We studied the fluorescence properties of nanoaggregates of pyrene ammonium derivative (PyAm) and their photoinduced dissolution and reaction in tetrahydrofuran (THF)/aqueous solution. The final concentration (dye concentration after reprecipitation) dependence of the fluorescence peak was measured. The fluorescence peak of PyAm that originated from the excimer is shifted to the long wavelength side increasing with the final concentration, which is ascribable to the characteristic fluorescence spectral changes depending on their size. The size-dependent fluorescence change in the nanoaggregates is related to some molecular conformation, packing, and elastic properties of the nanoparticles at the surface. To understand the fluorescence properties of the intermediates from aggregates to crystals is important for the studies of organic nanocrystals/aggregates prepared by the reprecipitation methods. We also determined the fluorescence spectra of the PyAm nanoaggregates in a THF/aqueous solution by photoinduced dissolution and reaction. It originated from the photochemical reaction between PyAm and THF. Changes in the perpendicular light scattering intensity by photoirradiation supports the photoinduced dissolution of PyAm in the THF/aqueous solution. These findings are important in the research field of photochemical reactions in organic nanocrystals. KeywordsOrganic nanocrystal; fluorescence properties; size dependence; photochemical reaction; photoinduced dissolution 2 IntroductionOrganic nano-materials are widely investigated in terms of their unique optical properties [1]. Kasai and Nakanishi et al. first reported that the fluorescence spectra of perylene nanocrystals, which were prepared by the reprecipitation method, depended on their size, and the fluorescence spectra of the nanoparticles differ from that of the bulk crystals [2,3]. There are many reports about organic nanoparticles in the past decade [4][5][6][7]. They proposed lattice softening of the nanoparticles as a cause of the size-dependent optical properties. Yao et al. reported the size dependence of the emission spectra for pyrazoline nanoparticles, which originated from the formation of charge transfer excitons due to the flexibility and various inter-and intramolecular overlapping structures in the nanoparticles [5].We have investigated the fluorescence properties of pyrene ammonium derivative (PyAm, Figure 1) in poly(vinylalcohol) (PVA) thin films [8]. The fluorescence spectra of PyAm in the PVA film at a lower concentration showed only a monomer emission. The fluorescence peak shifted to the long wavelength side with the increasing dye concentration, which was assigned to a dimer or excimer-like emission. The concentration-dependent fluorescence changes originated from the PyAm aggregates formed in the PVA matrix by phase separation. The size of the PyAm aggregates in the PVA thin films increased with the increasing dye concentration. The peaks of the dimer and/or excimer-like fluorescence originated from the aggregates that ...
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