Fragile arrangements of self-organized J aggregates of pseudoisocyanine dye at a glass/solution interface

Abstract

Self-organized J aggregates of a 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine: PIC) dye produced at a soda lime glass/solution interface (J_L aggregate) were characterized. The dye concentration dependence of J_L aggregation showed a quasi-adsorption behavior owing to formation of the aggregates at anionic sites on the glass surface. An increase in the PIC concentration accompanied a slight red shift of the J_L band as well as a decrease in the fluorescence lifetime. The results suggested that the J_L aggregate possessed relatively fragile arrangements. In order to discuss the structures of the aggregates, we proposed a new approach to estimating the number of interacting molecules (N) in the aggregate on the basis of a thermodynamic consideration for self-association of molecules and an adsorption isotherm. Then, we estimated N to be 6 in the J_L aggregates when N is assumed to be unchanged. According to extended dipole model calculations for a brickstone or staircase structure with N=6, the concentration dependence of the optical properties of the J_L band was reasonably explained by assuming loose and fragile arrangements of PIC molecules in the aggregates. The amorphous solid surface of the glass was shown to be the origin for fragile arrangements of PIC molecules in the J_L aggregates.