Influence of Conformation on Conductance of Biphenyl-Dithiol Single-Molecule Contacts

The conductance of a family of biphenyl-dithiol derivatives with conformationally fixed torsion angle was measured using the scanning tunneling microscopy (STM)-break-junction method. We found that it depends on the torsion angle Φ between two phenyl rings; twisting the biphenyl system from flat (Φ = 0°) to perpendicular (Φ = 90°) decreased the conductance by a factor of 30. Detailed calculations of transport based on density functional theory and a two level model (TLM) support the experimentally obtained cos2 Φ correlation between the junction conductance G and the torsion angle Φ. The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the π-π coupling dominates the transport under "off-resonance" conditions where the HOMO levels are well separated from the Femi energy.