Dipole moments in excited state. Solvatochromic equations based on different local-field models

Abstract

Two recent dielectric models, viz. the inverse exponential model of Block and Walker (BW) and the mean spherical model of Wertheim (W), were applied to the conceptually simple solvatochromic theory in the attempt to appreciate their ability to predict excited state dipole moments (μe) on the basis of the solvent dependence of electronic absorption spectra. Validity of the suggested equations was tested by standard methods of statistical analysis, as well as by comparison of the predicted μe values with those obtained independently from electrooptical measurements.All variants, including the original Onsager (O) formulation appear to be acceptable, the correlation coefficient varying from 0.920 to 0.982. However, they predict different μe values the order of magnitude being (for a given solute) invariably (μe)BW > (μe)O > (μe)W. In view of the large uncertainties in the cavity radius a, no unambiguous conclusions can be drawn on the preference of the respective models. Provided that the mean polarizability α is approximated by α = a3/2, the most satisfactory results (mean relative error in μe of about ±6%, R = 0.930-0.960) are obtained by using the reaction field model of Block and Walker.