Coupled Electron/Proton Transfer Studies of Benzoquinone-Modified Monolayers

Abstract

The electrochemical reduction of benzoquinone involves a coupled transfer of protons and electrons. Theoretical treatments of proton-coupled electron transfer (PCET) indicate that both thermodynamic (apparent formal potential) and kinetic (apparent standard heterogeneous rate constants) quantities should be highly pH-dependent. Although there are several reports of quinone-derivatized self-assembled monolayers (SAMs) in the literature, no systematic studies on the pH dependence of the PCET kinetics have been reported. In this work, we outline a method to produce SAM-aminobenzoquinone monolayers starting from diluted monolayers of an amine-terminated SAM. The electroactive monolayer behaves as a nearly ideal Nernstian system (no interactions between redox centers). In neutral to weakly acidic electrolytes, the aminobenzoquinone can be reduced in a kinetically slow, two-electron, two-proton (2e2H) process, but in lower pH solutions, the observed reaction is consistent with an overall 2e3H transfer. Kinetic measurements have been made using techniques that do not rely on the assumption of a potential-independent apparent transfer coefficient. The apparent standard rate constants range from 0.1 to 0.01 s−1, which is roughly an order of magnitude faster than previous reports for comparable systems. Plots of log(ks,app) versus pH are distorted variants of the expected W plots predicted from the theory of 2e2H PCET. These deviations may be caused by surface pKa effects and/or additional reaction pathways arising from the third proton transfer.