Through-Space-Conjugated Viologen Cyclophanes for Single-Molecule Junctions

Abstract

Molecular electronics offers a promising route toward device miniaturization beyond the limits of silicon-based technologies, but the rational design of molecular components with stable and tunable charge transport remains challenging. Through-space-conjugated (TSC) viologens are attractive candidates for single-molecule devices because they combine spatial π-electron delocalization, redox activity, and strong electron-accepting ability; however, their weak and conformation-sensitive electronic coupling leads to fluctuations in conductance behavior, limiting the effective utilization of conductance states in single-molecule junctions. Herein, we report a rigid folded TSC viologen cyclophane 5 that integrates strong electron-accepting viologen units, spatial π-electron delocalization, and conformationally constrained cyclophane architecture. Cyclophane 5 exhibits reversible redox behavior together with pronounced photoluminescence. In situ high-pressure fluorescence measurements confirm the conformational rigidity of 5, in contrast to the pressure-sensitive flexible half-box reference 7, accounting for their divergent device performance. Scanning tunneling microscopy break-junction measurements show that 5 forms stable voltage-responsive junctions with a conductance peak at 10–4.01G0 under biases above 0.3 V. Non-equilibrium Green’s function calculations reveal LUMO-dominated charge transport and show that the applied electric field strengthens through-space conjugation, thereby enhancing molecular conductance. This work establishes rigid folded TSC viologen cyclophanes as a promising platform for voltage-responsive single-molecule electronics and provides design guidelines for next-generation organic molecular electronic materials and devices.