The electron-acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence or absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation or reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.
