Tuning Conductance in π–σ–π Single-Molecule Wires

While the single-molecule conductance properties of π-conjugated and σ-conjugated systems have been well-studied, little is known regarding the conductance properties of mixed σ–π backbone wires and the factors that control their transport properties. Here we utilize a scanning tunneling microscope-based break-junction technique to study a series of molecular wires with π–σ–π backbone structures, where the π-moiety is an electrode-binding thioanisole ring and the σ-moiety is a triatomic α–β–α chain composed of C, Si, or Ge atoms. We find that the sequence and composition of group 14 atoms in the α–β–α chain dictates whether electronic communication between the aryl rings is enhanced or suppressed. Placing heavy atoms at the α-position decreases conductance, whereas placing them at the β-position increases conductance: for example, the C–Ge–C sequence is over 20 times more conductive than the Ge–C–Ge sequence. Density functional theory calculations reveal that these conductance trends arise from periodic trends (i.e., atomic size, polarizability, and electronegativity) that differ from C to Si to Ge. The periodic trends that control molecular conductance here are the same ones that give rise to the α and β silicon effects from physical organic chemistry. These findings outline a new molecular design concept for tuning conductance in single-molecule electrical devices.