A new amphiphilic boron-dipyrromethene (BODIPY) dye bearing a hydrophobic 2,3,4-tris (n-dodecyloxy) phenyl group at the meso-position and two hydrophilic cationic moieties at the boron was synthesized. This dye exhibited kinetic-controlled and thermodyanmic-controlled aggregation pathways in the aqueous solution, leading to the formation of two type of J-aggregates, i.e. the metastable Agg. I and thermodynamically more stable Agg. II. Further kinetic analysis of spontaneous transformation from Agg. I to Agg. II indicated that the Agg. I was an off-pathway aggregate species. Moreover, both of Agg. I and Agg. II displayed J-aggregate characteristics with enhanced fluoresence quantum yields and shortened fluoresence lifetime with respect to that of the monomeric dye. In particular, the Agg. II exhibited a narrowed and intensive J-band, reflecting the strong excitonic coherence in the aggregate. The morphological studies revealed that Agg. I was spherical nanoparticles while Agg. II had a multilamellar vesicular structure. In addition, in the fluorescence spectra of Agg. I and Agg. II an unexpected broad band at longer wavelength than that of the J-aggregate emission was observed, which could be ascribed to the possible formation of excimers in these J-aggregates in aqueous solution.

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