On the Estimation of Excited-State Dipole Moments from Solvatochromic Shifts of Absorption and Fluorescence Spectra

The effect of the solvent polarity parameters f (Ɛ, n) and ᵩ(Ɛ, n) appearing in solvatochromic theories, and the effect of the molecular shape of the Onsager cavity (sphere, ellipsoid of revolution) on the determined electric dipole moments μ_e in the singlet excited state are studied. It is found that the shape of the solute does not exhibit a significant effect on the determined values of μ_e, but only on the solvent parameters f (Ɛ, n) and ᵩ(Ɛ, n) as well as on the Onsager radius a. Passing from a sphere to an ellipsoid leads to such a change in the scale that induces a proportional change in the slope coefficients m₁ and m₂. Also the effect of α/a³ (a and a are the mean isotropic polarizability of the solute and the Onsager cavity radius in a homogeneous dielectric, respectively) on the determined values of m e has been studied, and it is found that the assumption α/a³ = 1/2 is valid in many cases.