Photothermal Switchable Single-Molecule Wires via Donor–Acceptor Side Chains

Despite considerable efforts being made to fine-tune single-molecule conductance, precise and reversible control of electron distribution at specific sites on a molecular backbone remains a significant challenge in molecular electronics studies. Herein, we design single-molecule wires with a carbazole donor backbone and an azobenzene-bridged acceptor side chain. Photoisomerization of the azobenzene unit enables partial switching to a steady state, which can be reset to the initial configuration via thermal relaxation. This process reversibly modulates the donor-acceptor interaction, thereby modulating the donor-acceptor electronic coupling and the spatial conjugation and, consequently, the molecular conductance. Scanning tunneling microscopy break junction measurements demonstrate distinct and reproducible conductance switching between the photogenerated and thermally recovered states. This side-chain engineering strategy, utilizing combined photo and thermal stimuli, provides a novel approach for reversible, state-dependent control of charge transport at the single-molecule level, offering valuable insights for developing functional molecular electronic devices.