SYMMETRY EFFECTS ON ELECTRON-TRANSFER REACTIONS - TEMPERATURE-DEPENDENCE AS A DIAGNOSTIC-TOOL

The temperature dependences of photoinduced intramolecular electron-transfer rate constants are used to determine the magnitude of the donor-acceptor electronic coupling matrix element, absolute value of V, across seven sigma bonds for three donor-spacer-acceptor molecules 1-3 in acetonitrile and tetrahydrofuran. The wmiclassical formulation of electron-transfer rate constants and a linearized approximation are used to analyze the rate constants. Charge-transfer emission spectra from shorter analogs of 1 and 2 are analyzed to obtain estimates of the vibrational reorganization energy, the "average" vibration frequency, and S, the electron-phonon coupling constant, which are required in order to extract absolute value of V from the rate constant analyses. The results confirm predictions bawd on electronic symmetry arguments that electron transfer is symmetry forbidden in 1 and symmetry allowed in 2 and 3. In addition, the analyses suggest that through solvent coupling is more effective in acetonitrile than in tetrahydrofuran. Additional sources of the apparent solvent dependence of absolute value of V are discussed.