Effects of molecular charge on photoinduced electron-transfer reactions

Fluorescence quenching due to full and partial electron transfer (ET) and b ack ET within geminate radical pairs are studied (i) in acetonitrile using N-methylacridinium ion (MeAc+) as a monocationic fluorescer and aromatic co mpounds as neutral quenchers and (ii) in methanol using aromatic compounds as neutral fluorescers and methyl and benzyl viologens as dicationic quench ers. By comparison with the results previously obtained for the system of n eutral fluorescer and quencher it is demonstrated that the free energy depe ndence of the rates of fluorescence quenching and back ET is not affected b y molecular charge, but the rate of geminate radical separation into free r adicals 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 lo ng-distance ET) when the free energy change Delta G(fet), in full ET is mor e negative than -0.5 eV and by a partial ET at contact distance (i.e., an e xciplex formation) when Delta G(fet),t is more positive than -0.4 eV. A cri terion to determine the ET fluorescence quenching mechanism is given.