Effect of solvent polarity on nonradiative processes in xanthene dyes: Rhodamine B in normal alcohols

The fluorescence lifetime of rhodamine B in a series of normal alcohols (C_nH_2n-1OH, n = 1-6) was measured as a function of temperature. The nonradiative rate constants were calculated from the fluorescence lifetimes and quantum yields. Activation energies were obtained from Arrhenius plots of the nonradiative rate constant. The variation of the nonradiative rate constant with solvent polarity and temperature was consistent with a photophysical mechanism that involves equilibrium between the planar and twisted configurations of the diethylamino groups on the xanthene ring of rhodamine B and internal conversion from the twisted configuration. The activation energy is equal to the free energy difference between the twisted and planar configurations. The solvent polarity dependence of the free energy difference and of the rate constant for internal conversion from the twisted configuration determines the variation of the nonradiative rate constant with solvent. When solvent polarity effects are taken into account by using the parameter E_T(30), the nonradiative rate constant shows weak or no dependence on the solvent viscosity.