Direct measurements of the charge-recombination dynamics of geminate ion pairs formed upon electron-transfer quenching at high donor concentration

The rate constants of back-electron-transfer (BET) reaction within geminate ion pairs generated upon static ET 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 grating spectroscopy. The free energy of BET, DeltaG(BET), was varied between -3.0 and -0.6 eV, a range corresponding, in principle, to the inverted, barrierless. and normal regimes. When plotted vs DeltaG(BET), the measured rate constants, k(BET), exhibit a large scattering. Good fits of the semiclassical expression for nonadiabatic ET are obtained if the rate constants are sorted according to the electron donor. The resulting electronic coupling matrix elements V are larger and the solvent reorganization energies smaller than those reported for BET within solvent-separated ion pairs, suggesting that BET takes place between ions in contact. However, in the low exergonicity region, the observed BET rate constants are slower than those reported for contact ion pairs formed by charge-transfer excitation. The dynamics of BET within radical pairs generated upon ET quenching of the N-methylacridinium cation has also been investigated, and the role of the electrostatic interaction within geminate ion pairs is discussed.