Determination of Energy Level Alignment and CouplingStrength in 4,4′-Bipyridine Single-Molecule Junctions.

We measure conductance and thermopowerof single Au–4,4′-bipyridine–Aujunctions in distinct low and high conductance binding geometriesaccessed by modulating the electrode separation. We use these datato determine the electronic energy level alignment and coupling strengthfor these junctions, which are known to conduct through the lowestunoccupied molecular orbital (LUMO). Contrary to intuition, we findthat, in the high-conductance junction, the LUMO resonance energyis further away from the Au Fermi energy than in the low-conductancejunction. However, the LUMO of the high-conducting junction is bettercoupled to the electrode. These results are in good quantitative agreementwith self-energy corrected zero-bias density functional theory calculations.Our calculations show further that measurements of conductance andthermopower in amine-terminated oligophenyl–Au junctions, whereconduction occurs through the highest occupied molecular orbitals,cannot be used to extract electronic parameters as their transmissionfunctions do not follow a simple Lorentzian form. [ABSTRACT FROM AUTHOR]