Designing Viologen Molecular Wires with Enhanced ElectronTransport Properties: Implication for Molecular Electronics

Achieving efficient charge transport across nanometers of organic matter remains a fundamental challenge in molecular electronics. In this work, thin layers of fully conjugated oligomers incorporating viologen units with thicknesses between 2 and 17 nm have been deposited on gold electrodes by electrochemical reduction of a diazonium salt and used as active layers in a solid-state large-area molecular junction (MJ) using titanium/gold top electrode. The devices show symmetric J–V curves, highly efficient long-range transport with an apparent attenuation factor as small as 0.3 ± 0.03 nm–1, and, most importantly, record high extrapolated current density at zero thickness. These outstanding performances are attributed to the fact that the viologen-based oligomers are fully conjugated, strongly coupled with the electrodes and that their LUMO energy lies between the energies of the Fermi levels of the two contacts. Consequently, the injection barrier at the interfaces is low and resonant tunneling combined with redox hopping prevails for all investigated thicknesses.