Effect of solvent on nonradiative processes in xanthene dyes: Pyronin B in alcohols and alcohol-water mixtures

The fluorescence lifetime π_f and quantum yield φ_f of pyronin B in water, n-alcohols, and alcohol-water mixtures were measured as a function of temperature. The nonradiative rate constant k_nr was calculated from τ_f, and φ_f. At 25°C, k_nr was equal to (4.1 ± 0.2) × 10⁸ s^-1 in water and ranged from (1.3 ± 0.3) × 10⁸ to (3.9 ± 0.4) × 10⁸ s^-1 in alcohols and from (3.6 ± 0.7) × 10⁸ to (6.0 ± 0.5) × 10⁸ s^-1 in mixtures. The natural logarithm of k_nr increased linearly with the solvent polarity parameter E_T(30) but with a steeper slope for alcohols than for mixtures. The nonradiative activation energy E_a, which was obtained from Arrhenius plots of k_nr, was equal to 4.0 ± 0.8 kcal mol^-1 in water, 5.1 ± 0.3 kcal mol^-1 in alcohol-water mixtures, and 6.0 ± 0.3 kcal mol^-1 in alcohols. The dependence of the nonradiative rate on solvent polarity can be explained by a model involving a planar-to-pyramidal change at the xanthene-amine bond. On the basis of this model, the dependence on solvent polarity is due to specific solute-solvent interactions at the amino group.