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

The electron-transfer kinetics of Betaine-SO (B-30), which have been widely studied in polar, aprotic solvents, are investigated for the first time in alcohols with femtosecond pump-probe spectroscopy. In alcohols, B-30 is believed to have a multidimensional solvent coordinate with components corresponding to solvent/solute hydrogen bonding. The observed back-electron transfer (b-ET) times are compared to predictions of the phenomenological electron-transfer model of Walker et al., which has previously been shown to work well for polar, aprotic solvents. This model fails for alcohols, presumably due to hydrogen-bonding interactions. However, if the model is modified to include a fast component of solvation corresponding to changes in hydrogen bonding, it agrees well for the linear alcohols over a large temperature range. Experimental evidence suggests that the newly identified component of the solvent coordinate should be associated with solvent/solute hydrogen-bonding displacements, rather than the inertial response of the solvent. The new experimental results in alcohols, combined with the previous results in polar, aprotic solvents, confirm and broaden the previous conclusion that both solvation dynamics and vibrational effects are important in the electron-transfer kinetics of B-30.