Switchable modes of azulene-based single molecule–electrode coupling controlled by interfacial charge distribution

Abstract

Molecule–electrode interactions are critical for determining transport mechanisms and device functionalities in both single-molecule electrochemistry and electronics. Crucial factors such as anchoring groups and local fields have been studied, but the role of electrolytes and interfacial charge distribution remains largely underexplored. The present research focuses on how the interfacial charge distribution in the electric double layer (EDL) controls single-molecule junctions anchored by azulene. This probe molecule is chosen for its distinct charge properties in its 5- and 7-membered condensed ring structures that impose unique sensitivity to the surrounding electric field. Using scanning tunneling microscopy break junction (STM-BJ) techniques, we systematically investigate the conductance, anchoring sites, and coupling strength of these junctions in organic liquid but non-electrolytic environments, in aqueous solution under varying ionic strengths, and across different electrode systems and potential profiles. Our results demonstrate that the conductance and molecule–electrode coupling modes can be effectively tuned through control of interfacial charge distribution, particularly by altering the ion distribution around the electrodes. Mechanical modulation experiments substantiate these trends, and theoretical calculations pinpoint ion distribution as a key driver of molecule–electrode interaction. This research introduces a novel approach to dynamic control of the azulene–electrode coupling through electrolyte manipulation, offering entirely new insight for the design of electrolyte-responsive, switchable single-molecule devices.