Effect of solvent polarity on non-radiative processes in xanthene dyes: the acid form of rhodamine B in nitrile solvents

The fluorescence lifetimes of the acid form of rhodamines B in nitrile solvents were measured as a function of temperature. Non-radiative rate constants were calculated from the fluorescence lifetimes and quantum yields. Arrhenius plots of the non-radiative rate constants were linear. The value of the non-radiative activation energy was, within experimental error, the same for all the solvents studied and equal to 3.4 ± 0.3 kcal mol^-1. However, at constant temperature, the natural logarithm of the non-radiative rate constant increased linearly with the solvent polarity parameter E_T(30). The dependence of the non-radiative rate on solvent polarity can be explained by a model involving a planar-pyramidal change at the xanthene-amine bond. On the basis of this model, a comparison of the non-radiative rate of the dye in alcohols and nitriles shows that the dependence on solvent polarity is due to specific solute-solvent interactions at the amino and carboxyphenyl groups.