Single-Molecule Junctions Based on Bipyridine: Impact of an Unusual Reorganization on Charge Transport

Abstract

The (4,4′)-bipyridine molecule (44bpy) has attracted particular interest in molecular electronics because single-molecule junctions can be directly formed via nitrogen–gold affinity, obviating the need of understanding nontrivial invasive effects due to extra anchoring groups. In a recent study, an apparent conundrum related to the transport through 44bpy junctions has been resolved by emphasizing the essential role of the environment (solvent vs ambient conditions). In the present paper, we demonstrate the robustness of the conclusion of that study, by introducing intramolecular reorganization as a new and essential element in the analysis. This extension is necessary in the light of recent investigations drawing attention to the unusual character of intramolecular reorganization in 44bpy as a molecule possessing a floppy, highly anharmonic degree of freedom, which is strongly and nonlinearly coupled to the molecular orbital dominating the charge transport. As a further important effect related to the significant and unusual intramolecular reorganization, we investigate the excess (shot) noise and find values substantially larger than in cases of molecular junctions wherein it has been measured so far. The noise power and Fano factor calculations demonstrate the importance of energy-dependent transmission, a fact disregarded in the interpretation of experimental data for nanojunctions and molecular junctions investigated so far. According to the theoretical results reported here, the intramolecular reorganization should have a more pronounced overall impact on the charge transport in 44bpy for bias voltages larger than those explored in existing experiments, but not much larger to become prohibitive. These findings should motivate companion experimental investigations in this direction