Length and temperature dependent conduction of ruthenium-containing redox-active molecular wires

We report the electrical transport characteristics of two series of linear ruthenium(II) bis(δ-arylacetylide) molecular wires, Ru_nM and Ru_nH (n = 1, 2, 3), consisting of multiple redox-active Ru(II) centers and different saturated side chains (M = -CH₂-, and H = -O(CH₂)₆-) with lengths up to 6.0 nm. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RA-FTIR), and cyclic voltammetry. The resistance and current (I)-voltage (V) characteristics of these molecular wires were measured as a function of length and temperature using conducting probe atomic force microscopy (CP-AFM). Both series of molecular wires exhibited very weak length dependence of the wire resistance, the β value of Ru _nM being 1.02 nm^-1, and that of Ru_nH being 1.64 nm^-1, indicating a high degree of electronic coupling between the redox centers. Further analysis of I-V characteristics revealed that the charge transport in Ru_nM junctions was direct tunneling, but in Ru _nH (n = 2, 3) junctions with the long chains the mechanism was thermally activated hopping, consistent with the temperature-dependent conduction measurement.