Frontier Orbital Gating of Rectification and Conductance in Tunneling Junctions Comprising Pyridine-Terminated Molecular Wires

This work experimentally investigates a mechanism of rectification in molecular junctions proposed by van Dyck and Ratner, supported by theoretical modeling. The defining feature of the mechanism is the spatial separation of frontier molecular orbitals such that each tracks the two leads independently. We achieve this orbital separation in oligophenyleneethylene molecular wires with electron-rich thiols and electron-poor pyridines at their termini. Density functional theory (DFT) calculations show localization of the frontier molecular orbitals at these termini that increases with the molecular length. Measurements of rectification ratios in molecular ensemble junctions using eutectic Ga-In (EGaIn) top-contacts and Au bottom-contacts reveal a length dependence that is almost completely insensitive to the insertion of a nonconjugated methylene spacer between the thiol anchor and conjugated backbone. Simulations using nonequilibrium Green's function + DFT methods show that transport is dominated by the lowest unoccupied molecular orbitals, which track the EGaIn electrode, leading to rectification. These results validate the approach of creating molecular rectifiers by spatially separating the frontier molecular orbitals and show an approach to modeling their behavior under bias in ensemble junctions.