The initial processes of the crystallization of a solute molecule, 1-cyano-trans-1,2-bis-(4'-methylbiphenyl)-ethylene (CN-MBE) in binary solution (water and acetone), were investigated by means of fluorescence spectroscopy as well as scanning electron microscopy (SEM). With an increase in the volume fraction (V) of the poor solvent (water) in the solution, a drastic change in the fluorescence spectra and intensity of CN-MBE was observed. This change was attributed to aggregation induced emission (AIE). By analyzing the evolution of AIE by multivariate curve resolution-alternating least squares (MCR-ALS), it was revealed that four main species appeared in the solution depending on the V values. On the basis of molecular exciton theory, we assigned these four emissive states to the monomer, H-dimer, J-dimer, and H-aggregates. Interestingly, the J-dimer state was observed only in a V range of 40% to 50%, just before the formation of the aggregate. This result suggests that the J-dimer plays an important role as the precursor for larger aggregates leading to crystal formation. By integrating the present results with previous work on the crystallization of CN-MBA through solvent evaporation, we discussed the dynamics of the crystallization from the viewpoint of the sequence of molecular species appearing in the aggregation in solution.
Understanding the polymorph phenomenon for organic crystals is essential for the development of organic solid materials. Here, the fluorescence study of the evaporative crystallization of 1,3-dipyrrol-2-yl-1,3-propanedione boron difluoride complex (1), which has three polymorphs showing different emission profiles, is reported. The droplet of 1 in 1,2-dichloroethane showed blue emission just after dropping. Solids with bluish-green emission were observed. As time elapsed, a solid with red or orange emission was observed around the droplet. Time evolution of the fluorescence spectra, observed for the first time, implied that the molten state of 1 was observed by emission of an intermediate, even at ambient temperature. These findings suggested that the liquid-like cluster incidentally forms an ordered array as the crystallites nucleate. The liquid-like cluster can be considered as the "crucible" in the nucleation of polymorphs.
5Size-dependent fluorescent properties of aggregates of a perylene ammonium derivative (PeryAm) were studied by steady-state and time-resolved spectroscopic methods. Quantitative analyses of aggregated states in aqueous solution indicated that the aggregation proceeded through dimer units of PeryAm. Fluorescence of the aggregate in the PVA film prepared from the aqueous solution continuously redshifted with an increase in the concentration of PeryAm in the mother liquor, while keeping the 10 absorption spectra almost the same band shapes. Fluorescence anisotropy values of aggregates in the PVA film were dependent on the monitoring wavelength and time profiles of the fluorescence in longer wavelength showed fast rise just after the pulsed excitation. These results indicated the efficient energy transfer to the stable sites in aggregates. Fluorescence microscope images showed aggregates were segregated in the PVA film and the color of the emission was dependent on the size of the aggregate. 15Under the steady-state irradiation, the emission color of the aggregates was changed from green to yellow, which was attributable to the association of a small cluster of PeryAm with the green emission resulting in the formation of yellow-colored aggregates. On the basis of these results, we discussed the mechanisms and the dynamics of the aggregation and size-dependent emission in aggregates.
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