Effects of molecular charge on photoinduced electron-transfer reactions

Fluorescence quenching due to full and partial electron transfer (ET) and back ET within geminate radical pairs are studied (i) in acetonitrile using N-methylacridinium ion (MeAc+) as a monocationic fluorescer and aromatic compounds as neutral quenchers and (ii) in methanol using aromatic compounds as neutral fluorescers and methyl and benzyl viologens as dicationic quenchers. By comparison with the results previously obtained for the system of neutral fluorescer and quencher it is demonstrated that the free energy dependence of the rates of fluorescence quenching and back ET is not affected by molecular charge, but the rate of geminate radical separation into free radicals is. In the system of aromatic fluorescer and quencher, in general, fluorescence quenching is induced by a full ET at long distance (i.e., a long-distance ET) when the free energy change Delta G(fet), in full ET is more negative than -0.5 eV and by a partial ET at contact distance (i.e., an exciplex formation) when Delta G(fet),t is more positive than -0.4 eV. A criterion to determine the ET fluorescence quenching mechanism is given.