SOLVENT EFFECTS ON THE PHOTOISOMERIZATION RATES OF THE ZWITTERIONIC AND THE CATIONIC FORMS OF RHODAMINE-B IN PROTIC SOLVENTS

Solvent effects on the nonradiative rate have been investigated for the zwitterionic form of rhodamine B dissolved in water-methanol mixtures, water-ethanol mixtures, monoalcohols (C3-C-10), ethylene glycol, and glycerol. The effect of solvent polarity is considered in the Arrhenius-type equation by introducing a polarity dependence into the activation energy and the effect of solvent viscosity is taken into account by including a fractional-viscosity factor in the preexponential coefficient. This empirical expression has been found to describe satisfactorily the photoisomerization rates of rhodamine B in the above solvents. We also studied solvent effects on the nonradiative rates of the cationic form of rhodamine B in alcoholic solutions. We found that the effect of solvent polarity is stronger for the zwitterionic form than for the cationic form. However, the effect of solvent viscosity is about the same for both forms.