High-Conductance Conformers in Histograms of Single-Molecule Current–Voltage Characteristics

Understanding charge transport across single molecular junctions is essential for the rational design and optimization of molecular device components. However, the correlation between calculated and experimental transport properties of single molecules probed by current–voltage (I–V) characteristics is often uncertain. Part of the challenge is that molecular conductance is sensitive to several factors that are difficult to control, including molecular orientation, conformation, aggregation, and chemical stability. Other challenges include the limitations of computational methodologies. Here, we implement the Σ-Extended Hückel (EH) nonequilibrium Green’s function (NEGF) method to analyze the histogram of I–V curves of 4,4′-diaminostilbene probed by break-junction experiments. We elucidate the nature of the molecular conformations with a widespread distribution of I–V curves, typically probed under experimental conditions. We find maximum conductance for molecules that are not at the minimum energy configuration but rather are aligned almost parallel to the transport direction. The increased conductance is due to the more favorable electronic coupling between the transport channel state and the electronic states in the contacts, as indicated by the broadening of bands in the transmission function near the Fermi level. These findings provide valuable guidelines for the design of anchoring groups that stabilize conformations of molecular assemblies with optimal charge transport properties.