Solvent effects on the energetics of intermolecular charge-transfer reactions

Article abstract of DOI:10.1021/ja00176a002

Picosecond absorption spectroscopy is used to examine the intermolecular electron-transfer reaction between the trans-stilbene radical cation and a series of olefin radical anions in polar solvents. Laser excitation of a ground-state complex between trans-stilbene and electron-deficient olefins selectively generates the contact ion pair within the time resolution of the experiment. By monitoring the time-dependent absorption of the trans-stilbene radical cation, the dynamics of electron back-transfer and ion-pair separation are studied. The rate of electron back-transfer is found to be temperature independent in the series of polar aprotic solvents over the temperature range from -8 °C to +80 °C. However, a linear correlation is observed between the charge-transfer rate and solvent polarity. With this result, the temperature data are interpreted in terms of solvent polarity effects on the activation energy for charge transfer. Once the rates are corrected for variation of solvent polarity, activation parameters are determined and the contributions of solvation and intramolecular modes to the reorganizational energy are evaluated. The dynamics of ion-pair separation are also briefly discussed.