Direct Visualization of the Two-step Nucleation Model by Fluorescence Color Changes during Evaporative Crystallization from Solution

The two-step nucleation model for crystal nuclei formation explains several experimental and theoretical results better than the classical nucleation theory. We report here direct visualization of the two-step nucleation model for organic molecular crystallization. Evaporative crystallization from a solution of a dibenzoylmethane boron complex that displays mechanofluorochromism, a fluorescence color change induced by mechanical perturbation, was probed by fluorescence change. The dependence of fluorescence change on dispersion concentration of the complex in a polymer matrix was also investigated. We detected transitional emission from the amorphous cluster state prior to crystallization. This is the first demonstration of the two-step nucleation model based on fluorescence color changes.