Bidirectional single-molecule photoconductors based on ESIPT

Single-molecule photoconductors capable of optically modulating molecular conductance hold great promise for molecular optoelectronics, yet challenges persist in modulating bidirectional photoconductance at the single-molecule level. Here, we present a rational design strategy for high-performance single-molecule photoconductors exhibiting either increased or decreased photoconductance, enabled by the synergistic interplay between excited-state intramolecular proton transfer (ESIPT) and quantum interference (QI) effects. Utilizing the scanning tunneling microscope break junction (STM-BJ) technique, we investigate two structurally related 2-(2-hydroxyphenyl)pyridines with para- and meta-SMe groups (PPOH and PMOH), achieving record-high photoconductance modulation. Upon continuous 365 nm irradiation, PPOH-based junctions exhibit the first inverse photoconductance with a remarkable ~120-fold decrease, among the most significant reported, while PMOH-based junctions show a ~1.78-fold enhancement, demonstrating bidirectional photoconductance within a shared molecular scaffold. Theoretical calculations reveal that in PPOH, ESIPT induces frontier orbital localization, which dominates over bandgap narrowing, leading to conductance suppression. In PMOH, in addition to these two factors, ESIPT drives a QI transition from destructive interference in the ground state to constructive interference upon photoexcitation, enhancing conductance. This work bridges macroscopic photoconductor materials and individual photoresponsive molecules, offering a new molecular design paradigm for ESIPT-driven QI effects.