FEMTOSECOND EXPERIMENTS ON SOLVATION DYNAMICS OF AN ANIONIC PROBE MOLECULE IN METHANOL

Fluorescence up-conversion measurements were conducted to investigate the solvation dynamics of a deprotonated coumarin 343 (C343) in a solution of CH3ONa-CH3OH. In the presence of a base at an appropriate concentration, C343 exists in an anionic form. The observed relaxation kinetics of the solvent polarization exhibit behavior similar to those with a neutral probe molecule with an analogous structure, C152, although the average solvation time for the ionic probe is slightly longer. Theoretical calculations employing three different models for the solvation dynamics were performed: the dielectric continuum model, the mean spherical approximation (MSA) for ionic solvation, and the MSA for dipole solvation. Although the MSA for ionic solvation gives the best prediction for the average solvation time, none of the theories can explain the overall time profile of the solvation dynamics satisfactorily.