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.

The thermally backward reaction involved in the mechanofluorochromism of dibenzoylmethanatoboron difluoride (BF₂DBM) derivatives, accompanied by an amorphous–crystalline phase transition, was quantitatively evaluated based on kinetics and thermodynamics. The kinetics was discussed by evaluation of the effect of temperature on the time-dependent changes of the fluorescence intensity for amorphous samples obtained by mechanical grinding. The thermodynamics was discussed based on data for the amorphous–crystalline phase transition obtained by differential scanning calorimetry. The enthalpy of activation (ΔH^‡) of BF₂DBM derivatives with MeO groups (2aBF₂) was larger than that of derivatives with alkyl groups (2b-dBF₂), whereas the entropy of activation (ΔS^‡) was smaller than that of the derivatives with alkyl groups. It is proposed that the reaction dynamics of 2aBF₂ will be governed by rotational motion around the C(methyl)–O bond. Interestingly, the Gibbs energies of activation (ΔG^‡) were comparable for the reactions of all members of the BF₂DBM series, though ΔH^‡ and ΔS^‡ were strongly dependent on the identity of the substituent. It is proposed that the substituent-dependent ΔS^‡ term is one of the key parameters for understanding the mechanofluorochromism of BF₂DBM derivatives associated with the amorphous–crystalline phase transition. These findings will also provide important insights into the process of formation of crystal nuclei in moving from the melted to the crystalline state.

We investigated the fluorescence properties of dye aggregates formed in a poly(vinylalcohol) (PVA) matrix by phase separation. Trimethyl-(2-oxo-2-pyrene-1-yl-ethyl)-ammonium bromide (PyAm) was used as a fluorescent dye molecule. The size of PyAm aggregates in the PVA thin films were increased with increasing dye concentration, which was confirmed by atomic force microscope (AFM) measurements. The fluorescence spectra of PyAm in the PVA film at a lower concentration of 0.001 mol% only showed the monomer emission. The fluorescence peak shifted to the red with increasing dye concentration, which was assigned to a dimer or excimer-like emission. Changes in the fluorescence spectra relate to the formation of aggregates in the films. The fluorescence anisotropy decay time constant increases with increasing PyAm concentration up to the order of 100 ps. It is suggested that the exciton efficiently diffuses within the aggregates, and then was trapped at the dimer sites. We also demonstrated the application for gas sensing of nitroaromatics: 2,4-dinitrotoulene (DNT) based on the fluorescence quenching by the photoinduced electron transfer. The quenching efficiency of PyAm fluorescence reached about 43% under concentration of 2.0 mol%. The fluorescence intensity efficiently quenched at the dimer or excimer-like band. These results indicated that the efficient fluorescence quenching increases the reaction probability between PyAm and DNT by the exciton diffusion in the aggregates, called “amplified quenching”. The nano-sized aggregates of PyAm formed in the PVA films are responsible for high sensitivity as an artificial fluorescent chemosensor for vapors of the nitroaromatics.