Investigation of the charge transport in model single molecule junctions based on expanded bipyridinium molecular conductors

In this work the charge transport and energetics of a photochemically addressable single molecule switch based on the expanded bipyridinium core linked to an anchoring group with a controlled torsion angle theta was investigated. Electrochemical and UV-VIS absorption spectroscopy techniques complemented by the analysis based on density functional theory (DFT) show that for selected molecules the energy and shape of the LUMO is insensitive to the value of theta, but the difference in torsion angle theta leads to a sizable shift of the LUMO energy and single molecule conductance value in a metal-molecule-metal junction for these molecules as shown by the combination of experimental single molecule break junction technique and theoretical non-equilibrium Green's function (NEGF)/DFT approach. The conductance switching ratio calculated from the cos(2) theta law is in a perfect agreement with the value obtained from the NEGF approach. Our combined experimental and theoretical approach paves the way for investigating expanded bipyridinium systems with multiple photochemically addressable units potentially achieving greater conductance switching ratios. (C) 2019 Elsevier Ltd. All rights reserved.