When Dielectric Constants Deceive: Interrogating Solvation in Ionic Liquids with Cyclic Voltammetry

Ionic liquids are increasingly discussed as alternatives to conventional organic solvents for applications based on photoinduced electron transfer. For the rational design of such applications, reliable estimates of electron-transfer driving forces are essential. Based on the Born model of solvation, the moderate dielectric constants of ionic liquids (εr ≈ 8 - 15) suggest that they should resemble medium-polarity solvents such as dichloromethane or pyridine in photoinduced electron transfer and exhibit comparable solvation energies. Here, we test this assumption by experimentally comparing the solvation energies of three small organic solutes relevant to photochemistry in several imidazolium-based ionic liquids and in conventional dipolar solvents. Solvation energies were inferred from shifts of half-wave reduction potentials obtained from cyclic voltammetry. We find that, for the investigated solutes, ionic liquids provide solvation energies comparable to those of strongly polar solvents such as acetonitrile or dimethyl sulfoxide. While the organic solvents follow the qualitative trend predicted by the Born equation, ionic liquids deviate from it and yield much larger solvation energies compared to dipolar solvents of the same dielectric constant. This behavior is attributed to the intrinsically high ionic strength of ionic liquids, which enhances electrostatic screening and results in substantially larger solvation energies and consequently much larger driving forces for photoinduced electron transfer than would be expected based on their dielectric constants alone.