Synergetic Control of Single-Molecule Proton Transfer: Electrode Geometry Meets Solvent Proton Activity

Understanding the hydrogenation/dehydrogenation of proton-active molecules on solid surfaces is fundamental in broad fields such as molecular electronics, catalysis, and biology. However, elucidating the proton transfer mechanism at such interfaces remains challenging due to the complex interplay between dynamic electrode reconstruction and the aqueous environment. Here, combining scanning tunneling microscope break junction experiments with density functional theory (DFT) calculations, we reveal the synergetic interplay between proton concentration and electrode configuration that regulates the interfacial proton transfer of octanedioic acid and its single-molecule conductance. Our DFT calculations successfully reproduce the experimentally observed high- and low-conductance plateaus, primarily attributing them to the promoting effect of hydrogen-bonded water networks and diatomic sites on carboxyl deprotonation. Furthermore, ab initio molecular dynamics simulations uncover a structural evolution of the electrode terminals during junction stretching (transitioning from a diatomic to a monatomic configuration). This evolution induces a deprotonation-to-protonation transition, manifesting as a distinct two-step plateau in individual conductance traces. This work establishes a framework connecting solution proton concentration, interfacial proton transfer kinetics, and single-molecule conductance, providing fundamental insights for the rational design of electrochemical interfaces and devices.