DIPOLE SOLVATION IN NONDIPOLAR SOLVENTS - EXPERIMENTAL STUDIES OF REORGANIZATION ENERGIES AND SOLVATION DYNAMICS

Steady-state and time-resolved emission measurements of the solvatochr omic probe coumarin 153 are used to study solvation of a dipolar solut e in nondipolar solvents such as benzene and 1,4-dioxane. Contrary to the predictions of dielectric continuum theories, the Stokes shifts (o r nuclear reorganization energies) that accompany electronic excitatio n of this solute are substantial in such solvents (similar to 1000 cm( -1)). The magnitudes of the shifts observed in both nondipolar and dip olar solvents can be consistently understood in terms of the relative strength of the interactions between the permanent charge distribution s of the solute and solvent molecules. (Information concerning these c harge distributions is derived from extensive ab initio calculations o n the solute and 31 common solvents.) The dynamics of solvation in non dipolar solvents, as reflected in the time dependence of the Stokes sh ifts, is qualitatively Like that observed in polar solvents. But, wher eas the dynamics in polar solvents can be rather simply modeled using the solvents dielectric response as empirical input, no simple theorie s of this sort are currently capable of predicting the solvation dynam ics in nondipolar solvents.