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The fluorescence lifetimes of N-(2-tolyl)-N'-(2-tolyl-5-sulphon~te) 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 (zf) was found to vary with solvent viscosity (q) according to a Forster-Hoffmann type equation: zf = CV''~, 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.

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