Experimental and theoretical identification of valence energy levels and interface dipole trends for a family of (oligo)phenylene-ethynylenethiols adsorbed on gold

Metal-molecule-metal junctions composed of organic molecular wires formed via self-assembly are of relevance in the empirical evaluation of single-molecule electronics. Key to understanding the effects of these monolayer structures on the transport through single molecules, however, is discerning how the molecular electronic levels evolve under the influence of the metal substrate and intermolecular interactions. We present a joint experimental and computational investigation of the electronic structure and electrostatic properties of a series of self-assembled donor- and acceptor-substituted (oligo)pheneylene-ethynylenethiols (OPEs) on gold. Photoemission spectroscopy is employed to determine the energy-level alignment for these monolayers. Isolated molecule and small cluster calculations are performed to assess changes in geometry, electronic structure, and charge distribution upon chemisorption. The calculated densities of electronic states allow assignment of the higher-lying occupied states mapped by experimental photoemission data. Calculated estimates of the surface, bond dipole, and image potential energies are used to estimate contributions to the measured work function changes; good correlations between the experimental and theoretical values are found. Importantly, these results point to a dependence of the dipole contributions on the orientational order of the SAM.