Charge transport DOI: 10.1002/smll.200800390 Charge Transport Through a Cardan-Joint Molecule Mario Ruben, * Aitor Landa, Emanuel Lo ¨rtscher, * Heike Riel, Marcel Mayor, Helmar Go ¨rls, Heiko B. Weber, Andreas Arnold, and Ferdinand Evers* The charge transport through a single ruthenium atom clamped by two terpyridine hinges is investigated, both experimentally and theoretically. The metal-bis(terpyridyl) core is equipped with rigid, conjugated linkers of para-acetyl-mercapto phenylacetylene to establish electrical contact in a two-terminal configuration using Au electrodes. The structure of the [Ru II (L) 2 ](PF 6 ) 2 molecule is determined using single-crystal X-ray crystallography, which yields good agreement with calculations based on density functional theory (DFT). By means of the mechanically controllable break-junction technique, current–voltage (I–V), characteristics of [Ru II (L) 2 ](PF 6 ) 2 are acquired on a single-molecule level under ultra-high vacuum (UHV) conditions at various temperatures. These results are compared to ab initio transport calculations based on DFT. The simulations show that the cardan-joint structural element of the molecule controls the magnitude of the current. Moreover, the fluctuations in the cardan angle leave the positions of steps in the I–V curve largely invariant. As a consequence, the experimental I–V characteristics exhibit lowest-unoccupied-molecular-orbit-based conductance peaks at particular voltages, which are also found to be temperature independent. 1. Introduction A detailed understanding of charge transport through single-molecule junctions is a key prerequisite for the design and development of molecular electronic devices. [1] Various experimental techniques have been used to demonstrate transport through single molecules, for example, mechanically controllable break junctions (MCBJs), [2] electro-migrated junctions (EMJs), [3] and scanning tunneling microscopy/ spectroscopy measurements (STM/STS). [4] Among the mole- cules of interest, the class of metal-ion complexes attracts special attention because the electron and spin densities of these complexes are highly confined, both electronically and spatially, around the metal centers, which means that they have a large potential for use in ‘‘ion-dot’’ concepts, analogous to larger quantum dots. [5] Pt II and Fe II compounds have been investigated in MCBJ experiments, and exhibited conductance values ranging from insulator like [6a] to metal like. [6b] Co II and Os II complexes bearing insulating linkers of different chain lengths have been studied in electro-migrated nano-gaps [7] and in in-situ STM/STS experiments, which revealed transistor- like behavior. [8] Charge Transport Through a Cardan-Joint Molecule [  ] Dr. M. Ruben, Dr. F. Evers, Dr. A. Landa, Dr. A. Arnold Institute of Nanotechnology, Karlsruhe Institute of Technology PF 3640, 76021 Karlsruhe (Germany) E-mail: ruben@int.fzk.de, evers@int.fzk.de Dr. E. Lo ¨rtscher, Dr. H. Riel IBM Zurich Research Laboratory Sa ¨umerstrasse 4, 8803 Ru ¨schlikon (Switzerland) E-mail: hei@zurich.ibm.com Dr. F. Evers Institut fu ¨r Theorie der Kondensierten Materie Karlsruhe Institute of Technology PF 6980, 76128 Karlsruhe (Germany) Prof. M. Mayor Department of Chemistry, Universita ¨t Basel St. Johanns-Ring 19, 4056 Basel (Switzerland) Dr. H. Go ¨rls Institut fu ¨r Anorganische und Analytische Chemie, Universita ¨t Jena Agust-Bebel-Strasse 1, 07743 Jena (Germany) Prof. H. B. Weber Institut fu ¨r Angewandte Physik, Universita ¨t Erlangen-Nu ¨rnberg Staudt- strasse 7, 91058 Erlangen (Germany) : Supporting Information is available on the WWW under http:// www.small-journal.org or from the authors. Keywords:  break junctions  molecular electronics  ruthenium complexes  single-molecule studies small 2008, 4, No. 12, 2229–2235 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2229