NONADIABATICITY IN THE PHOTOINDUCED ELECTRON-TRANSFER REACTIONS OF METAL-COMPLEXES

The rate of electron transfer (ET) in a variety of chemical and biolog ical processes is influenced by factors like the free energy change (D ELTAG), the donor-acceptor electronic coupling and the medium. The eff ect of donor-acceptor electronic coupling on the rate of photoinduced intermolecular electron transfer is considered by taking Ru(II) and Cr (III) metal complexes in the excited state as electron acceptors and o rganic compounds as electron donors. The electronic coupling between t he donor and acceptors depends strongly on donor-acceptor distance. Th e electron transfer distance is varied by introducing alkyl groups of different sizes either on the bipyridine ligand of the metal complex o r on the quencher. The semiclassical theory of electron transfer expre sses k(ET) as the product of a nuclear and an electronic transmission coefficient (K(n) and K(el) respectively) and an effective nuclear-vib ration frequency (v(n)), k(ET) = v(n)K(el)K(n). The electron transfer reaction becomes nonadiabatic if the donor-acceptor distance is long. The change of electron transfer mechanism from adiabatic to nonadiabat ic due to the introduction of bulky groups is explained in terms of se miclassical theory and from the temperature-dependence study of photoi nduced electron transfer reactions of metal complexes.