A. Mishchenko, D. Vonlanthen, V. Meded, M. Buerkle, C. Li, I.V. Pobelov, A. Bagrets, J.K. Viljas, F. Pauly, F. Evers, M. Mayor, and T. Wandlowski

Nano Letters 10, 156-163 (2010)

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.