Viscosity dependent internal conversion in the rhodamine dye, fast acid violet 2R

Abstract

The fluorescence lifetimes of N-(2-tolyl)-N′-(2-tolyl-5-sulphonate) rhodamine, Fast Acid Violet 2R, in a series of alcohols, range from 10 to 160 ps as a result of viscosity dependent ultrafast internal conversion. Rates of internal conversion are shown to be dependent upon the microviscosity, as opposed to the bulk viscosity of the solvent. Within a given homologous series of alcohols, the fluorescence lifetime (τ_f) was found to vary with solvent viscosity (η) according to a Forster–Hoffmann type equation: τ_f=C_η^2/3, but the value of C was different between the monohydric alcohols, dihydric alcohols and glycerol. This dye may be a good candidate as a fluorescent probe of solvent structure.