Extreme\nAnomalous Conductance Enhancement in Neutral\nDiradical Acene-like Molecular Junctions

We\nachieve, at room temperature, conductance enhancements over\n2 orders of magnitude in single molecule circuits formed with polycyclic\nbenzoquinoidal (<b>BQ<sub>n</sub></b>) diradicals upon increasing\nmolecular length by ∼5 Å. We find that this extreme and\natypical anti-ohmic conductance enhancement at longer molecular lengths\nis due to the diradical character of the molecules, which can be described\nas a topologically nontrivial electronic state, and results in constructive\ninterference between the frontier molecular orbitals. The distinct\nfeature of the compounds studied here as molecular wires is that they\nare characterized by moderate diradical character in the neutral state,\nallowing for robust and facile measurements of their transport properties.\nWe adapt the 1D-SSH model, originally developed to examine electronic\ntopological order in linear carbon chains, to the polycyclic systems\nstudied here and find that it captures the anti-ohmic trends in this\nmolecular series. Specifically, our model reveals that the mechanism\nof conductance enhancement with length in polycyclic systems is constructive\nquantum interference between the frontier orbitals with nontrivial\ntopology, which is present in acene-like, but not in linear, molecular\nsystems. Importantly, we use our model to predict and experimentally\nvalidate that anti-ohmic trends can be engineered through synthetic\nadjustments of the diradical character of the acene-like molecules.\nOverall, we achieve extreme anti-ohmic enhancement and mechanistic\ninsight into electronic transport in a class of materials that we\nidentify here as promising candidates for creating highly conductive\nand tunable nanoscale wires.