changes depending on the molecular state, such as monomer, amorphous, and crystal polymorph. This method could be easily used to detect assembly processes by measuring the real-time fluorescence changes under ambient conditions. Furthermore, the formation of polymorphs is most likely affected by the cluster structure prior to nucleation. Therefore, insights into the nuclei precursor clusters in polymorph formation measured by fluorescence changes will enable us to predict the outcome of polymorph formation.