Control over Molecular Orbital Gating and Marcus Inverted Charge Transport in Molecular Junctions with Conjugated Molecular Wires

Recently it is discovered that molecular junctions can be pushed into the Marcus Inverted region of charge transport, but it is unclear which factors are important. This paper shows that the mechanism of charge transport across molecular wires can be switched between the normal and Marcus Inverted regions by fine-tuning the molecule–electrode coupling strength and the tunneling distance across oligophenylene ethynylene (OPE) wire terminated with ferrocene (Fc) abbreviated as S-OPE_nFc (n = 1–3). Coherent tunneling dominates the mechanism of charge transport in junctions with short molecules (n = 1), but for n = 2 or 3 redox reactions become important. By weakening the molecule—electrode interaction by interrupted conjugation, S-CH₂-OPE_nFc, intramolecular orbital gating can occur pushing the junctions completely into the Marcus Inverted region. These results indicated that weak molecule—electrode coupling is important to push junctions into the Marcus Inverted Region.