Quantifying Molecular Structure-Tunneling Conductance Relationships: Oligophenylene Dimethanethiol Vs Oligophenylene Dithiol Molecular Junctions

We report quantitative analysis of tunneling conductance in molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene dimethanethiols (OPDMn) in which -CH2- spacers flank either side of the phenylene (n = 1), biphenylene (n = 2), and terphenylene (n = 3) aromatic cores. The current-voltage (I-V) characteristics for the OPDMn junctions with Au and Pt contacts are analyzed quantitatively with a previously validated single level model (SLM) to extract key junction metrics, namely the HOMO-to-Fermi-level offset, epsilon(h), and the electronic coupling, Gamma. Independent determination of epsilon(h) by ultraviolet photoelectron spectroscopy (UPS) corroborates the estimation of epsilon(h) from the I-V characteristics and provides strong evidence for the validity of the SLM analysis. Further, comparison of the results for OPDMn junctions with those for oligophenylene dithiol (OPDn) junctions, which do not have -CH2- spacers, reveals that the much larger resistance for OPDMn (>1000-fold) is primarily due to a similar to 50-fold decrease in G and not to any significant change in epsilon(h); epsilon(h) is nearly identical for OPDMn and OPDn junctions for each value of n. Overall, our results provide a clear delineation of the influence of -CH2- spacers on epsilon(h) and Gamma and give further evidence that the analytical SLM is a useful tool for determining structure-transport relationships in molecular tunnel junctions.