The Dynamics of Preferential Solvation

Introduction Molecules are stabilized by their interaction with the solvent, and this induces a shift in their absorption or emission spectra as compared with that in the gas phase. When a dipolar or charged solute is introduced into a neat polar solvent, the solvent responds predominantly by librational motions to optimize the solute-solvent interactions. If the dipole is created (or enhanced) by photoexcitation of a fluorescent probe molecule, the solvent response manifests itself in a fast time-dependent Stokes shift for the average fluorescence frequency, #(t). The situation is different in a mixture of polar and nonpolar solvents, in which polar solvent molecules tend to aggregate around the polar solute, a phenomenon known as "preferential solvation". Equilibrium electrostatic models have been utilized to explain the mole-fraction dependence of the shift emanating from such a "dielectric enrichment" process, 4 and the theory was applied to solvatochromic shifts in st