Origin of the Reversed\nConductance Decay in Radical\nCationic Molecular Wires: A Molecular Orbital Perspective

One\nof the prerequisites for designing and manufacturing functional\nradical-based electronic devices is a thorough understanding of their\nconducting mechanism. By employing the first-principles quantum transport\ncalculations, we have investigated the electronic transport properties\nof a series of single-molecule junctions incorporating oligophenylene-bridged\nbis(triarylamines) (Bn, <i>n</i> = 1–4) in their\nthree different charge states: neutral, monocation, and dication.\nThe low-bias conductance of the neutral molecular junctions is mainly\ndetermined by the level alignment of the highest occupied molecular\norbital (HOMO) relative to the electrode Fermi energy and the destructive\nquantum interference (DQI) between the HOMO and HOMO–1. These\ntwo factors together lead to the exponential decay of the junction\nconductance with the molecule length. In contrast, the transmission\npeaks dominated by the SUMO (singly unoccupied molecular orbital)\nand LUMO (lowest unoccupied molecular orbital) make decisive contributions\nto the low-bias conductance of the mono- and dication junctions, respectively.\nThe Bn<sup>+</sup> SUMOs (Bn<sup>2+</sup> LUMOs) are close enough\nto the electrode Fermi energy, and their coupling strength to the\ntwo gold electrodes weakens insignificantly with increasing molecule\nlength. This results in a reversed exponential conductance decay for\nthese cation junctions within a certain length range (<i>n</i> ≤ 3). The much reduced SOMO–SUMO (HOMO–LUMO)\ngaps of Bn<sup>+</sup> (Bn<sup>2+</sup>) and the electron transfer\nfrom the two gold electrodes to the central cations in these junctions\npromote the alignment of the Bn<sup>+</sup> SUMOs (Bn<sup>2+</sup> LUMOs) to the electrode Fermi energy. Our findings elucidate the\nmechanism underpinning the opposite length dependence of the junction\nconductance on the charge and spin state of this series of molecules\nand are helpful for the design and application of radical-based electronic\ndevices.