Stereoelectronic Modulation of a Single-Molecule Junction through a Tunable Metal-Carbon d?-p? Hyperconjugation

Conjugated molecules play a critical role in the construction of single-molecule devices. However, most conventional conjugated molecules, such as hydrocarbons, involve only a pir-pir conjugation of light elements. While the metal d-orbitals can introduce abundant electronic effects to achieve novel electronic properties, it is very scarce for the charge transport study of dir-pir conjugated pathways with a metal involved. Here, we employed the single-molecule break junction technique to investigate the charge transport through dir-pir conjugated backbones with metal-carbon multiple bonds integrated into the alternative conjugated pathways. The involved dir-pir conjugation not only supports high conductivity comparable to that of conjugated hydrocarbons but also significantly enhances the tunable diversity in electronic properties through the metal-induced secondary interaction. Specifically, the introduction of the metal brings an unconventionally stereoelectronic effect triggered by metal-carbon dir-pir hyperconjugation, which can be tuned by protonation taking place on the metal-carbon multiple bonds, collectively modulating the single-molecule rectification feature and transmission mechanism. This work demonstrates the promise of utilizing the diverse electronic effect of metals to design molecular devices.