DYNAMIC SOLVENT EFFECT ON BETAINE-30 ELECTRON-TRANSFER KINETICS IN ALCOHOLS

The electron-transfer kinetics of Betaine-SO (B-30), which have been w idely studied in polar, aprotic solvents, are investigated for the fir st time in alcohols with femtosecond pump-probe spectroscopy. In alcoh ols, B-30 is believed to have a multidimensional solvent coordinate wi th components corresponding to solvent/solute hydrogen bonding. The ob served back-electron transfer (b-ET) times are compared to predictions of the phenomenological electron-transfer model of Walker et al., whi ch has previously been shown to work well for polar, aprotic solvents. This model fails for alcohols, presumably due to hydrogen-bonding int eractions. However, if the model is modified to include a fast compone nt of solvation corresponding to changes in hydrogen bonding, it agree s well for the linear alcohols over a large temperature range. Experim ental evidence suggests that the newly identified component of the sol vent coordinate should be associated with solvent/solute hydrogen-bond ing displacements, rather than the inertial response of the solvent. T he new experimental results in alcohols, combined with the previous re sults in polar, aprotic solvents, confirm and broaden the previous con clusion that both solvation dynamics and vibrational effects are impor tant in the electron-transfer kinetics of B-30.