Highly conducting single-molecule topological insulators based on mono- and di-radical cations

Abstract

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 (2e 2 /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.

Reversed Conductance Decay of 1D Topological Insulators by Tight-Binding Analysis

“…As the chain length grows, the molecule-electrode coupling becomes increasingly important. Eventually a long-chain regime is reached where the conductance-length relationship reverts to exponential decay (eq ), as indicated by the complex band theory. − This transition between the two conductance-length regimes has recently been shown in experiments. , Here, we examine systems of molecular wires terminated by radicals in molecular junctions and use a tight-binding approach to investigate the full evolution of the conductance with length. We further explain how the molecule-electrode coupling and the on-site energy of the edge sites affect the conductance trends, to highlight the role of the electrode-molecule interaction on electron transmission.…”

“…21−23 This transition between the two conductance-length regimes has recently been shown in experiments. 13,24 Here, we examine systems of molecular wires terminated by radicals in molecular junctions and use a tight-binding approach 25 to investigate the full evolution of the conductance with length. We further explain how the molecule-electrode coupling and the on-site energy of the edge sites affect the conductance trends, to highlight the role of the electrode-molecule interaction on electron transmission.…”

“…We next expand our model to capture a poly-p-phenylene wire terminated with noncarbon atoms (X) to explore whether reversed conductance decay can be achieved in synthetically accessible structures. 24 A diradical character emerges when polyphenylene is oxidized to the quinone that can support edge states (Figure 5B). In this system, a single parameter δ is insufficient to probe the structural transition between the quinone form and diradical form because there are effectively three types of C−C bonds as illustrated in Figure 5A and 5B.…”

“…Therefore, this conclusion supports the design of reversed conductance decay in more realistic molecular wires, which has been confirmed in experiments. 24 We note here that these models and derivations rely on a single-particle approach which does not include electron− electron interactions or the electron spin. In general, electron− electron interactions lead to Coulomb blockade which results in a splitting of the spin-up and spin-down density of states by a charging energy ΔU when one electron occupies a spindegenerate two-electron level.…”

“…We next expand our model to capture a poly- p -phenylene wire terminated with noncarbon atoms (X) to explore whether reversed conductance decay can be achieved in synthetically accessible structures . A diradical character emerges when polyphenylene is oxidized to the quinone that can support edge states (Figure B).…”