Effect of external pressure on photoinduced electron-transfer reactions in the Marcus inverted region

The effect of pressure on the quantum yields for formation of free-radical ions (Φ_ions) in the photoinduced bimolecular electron-transfer reactions of the excited-state acceptors 9,10-dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and N-methylacridinium (MA⁺), with simple alkyl-substituted benzene donors, has been studied in acetonitrile at room temperature The values of Φ_ions, which are determined by the competition between return electron transfer (k_-et) and separation (k_sep) within the initially formed geminate radical ion pairs, are found to decrease with increasing pressure. The results are discussed in terms of the pressure effects on the individual rate constants k_-et and k_sep. The k_-et process is unique among those electron-transfer reactions whose pressure dependence has been studied since the reactions are in the Marcus inverted region and are nonadiabatic. Measurements of the relative rates of electron transfer as a function of reaction exothermicity allow the effect of pressure on the reorganization energy to be determined. Simple dielectric continuum models predict that the solvent reorganization energy should significantly decrease with applied pressure due to changes in the solvent dielectric constant and refractive index, whereas small increases are observed. Possible reasons for this discrepancy are discussed. Electron transfer from a neutral donor to DCA or TCA forms a radical anion/radical cation pair, whereas electron transfer to MA⁺ forms a radical/radical cation pair. Different pressure dependencies are observed for Φ_ions when the cyanoantnracene and MA⁺ acceptors are used which are attributed to differing influences of electrostriction on k_-et for the two acceptors.