Photoconductance Induced by Excited-State Intramolecular Proton Transfer (ESIPT) in Single-Molecule Junctions

Excited-state intramolecular Proton Transfer (ESIPT) molecules have been drawing considerable attention due to their unique photophysical properties and potential applications in optoelectronic devices. Although ground and excited-state tautomerism in various proton transfer systems associated with ESIPT has been extensively studied both experimentally and theoretically, the charge-transport characteristics of ESIPT molecules at the single-molecule level has been little investigated. In this work, scanning tunneling microscope-based fixed junction technique (STM-FJ) is employed with theoretical calculations to explore the electronic properties of SMe-PhOH (with ESIPT properties), together with its photoconductance induced by ESIPT photocycle processes under continuous light illumination (254/275/295/310 nm). The conductance variation of SMe-PhOH with different UV wavelengths exhibits a continuous photoconductance distribution, which is highly consistent with the results of its UV-vis absorption spectrum. Theoretical calculations indicate that the interaction between localized HOMO and delocalized LUMO of SMe-PhOH K^*-state gives rise to Fano resonance, thereby leading to enhanced conductance compared with its E-state. It reveals the microscopic mechanism of ESIPT process at the nanoscale and provides a constructive perspective for optimizing the photoresponsive properties of ESIPT-type molecules, as well as designing high-performance single-molecule devices.