Probing charge transport of ruthenium-complex-based molecular wires at the single-molecule level

A ruthenium(II) bis(sigma-arylacetylide)-complex-based molecular wire functionalized with thiolacetyl alligator clips at both ends (OPERu) was used to fabricate gold substrate-molecular wire-conductive tip junctions. To elucidate the ruthenium-complex-enhanced charge transport, we conducted a single-molecule level investigation using the technique-combination method, where electronic decay constant, single-molecular conductance, and barrier height were obtained by scanning tunneling microscopy (STM) apparent height measurements, STM break junction measurements, and conductive probe-atomic force microscopy (CP-AFM) measurements, respectively. A quantitative comparison of OPERu with the well-studied pi-conjugated molecular wire oligo(1,4-phenylene ethynylene) (OPE) indicated that the lower electronic decay constant as well as the higher conductance of OPERu resulted from its lower band gap between the highest occupied molecular orbital (HOMO) and the gold Fermi level. The small offset of 0.25 eV was expected to be beneficial for the long-range charge transport of molecular wires. Moreover, the observed cross-platform agreement proved that this technique-combination method could serve as a benchmark for the detailed description of charge transport through molecular wires.