Highly Conducting Single-Molecule Topological Insulators Based on Mono- and Di-Radical Cations

Single-molecule topological insulators are promising candidates as conducting wires over nanometer length scales. In past, most conjugated molecular wires exhibit low conductance that decays as the wire length increases. To overcome this limitation, we studied a family of oligophenylene-bridged bis(triarylamines) with tunable and stable (mono-/di-)radicaloid character. The wires can undergo one- and two-electron chemical oxidations to the corresponding monocation and dication, respectively. We found that the oxidized wires exhibit high reversed conductance decay with increasing length, consistent with the expectation for the Su-Schrieffer-Heeger-type one-dimensional (1D) topological insulators. The champion 2.6 nm long dication displays a significantly high conductance greater than 0.1 G0 (2e2/h, the conductance quantum), 5400-fold greater than the neutral form. The observed conductance-length relationship is similar between monocation and dication series. DFT calculations elucidate how the frontier orbitals and delocalization of radicals facilitate the observed nonclassical quasi-metallic behavior. These findings offer new insights into molecular design of highly conducting 1D topological insulators.