Energetics of Electron Transfer and Charge Separation in Moderate to Low Polarity Solvents

Abstract

The absorption of light in natural photosynthesis occurs in a low polarity region allowing photoelectrons to store absorbed energy in chemical bonds instead of emitting heat via solvent interaction. Consequently, studying electron transfer and charge separation in low polarity solvents may have application in solar energy storage as such a study will make electron transfer in low polarity become more thoroughly understood. Conducted in solvents of various dielectric constants, the study combines strong electron donors and electron acceptors to promote ion formation by thermal equilibria. The extent to which these molecules formed ion pairs and free ions was measured via conductivity and UV-Vis spectroscopy. Gibbs free energy changes were calculated for ion formation and separation using acetonitrile as a standard highly polar solvent. With values of ionization potentials and electron affinities, free energy changes observed in the moderately polar solvent, tetrahydrofuran, were fit to theoretical Born solvation and Coulomb potential models by determining effective ion radii. Model predictions for free energy changes in other solvents disagreed with experimental data because unexpected charge transfer complexes become significant in concentration. Approximations of free energy changes were difficult in low polarity solvents due to low conductivity measurements and charge transfer complexes that appear in UV-Vis absorption spectra. Observed free energy changes were found to be significantly less favorable than redox potentials of the same reactions determined electrochemically not in this study. The difference is attributed to the unintended stabilization of ions caused by salt in solution required for the electrochemical method.