Contact and solvent-separated geminate radical ion pairs in electron-transfer photochemistry

The two primary intermediates that play a major role in determining the efficiencies of bimolecular photoinduced electron-transfer reactions are the contact (A^.-D^.+) and the solvent-separated (A^.-(S)D^.+ radical ion pairs, CRIP and SSRIP, respectively. These two species are distinguished by differences in electronic coupling, which is much smaller for the SSRIP compared to the CRIP, and solvation, which is much larger for the SSRIP compared to the CRIP. The present work addresses the quantitative aspects of these and other factors that influence the rates of energy-wasting return electron transfer within the ion-pair intermediates. The electron acceptor tetracyanoanthracene (TCA) forms ground-state charge-transfer complexes with alkyl-substituted benzene donors. By a change of the excitation wavelength and/or donor concentration, either the free TCA or the CT complex can be excited. Quenching of free ¹TCA* by the alkylbenzene donors that have low oxidation potentials, such as pentamethylbenzene and hexamethylbenzene, in acetonitrile solution leads to the direct formation of geminate SSRIP. Excitation of the corresponding charge-transfer complexes leads to the formation of geminate CRIP. Rates of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radical ions together with the CRIP fluorescence decay lifetimes. The rates of return electron transfer within both sets of radical ion pairs depend upon the reaction exothermicity in a manner consistent with the Marcus inverted region. The data are analyzed by using a golden rule model in which the rate is given as a function of an electronic coupling matrix element, reorganization energies for the rearranged high-frequency (skeletal vibration) and low-frequency (mainly solvent and libration) motions, and an averaged frequency for the skeletal modes. Estimates for the reorganization energies and the skeletal frequency for the CRIP are obtained independently by analysis of the spectral distribution of the CRIP (exciplex) emission spectrum. A good fit to the return electron-transfer rate data for the CRIP is obtained by using the values for these parameters obtained from the emission spectrum. It is found that the electronic coupling in the CRIP is ca. 2 orders of magnitude higher than in the SSRIP and that the intermolecular (mainly solvent) reorganization energy for the contact pair is ca. 1 eV lower than that of the solvent-separated pair. The relevance of these observations to the photophysical and photochemical properties of contact radical ion pairs is discussed.