DYNAMICS OF PHOTOINDUCED ELECTRON TRANSFER IN SOLUTION

Abstract

Three key intermediates can be identified in photoinduced bimolecular electron transfer reactions. These are the encounter pair (A * (S)D), the contact radical-ion pair (CRIP, A • D •+ ) or exciplex/excited CT complex (A • D ↔ A • D •+ ), and the solvent-separated radical-ion pair (SSRIP, A •− (S)D •+ ). The efficiency with which such reactions result in the formation of free radical ions in solution (A •− + D •+ ) depends upon the competition between an electron transfer reaction, and a reaction involving a change in solvation within each of these three intermediates. In the encounter pair, the competition is between solvent displacement to form contact species, and direct formation of the SSRIP via a “long distance” electron transfer (ca. 6–8 A) reaction. For the present systems, CRIP formation occurs with near unit efficiency in low polarity solvents, but direct formation of A •- (S)D •+ is observed in the medium polarity o -dichlorobenzene, in which charge-transfer interactions between the excited acceptor (A * ) and the solvent appear to enhance the long distance electron-transfer rate via a superexchange type mechanism. In acetonitrile the probability of direct formation of A •- (S)D •+ depends upon the energetics of the long distance electron transfer reaction, consistent with Marcus “normal” region behavior. In the contact and solvent separated radical-ion pairs the competition is between return electron transfer to reform the neutral starting materials and solvation and separation processes. The rate constants for the return electron transfer reactions are in the Marcus “inverted” region. The electronic coupling matrix element which characterizes these reactions is ca. 2 orders of magnitude lower in the solvent-separated pair compared to the contact pair. Conversely, in acetonitrile the solvent reorganization energy for the SSRIP is ca. 1 eV larger than that of the corresponding CRIP. Estimates for the reorganization energies for the CRIP reactions are obtained independently by using CRIP emission spectroscopy.