**Fuyuki Ito**

Department of Chemistry, Institute of Education, Shinshu University, Nagano 380-8544, Japan; fito@shinshu-u.ac.jp; Tel.: +81-26-238-4114

Received: 25 September 2020; Accepted: 16 October 2020; Published: 19 October 2020

**Abstract:** We have used the fluorescence detection of phase transformation dynamics of organic compounds by photochemical methods to observe a real-time symmetry breaking process. The organic fluorescent molecules vary the fluorescence spectra depending on molecular aggregated states, implying fluorescence spectroscopy can be applied to probe the evolution of the molecular-assembling process. As an example, the amorphous-to-crystal phase transformation and crystallization with symmetry breaking at droplet during the solvent evaporation of mechanofluorochromic molecules are represented in this review.

**Keywords:** amorphous-to-crystal phase transformation; detection of real-time symmetry breaking; mechanofluorochromism; fluorescence spectroscopy; liquid-like cluster; evaporative crystallization; quartz crystal microbalance; two-step nucleation model
