Enhanced charge transport via d(δ)–p(π) conjugation in Mo2-integrated single-molecule junctions

Abstract

A trans-dimolybdenum nicotinate (m-Mo₂) complex and its isonicotinate isomer (p-Mo₂) were synthesized and characterized crystallographically, and their single-molecule charge transport properties were investigated using the STM break junction (STM-BJ) technique. With a quadruply bonded Mo₂ complex unit integrated into molecular backbones, the single-molecule conductance for complex molecules was increased by more than one order of magnitude compared with that of the organic π-conjugated analogues 1,4-bis(4-pyridyl)benzene (p-Ph) and 1,4-bis(3-pyridyl)benzene (m-Ph). More interestingly, unlike m-Ph, m-Mo₂ with meta connected pyridyl anchors presents larger conductance than that of p-Mo₂ with two para connected pyridyl groups. DFT-based transmission calculations revealed that the significant conductance enhancement of Mo₂ molecules originates from the largely reduced HOMO–LUMO gap, and the unique d(δ)–p(π) conjugation between the Mo₂ unit and the pyridine rings gives rise to a delocalized electronic structure that endows the Mo₂ molecules with an unexpected high conductance.