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 phi between two phenyl rings: twisting the biphenyl system from flat (phi = 0 degrees) to perpendicular (phi = 90 degrees) 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 cos(2) phi correlation between the junction conductance G and the torsion angle phi. The TLM describes the pair of hybridizing highest occupied molecular orbital (HOMO) states on the phenyl rings and illustrates that the pi-pi coupling dominates the transport under off-resonance conditions where the HOMO levels are well separated from the Femi energy.