Subpicosecond resolution studies of solvation dynamics in polar aprotic and alcohol solvents

Subpicosecond resolution measurements of the kinetics of dipolar solvation have been made. The time resolved Stokes shift of a dye molecule, LDS-750 was measured using the fluorescence upconversion technique in the solvents acetonitrile, DMSO, nitrobenzene, methanol, and n-butanol. The solvation dynamics in both aprotic and alcohol solvents occur on a time scale roughly given by the longitudinal relaxation time as predicted by simple continuum theories. The relaxation in nitrobenzene and butanol is nonexponential and the relaxation in methanol is significantly faster than the calculated time. These deviations from simple theory are discussed in the context of (i) the significance of high frequency dispersions in the dielectric response, (ii) translational contributions to the solvent relaxation, and (iii) molecular aspects of the solvation not accounted in the continuum description.