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.