Length and Temperature Dependent Conduction of Ruthenium-Containing Redox-Active Molecular Wires

Abstract

We report the electrical transport characteristics of two series of linear ruthenium(II) bis(σ-arylacetylide) molecular wires, Ru n M and Ru n H ( n = 1, 2, 3), consisting of multiple redox-active Ru(II) centers and different saturated side chains (M = −CH 2 –, and H = −O(CH 2 ) 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 n M being 1.02 nm –1, and that of Ru n H 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 n M junctions was direct tunneling, but in Ru n H ( n = 2, 3) junctions with the long chains the mechanism was thermally activated hopping, consistent with the temperature-dependent conduction measurement.