Antiaromaticity-Driven Enhancement of Single-Molecule Electrical Conductance by s-Indacene Molecular Bridges

High electrical conductance of antiaromatic molecules compared with aromatic counterparts at the single-molecule level has been a topic of controversy, with only a few previous experimental proofs. We synthesized length-matched molecular wires incorporating 12 π-electron antiaromatic s-indacene (Ind) and 14 π-electron aromatic benzodithiophene (BDT) cores with different anchoring modes and measured the single-molecule electrical conductance (SMEC) by scanning tunneling microscopy-break junction method, validating that antiaromatic cores significantly enhance the conductance. For the diagonally anchored systems, the antiaromatic Ind derivative exhibits SMEC approximately 10 times higher than that of its aromatic BDT analogue, thanks to the cooperative effects of a linear conjugation pathway and the narrow gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) that brings favorable frontier molecular orbital (FMO) alignment with the electrode Fermi level. Notably, for the horizontally anchored systems, the Ind derivative exhibits SMEC 2.8 times higher than that of its BDT counterpart, as its antiaromaticity-driven FMO alignment overtakes the cross-conjugation pathway. Additionally, introduction of electron-withdrawing trifluoromethyl groups to the Ind core shifts the LUMO closer to the Fermi level, resulting in 1.5 times higher conductance. These results definitively demonstrate the enhancement of SMEC driven by antiaromaticity.