Zero-Bias Anti-Ohmic Behaviour in Diradicaloid Molecular Wires

Open-shell materials bearing multiple spin centres hold the key to efficient charge transport in singlemolecule electronic devices. They have very narrow bandgaps, and their partially occupied molecular orbitals align very efficiently to the Fermi level of the metallic electrodes of the single-molecule junction, thus allowing transparent electronic transport and higher conductance. Maintaining and stabilising multiple open-shell states, especially in contact with metallic electrodes is however very challenging, generally requiring a continuous chemical or electrochemical potential to avoid self-immolation of the open-shell character. To overcome this issue, we designed, synthesised, and measured the conductance of a series of bis(indeno) fused acenes, having a diradicaloid structure in resonance with a close-shell quinoidal conformation, providing steric protection with 3,5-dimethylthioanisole anchors to the electrodes and electronic protection against oxidation with tris(isopropyl)ethynyl substituents at the heart of the acene. We show here that these compounds have extremely anti-ohmic behaviour, with conductance increasing with increasing length at an unprecedented rate, across the entire bias window (±1.3 𝑉). Density Functional Theory (DFT) calculations support our findings, showing the rapidly narrowing bandgap unique to these diradicaloid structures is responsible for the observed behaviour. Our results provide a framework for achieving efficient transport in neutral compounds and demonstrate the promise that diradicaloid materials have in single-molecule electronics, owing to their great stability and unique electronic structure.