Efficiencies of photoinduced electron-transfer reactions: role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs

摘要: In photoinduced electron-transfer processes the primary step is conversion of electronic energy an excited state into chemical retained in form a redox (geminate radical-ion) pair (A + D A'-/D'+). polar solvents, separation geminate occurs with formation free radical ions solution. The quantum yields product formation, from reactions either ions, or pair, are often low, however, due to return electron transfer reaction (A'-/D'+ - A D), energy-wasting that competes useful ion pair. present study was undertaken investigate parameters controlling rates these reactions. Quantum were measured for pairs formed upon quenching first singlet states cyanoanthracenes by simple aromatic hydrocarbon donors aceonitrile at room temperature. free-ion determined competition between and transfer. By assuming constant rate separation, process obtained. These highly exothermic (-AG,, = 2-3 eV) found be strongly dependent on exothermicity. showed marked decrease (ea. 2 orders magnitude this AG, range) increasing This effect represents clear example Marcus "inverted region". Semiquantum mechanical theories used analyze data quantitatively. also depend degree charge delocalization within which attributed variations solvent reorganization coupling matrix element. Accordingly, mostly basis potentials, one can vary yield few percent values approaching unity. Use strong donor acceptor induce based likely inefficient because fast resulting low-energy system smallest possible driving force initial charge-separation results high-energy, therefore long-lived allows desired occur more efficiently. use indirect path secondary transfer, concept called "cosensitization", efficient radical-ion even when direct very low yield.