| Abstract
| - The rate constants of back-electron-transfer (BET) reaction within geminate ion pairs generated upon staticET quenching of cyano-substituted anthracenes by aromatic amines and methoxy-substituted benzenes (MSB)at high concentration in acetonitrile have been measured directly using ultrafast multiplex transient gratingspectroscopy. The free energy of BET, ΔGBET, was varied between −3.0 and −0.6 eV, a range corresponding,in principle, to the inverted, barrierless, and normal regimes. When plotted vs ΔGBET, the measured rateconstants, kBET, exhibit a large scattering. Good fits of the semiclassical expression for nonadiabatic ET areobtained if the rate constants are sorted according to the electron donor. The resulting electronic couplingmatrix elements V are larger and the solvent reorganization energies smaller than those reported for BETwithin solvent-separated ion pairs, suggesting that BET takes place between ions in contact. However, in thelow exergonicity region, the observed BET rate constants are slower than those reported for contact ion pairsformed by charge-transfer excitation. The dynamics of BET within radical pairs generated upon ET quenchingof the N-methylacridinium cation has also been investigated, and the role of the electrostatic interaction withingeminate ion pairs is discussed.
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