An Electrochemical Scanning Tunneling Microscopy Study of 2-(6-Mercaptoalkyl)hydroquinone Molecules on Au(111)Molecular Junctions

The hydroquinone/benzoquinone redox couple involves the exchange of two electrons and two protons in its oxidation/reduction reaction in aqueous buffered solutions. In this work, we employed Electrochemical Scanning Tunneling Microscopy and Spectroscopy (EC-STM, EC-STS) to study the interfacial electron transfer properties of hydroquinone incorporated in a Self Assembled Monolayer on a Au(111) substrate. The exchange of electrons between the STM tip and the substrate is regulated by the redox levels of the sandwiched molecule and showed the presence of two regions of tunneling enhancement in the tunneling current/overvoltage relationship. The two regions can be attributed to the presence of two one-electron transfer processes whose equilibrium positions shift upon pH variations. This is the first time a redox molecule involving the exchange of both electrons and protons is studied by EC-STM and EC-STS. The hydroquinone/benzoquinone redox couple can be exploited to obtain an electrochemically or a pH gated transistor.