Proton-Coupled Electron Transfer at Anthraquinone Modified Indium Tin Oxide Electrodes

The molecular modification of electrode surfaces enables synthesis of materials with atomic-level specificity. In order to fully realize the benefits of molecularly modified materials, immobilized molecules must maintain their solution-phase reactivity upon attachment to surfaces. We report the noncovalent immobilization of a pyrene-appended anthraquinone derivative onto indium tin oxide electrodes. X-ray photoelectron spectroscopy and cyclic voltammetry confirm the attachment of anthraquinone on the electrode surface. Cyclic voltammetry is reported between pH 5 and 9. The reduction potential of immobilized anthraquinone shifts cathodically with increasing pH by 60 mV/pH unit, consistent with proton-coupled electron transfer. The electron transfer rate constant also increases with increasing pH. Analogous electrochemical experiments performed using a water-soluble anthraquinone derivative freely diffusing in aqueous solution exhibit the same pH dependence in reduction potential. Thus, anthraquinone maintains its proton-coupled electron-transfer behavior upon noncovalent immobilization onto indium tin oxide electrodes.