Relation between charge transfer absorption and fluorescence spectra and the inverted region

The theoretical expression for the plot of a charge transfer (CT) band intensity vs frequency is known to parallel that for the plot of electron-transfer rate constant k^ (ET) vs –ΔG⁰. We use this parallelism to explore a recent model in the literature on the possible role, if any, of partial dielectric saturation of the solvent on the “inverted region” of electron-transfer reactions. A simple approximate expression is obtained for the full width at half-maximum for the k^ (ET) and spectral plots and is tested using recent numerical results for calculated k^ (ET) vs –ΔG⁰ curves. Studies of experimental widths of CT bands are used, thereby, to test ideas on the possible presence and effects of partial dielectric saturation and on the observation of the inverted region. Further uses of charge-transfer spectra, when both the absorption and fluorescence data are available, are also described, including the determination of the “0 → 0 transition energy” ΔE_ (0 → 0) for systems displaying little or no vibrational structure.