Length dependence of frontier orbital alignment in aromatic molecular junctions

Abstract

We report on experiments and computations performed on a series of aromatic monothiol molecular junctions (AMMJs) to ascertain both the identity of the frontier molecular orbitals (FMOs) and their approximate energetic separation from the chemical potential. Joint transition voltage spectroscopy and thermoelectric measurements unambiguously show that the FMOs in all the studied junctions are the highest occupied molecular orbitals and that the energetic separation decreases with increasing molecular length. Our computational studies of energetic separations and Seebeck coefficients of these AMMJs are in agreement with the experimentally obtained values and elucidate the electronic structure origins of the observed length dependence.