Influence of Conformation on Conductance of Biphenyl-Dithiol Single-Molecule Contacts Artem Mishchenko, † David Vonlanthen, ‡ Velimir Meded, § Marius Bu ¨ rkle, | Chen Li, † Ilya V. Pobelov, † Alexei Bagrets, § Janne K. Viljas, | Fabian Pauly,* ,| Ferdinand Evers,* ,§,⊥ Marcel Mayor,* ,‡,§ and Thomas Wandlowski* ,† † Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland, ‡ Department of Chemistry, University of Basel, 4003 Basel, Switzerland, § Institute of Nanotechnology, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany, | Institut fu ¨ r Theoretische Festko ¨ rperphysik, ⊥ Institut fu ¨r Theorie der Kondensierten Materie, and University of Karlsruhe, 76128 Karlsruhe, Germany ABSTRACT 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  between two phenyl rings; twisting the biphenyl system from flat ( ) 0°) to perpendicular ( ) 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 cos 2  correlation between the junction conductance G and the torsion angle . 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. U nderstanding of the charge transport characteristics of molecules in nanoscale metal-molecule-metal junctions is of fundamental interest and represents akeysteptowardtherealizationofmolecule-basedelectronics. 1,2 Several experimental approaches have been employed to measure transport through single and small groups of molecules. Examples are mechanical 3–6 or electromigration 7,8 break junctions, nanopores, 9 crossed-wire junctions, 10 mer- cury drop electrodes, 11 and a variety of scanning probe methods based on either scanning tunneling spectroscopy (STS) 12–18 or conducting probe atomic force microscopy (CP-AFM). 19,20 These measurements differ in the following criteria: (i) formation of reproducible contacts between molecules and two probing electrodes, (ii) access of “signa- tures” of single molecules, and (iii) algorithm of data analysis. These experimental results and numerous contributions on theoretical aspects of charge transport through molecular junctions 1,21 suggest that the transport characteristics are controlled by the intrinsic properties of the molecules, the contacts (“alligator clips”), and the metal leads. These include the molecular length, conformation, the gap between HOMO and LUMO, the alignment of this gap to the metal Fermi level, and the metal-molecule coordination geometry. Other factors of influence comprise temperature, mechanical stress, environment (UHV, gas, or solution phase), and the applied potential. Understanding of relationships between molecular structure and electronic transport characteristics of single- molecule junctions is a major challenge. Progress may lead to the rational design of functional molecules as active components in future electronic circuits. To date, the prop- erties of a specific molecular configuration and binding geometry with respect to stability and transmitivity have not been observed directly. Most experimental strategies are instead based on statistical ensemble measurements of individual junctions. 5,6,14–17,19 An important step to bridge our understanding between ensemble and single-junction characteristics represents the concept of “molecular fami- lies”. 22 The ideal strategy is based on the systematic varia- tion of one structure element of the junction, such as length, 14,17,19,23,24 anchoring group, 25 electronic structure, 26,27 or molecular conformation. 16,17,22,28 In combination with carefully designed transport experiments on the single- molecule level corresponding changes in physical observ- ables are analyzed. The present study aims at exploring experimentally and theoretically the correlation between torsion angle  and conductance in a series of single-molecule BPDT junctions as formed between two gold electrodes. Chemical tuning by incorporation of an alkyl chain of variable length in 2,2′- position (“molecular strap”) controls the torsion angle be- tween the two interconnected aromatic rings, guaranteeing minimum motion and conformational freedom of the bridge without changing the electronic character of the substituents and the length of the bridging molecule. For instance, the distance between the two -SH anchoring groups remains rather constant and amounts to ∼1.06 nm. The choice of the thiol anchoring groups ensures a strong chemical bond- ing to the leads with the current flow mainly modulated by the molecular HOMO level. 29 π-π coupling dominates electron transport in aromatic bridges and transport can be manipulated and tailored by controlling the degree of elec- * To whom correspondence should be addressed. E-mail: (F.P.) fabian.pauly@kit.edu; (F.E.) ferdinand.evers@int.fzk.de; (M.M.) marcel.mayor@unibas.ch; (T.W.) thomas.wandlowski@dcb.unibe.ch. Received for review: 09/17/2009 Published on Web: 12/21/2009 pubs.acs.org/NanoLett © 2010 American Chemical Society 156 DOI: 10.1021/nl903084b | Nano Lett. 2010, 10, 156-163