Stokes shift as a tool for probing the solvent reorganization energy

The difference between the maxima of absorption and fluorescence spectra, 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 examined. We report that the experimentally observed dependence of E-s on the solvent dielectric properties can be explained only when two effects are taken into account. The first effect is the distortion of dielectric properties of a uniform solvent by embedding a bulk solute molecule in the solvent. The second effect is the influence of the solvent polarity on the electronic density redistribution upon the electronic transition. Our model evaluations of E-s are based on quantum chemical calculations of solvent effects on charge redistribution of the dye upon 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 continuum approach is able in the present case to provide a reasonable correlation between E-s and the dielectric properties of the solvent. An experimentally observed 15%-30% difference in the values of E-s for polar aprotic solvents and for alcohols cannot be explained in terms of the continuum approach. Our data show that commonly adopted equation for Stokes shift that establishes equality between Stokes shift and the doubled reorganization energy is generally not rigorous. The experimental data obtained are consistent with a more rigorous approximation of Stokes shift that takes into account the quantum nature of the solute local modes.