EFFECT OF SOLVENT ON NONRADIATIVE PROCESSES IN XANTHENE DYES - PYRONIN-B IN ALCOHOLS AND ALCOHOL WATER MIXTURES

The fluorescence lifetime tau(f) and quantum yield phi(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 tau(f) and phi(f). At 25-degrees-C, k(nf) was equal to (4.1 +/- 0.2) X 10(8) s-1 in water and ranged from (1.3 +/- 0.3) x 10(8) to (3.9 +/- 0.4) x 10(8) s-1 in alcohols and from (3.6 +/-0.7) X 10(8) to (6.0 +/- 0.5) X 10(8) 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.