Identifying Carbon–Carbon Triple Bonds from Double Bonds via Single-Molecule Conductance

Molecular-scale electronics focuses on understanding and utilizing charge transport through individual molecules. A key issue is the charge transport capability of a single molecule characterized by current decay. We visualize the on-site formation of conjugated polymers with varying carbon–carbon bond orders by using scanning tunneling microscopy and noncontact atomic force microscopy. Although carbon–carbon double bonds and triple bonds exhibit similar electronic characteristics, single-molecule conductance measurements reveal distinct features based on different levels of conjugation. These findings, supported by density functional theory calculations, indicate that a higher bond order results in greater electron density and more symmetric molecular orbitals, leading to larger transmission rates and more rigid frontier orbitals. Consequently, this contributes to a higher conductance and a lower decay constant. These findings enhance the understanding of bond orders in molecular electronics and should facilitate the development of single-molecule devices and the applications of nanoscale circuitry.