Abstract
The kinetic behavior of the first excited singlet state of an ionic dye, Rhodamine 3B perchlorate, was studied in the region of total miscibility of water:toluene mixtures. The solute exists in two forms: the associated ion pair and cationic, both preferentially solvated by the more polar component (Chem. Phys. Lett. 307 (1999) 139). The transient photokinetics of the excited state hydrated ion pair’s conversion into the cationic species is studied. An anisotropic distribution of the nearest neighbor accounts satisfactorily for the transient effects induced in the diffusional behavior of water molecules in a dielectric cluster solvation sphere around the solute. A kinetic model is proposed to evaluate the dependence of the reactional rate constant k a and the mutual diffusion coefficient D on the water concentration, temperature, and the effect of friction. It is found that both k a and D decrease with the water concentration but, while the overall friction ( k B T/ D) decreases with the temperature, the rate constant k a is not affected. Overall, the kinetic data is in good agreement with the interpretation derived on the basis of steady-state results, and the recovered lifetimes of each individual species are similar to those observed in pure solvents.
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