Geometry-Dependent Suppression of Quantum Interference in Thiolate- and Nitrile-Terminated Naphthalene Junctions

Destructive quantum interference (DQI) in single-molecule junctions can drastically suppress electron transmission, offering a powerful mechanism for modulating molecular conductance. While DQI is often dominated by molecular connectivity, it is also sensitive to molecule-electrode coupling geometry. In this study, we investigate the impact of molecule-electrode coupling geometry, especially those induced by different anchoring motifs, on the manifestation of DQI using naphthalene derivatives substituted at the 2,6- or 2,7-positions and terminated with thiolate or nitrile groups. Conductance measurement using the break junction method revealed that the 2,7-substituted thiolate-anchored molecular junction exhibited strong DQI, while the corresponding nitrile-anchored molecular junction did not, showing conductance level comparable to the 2,6-isomer. Flicker noise analysis and density functional theory-based transmission calculations suggest that the suppression of DQI in the nitrile system arises from a face-on adsorption geometry, which induces direct π-system overlap between the molecule and the Au electrodes. This overlap effectively introduces a through-space conduction pathway that bypasses DQI features present in the through-bond channel. Our findings demonstrate that subtle variations in molecule-electrode geometry can strongly influence quantum interference and provide valuable guidelines for designing molecular devices with tailored transport properties.