Electron transfer of functionalized quinones in acetonitrile

Quinones are redox active organic molecules that have been proposed as an alternative choice to metal-based materials in electrochemical energy storage devices. Functionalization allows one to fine tune not only their chemical stability but also the redox potential and kinetics of the electron transfer reaction. However, the reaction rate constant is not only determined by the redox species but also impacted by solvent effects. In this work, we show how the functionalization of benzoquinone with different functional groups impacts the solvent reorganization free energies of electron transfer half-reactions in acetonitrile. The use of molecular density functional theory, whose computational cost for studying the electron transfer reaction is considerably reduced compared to the state-of-the-art molecular dynamics simulations, enables us to perform a systematic study. We validate the method by comparing the predictions of the solvation shell structure and the free energy profiles for electron transfer reaction to the reference classical molecular dynamics simulations in the case of anthraquinone solvated in acetonitrile. We show that all the studied electron transfer half-reactions follow the Marcus theory, regardless of functional groups. Consequently, the solvent reorganization free energy decreases as the molecular size increases.