Reorganization Parameters of Electronic Transitions in Electronically Delocalized Systems. 2. Optical Spectra

Abstract

This is a modification of the condensed-phase band shape analysis of optical spectra to treat electronically delocalized systems. It incorporates the dependence of the optical observables on the optical transition dipole. Absorption and emission band shapes are calculated on the basis of adiabatic free energy surfaces defined as the functions of the reorganization parameters invariant to electronic delocalization and the parameter of electronic delocalization. The latter is given through the ratio of the transition and differential chromophore dipoles. The linear response relations commonly used to connect optical observables to reorganization parameters of electron transfer reactions break down for electronically delocalized systems. The optical Stokes shift depends strongly on the delocalization parameter deviating downward from twice the adiabatic reorganization energy with electronic delocalization. The Stokes shift is a rising function of solvent polarity for localized systems. An inverted solvent dependence develops for delocalized complexes: the Stokes shift decreases with solvent polarity. The spectrum of permissible solvent fluctuations is limited from the low-energy side by the adiabatic splitting of the free energy terms inducing narrowing of the emission optical bands compared to the absorption bands.