Conductance Evolution of Photoisomeric Single-Molecule Junctions under Ultraviolet Irradiation and Mechanical Stretching

A comprehensive understanding of carrier transport in photoisomeric molecular junctions is crucial for the rational design and delicate fabrication of single-molecule functional devices. It has been widely recognized that the conductance of azobenzene (a class of photoisomeric molecules) based molecular junctions is mainly determined by photoinduced conformational changes. In this study, it is demonstrated that the most probable conductance of amine-anchored azobenzene-based molecular junctions increases continuously upon UV irradiation. In contrast, the conductance of pyridyl-anchored molecular junctions with an identical azobenzene core exhibits a contrasting trend, highlighting the pivotal role that anchoring groups play, potentially overriding (even reversing) the effects of photoinduced conformational changes. It is further demonstrated that the molecule with cis-conformation cannot be fully mechanically stretched into the trans-conformation, clarifying that it is a great challenge to realize a reversible molecular switch by purely mechanical operation. Additionally, it is revealed that the coupling strength of pyridyl-anchored molecules is dramatically weakened when the UV irradiation time is prolonged, whereas it is not observed for amine-anchored molecules. The mechanisms for these observations are elucidated with the assistance of density functional theory calculations and UV-Vis spectra combined with flicker noise measurements which confirm the photoinduced conformational changes, providing insight into understanding the charge transport in photoisomeric molecular junctions and offering a routine for logical designing synchro opto-mechanical molecular switches.