Origin of the Reversed Conductance Decay in Radical Cationic Molecular Wires: A Molecular Orbital Perspective

One of the prerequisites for designing and manufacturing functional radical-based electronic devices is a thorough understanding of their conducting mechanism. By employing the first-principles quantum transport calculations, we have investigated the electronic transport properties of a series of single-molecule junctions incorporating oligophenylene-bridged bis(triarylamines) (Bn, n = 1–4) in their three different charge states: neutral, monocation, and dication. The low-bias conductance of the neutral molecular junctions is mainly determined by the level alignment of the highest occupied molecular orbital (HOMO) relative to the electrode Fermi energy and the destructive quantum interference (DQI) between the HOMO and HOMO–1. These two factors together lead to the exponential decay of the junction conductance with the molecule length. In contrast, the transmission peaks dominated by the SUMO (singly unoccupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) make decisive contributions to the low-bias conductance of the mono- and dication junctions, respectively. The Bn+ SUMOs (Bn2+ LUMOs) are close enough to the electrode Fermi energy, and their coupling strength to the two gold electrodes weakens insignificantly with increasing molecule length. This results in a reversed exponential conductance decay for these cation junctions within a certain length range (n ≤ 3). The much reduced SOMO–SUMO (HOMO–LUMO) gaps of Bn+ (Bn2+) and the electron transfer from the two gold electrodes to the central cations in these junctions promote the alignment of the Bn+ SUMOs (Bn2+ LUMOs) to the electrode Fermi energy. Our findings elucidate the mechanism underpinning the opposite length dependence of the junction conductance on the charge and spin state of this series of molecules and are helpful for the design and application of radical-based electronic devices.