STOKES SHIFT AS A TOOL FOR PROBING THE SOLVENT REORGANIZATION ENERGY

The difference between the maxima of absorption and fluorescence spect ra, the Stokes shift, is affected by dynamic properties of the solvent , namely by the solvent reorganization free energy, E-s. Stokes shift a dye, proflavine, in polar aprotic solvents and in alcohols is examin ed. We report that the experimentally observed dependence of E-s on th e solvent dielectric properties can be explained only when two effects are taken into account. The first effect is the distortion of dielect ric properties of a uniform solvent by embedding a bulk solute molecul e in the solvent. The second effect is the influence of the solvent po larity on the electronic density redistribution upon the electronic tr ansition. Our model evaluations of E-s are based on quantum chemical c alculations of solvent effects on charge redistribution of the dye upo n excitation and on subsequent calculations of E-s in the framework of a model of a cavity in a continuum dielectric. We show that the conti nuum approach is able in the present case to provide a reasonable corr elation between E-s and the dielectric properties of the solvent. An e xperimentally observed 15%-30% difference in the values of E-s for pol ar aprotic solvents and for alcohols cannot be explained in terms of t he continuum approach. Our data show that commonly adopted equation fo r Stokes shift that establishes equality between Stokes shift and the doubled reorganization energy is generally not rigorous. The experimen tal data obtained are consistent with a more rigorous approximation of Stokes shift that takes into account the quantum nature of the solute local modes.