GOLD ELECTRODE MODIFICATION WITH A MONOLAYER OF 1,4,5,8-NAPHTHALENETETRONE AS A 4-ELECTRON TRANSFER MEDIATOR - ELECTROCHEMICAL REDUCTION OF DIOXYGEN TO WATER

The electrochemical properties of an unusual diquinone containing four carbonyl groups in the molecule (a carboxylic derivatized 1,4,5,8-naphthalenetetrone) were studied in aqueous solution and in monolayer form on a gold electrode surface. Monolayer covalent immobilization of the diquinone was achieved owing to the carbodiimide coupling of its carboxylic group with amino groups of cystamine chemisorbed previously onto the gold electrode surface. Cyclic voltammetry, constant potential electrolysis and rotating ring-disk electrode measurements were used to reveal the four-electron transfer process for dioxygen reduction mediated by the monolayer of immobilized diquinone. A very small amount of hydrogen peroxide was produced and a very high current efficiency, ca. 95%, for water formation was evaluated. These four-electron transfer properties are observed for quinonoid compounds for the first time. However, the diquinone redox potential is too negative (-0.48 V vs. SCE, pH 7.0) to provide efficient electrocatalysis for the four-electron reduction of dioxygen and the diquinone mediated electron transfer process could be observed only on the electrode surface blocked for the normal non-mediated dioxygen reduction. New compounds with a similar structure but with much more positive redox potentials are suggested as a new class of catalyst for the four-electron reduction of dioxygen.