**1. Introduction**

The phenomena of phase transformation with structural changes are important as both basic findings and applications for materials science. Studies have been focused on thermodynamics for the macroscopic phase or phase transition dynamics based on computer simulations. Based on the molecular science of chemistry, various phase transformation dynamics are needed to understand the variety depending on the molecular individuality and to develop smart materials. We have been utilizing fluorescence spectroscopy to clarify phase transformation dynamics of organic fluorescent molecules, which relies on the molecular aggregated state. Therefore, fluorescence spectroscopy can be applied to probe the process of molecular assembly. As an example, Yu et al. [1] demonstrated the fluorescence visualization of an amorphous-to-crystalline transformation in situ microscopic observation of the crystallization of molecules in microparticles through fluorescence color changes. Heterogeneous crystallization of amorphous microparticles was clearly observed by this method. This study can provide a picture based on real-time detection of the crystallization kinetics that occur spontaneously by external stimuli, such as mechanochromic behavior and solid–solid transitions.

Based on this research report on the phase transition phenomenon evaluation by fluorescence detection, we utilized mechanofluorochromic molecules to evaluate the transition state dynamics during the amorphous-to-crystal phase transition process. In addition, by utilizing this knowledge, we have started research on the solvent evaporative crystallization process of organic fluorescent molecules, which will be reviewed.
