Temperature and Length Dependence of Charge Transport in Redox-Active Molecular Wires Incorporating Ruthenium(II) Bis(σ-arylacetylide) Complexes

We report the electrical transport behavior of a series of redox-active conjugated molecular wires as a function of temperature and molecular length. The wires consist of covalently coupled ruthenium(II) bis(σ-arylacetylide) complexes (Ru1-Ru3) and range in length from 2.4 to 4.9 nm. The molecules are unique in that they contain multiple metal-redox centers that are well-coupled by conjugated ligands. The molecules were self-assembled and their films were extensively characterized using ellipsometry, X-ray photoelectron spectroscopy, reflectionabsorption infrared spectroscopy, and cyclic voltammetry. We probed their electrical properties using conducting probe atomic force microscopy and crossed-wire junctions. At room temperature, we found a very weak dependence of the wire resistance with molecular length, consistent with a high degree of electronic communication along the molecular backbone. In low-temperature (5 K) experiments, Coulomb blockadelike behavior was observed in junctions incorporating Ru3; direct tunneling appears to be the dominant transport mechanism in Ru1 and Ru2 junctions.