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Wir berichten hier uber ein neuartiges Einzelmolekul-Schaltkonzept, das auf dem Koordinationsspharen-abhangigen Spinzustand von FeII-Komplexen beruht. Die lotrechte Anordnung zweier Terpyridin(tpy)-Liganden eines heteroleptischen Komplexes wird durch das angelegte elektrische Feld verzerrt. Wahrend der eine Ligand den Komplex zwischen beiden Elektroden fixiert, erfuhlt der zweite Ligand durch ein intrinsisches Dipolmoment das angelegte elektrische Feld, was zur Verzerrung der Koordinationssphare und dadurch zum Spinubergang fuhrt. Eine Serie von Komplexen mit unterschiedlichen Dipolmomenten wurde hergestellt und auf ihre Transporteigenschaften in mechanisch kontrollierten Bruchkontakten untersucht. Die statistische Auswertung stutzt die Hypothese des Schaltmechanismus, bei dem mit zunehmender Starke des Dipolmomentes des Eisenkomplexes die Anzahl der Einzelmolekulkontakte mit spannungsabhangigen Bistabilitaten zunimmt. Ein konstanter Schwellwert der E-Feldstarke, die zum Schalten eines Kontaktes erforderlich ist, spricht ebenfalls fur den postulierten Schaltmechanismus.

The use of porphyrin molecules as building blocks of functional molecular devices has been widely investigated. The structural flexibility and well-developed synthetic chemistry of porphyrins allows their physical and chemical properties to be tailored by choosing from a wide library of macrocycle substituents and central metal atoms. Nature itself offers magnificent examples of processes that utilize porphyrin derivatives, such as the activation and the transport of molecular oxygen in mammals and the harvesting of sunlight in plant photosynthetic systems. In order to exploit the highly desirable functionality of porphyrins in artificial molecular devices, it is imperative to understand and control the interactions that occur at the molecule–substrate interface. Such interactions largely depend on the electronic and conformational structures of the adsorbed molecules, which can be studied using techniques such as scanning tunneling microscopy, UV photoemission spectroscopy, and X-ray photoemission spectroscopy, and on a theoretical level with density functional calculations. Recent studies on conjugated rod-like molecules have shown that molecular conductance measurements can be significantly affected by the binding geometry, coupling of the p orbitals to the leads, or p–p stacking between adjacent molecules. Herein, we present the results of a study of the interaction of laterally extended p-conjugated porphyrin molecules with electrodes by means of timeand stretching-dependent conductance measurements on molecular junctions. We further investigate strategies to reduce interactions of the molecular p electrons with the metal electrodes by modifying the chemical structure of the porphyrin molecules. We used the series of molecules represented in Figure 1a– c to examine the influence of the molecular structure on the formation of porphyrin single-molecule junctions. Since the thiol group is most commonly used to contact rod-like molecules to form straight molecular bridges, we first compared 5,10,15,20-tetraphenylporphyrin without thiol termination (H2-TPP; Figure 1a) to a nearly identical molecule with two thiol groups on opposite sides of the molecule (5,15di(p-thiophenyl)-10,20-di(p-tolyl)porphyrin (H2-TPPdT); Figure 1b). To investigate the influence of the molecular backbone geometry on the junction formation we further studied a thiol-terminated porphyrin molecule with two bulky pyridine axial groups attached through an octahedral Ru ion ([Ru{5,15-di(p-thiophenyl)-10,20-diphenylporphyrin}(py)2] (Ru-TPPdT); Figure 1c). As a consequence of steric hindrance, the pyridine groups in Ru-TPPdT reduce the direct interaction of the metal electrodes with the p face of the porphyrin. A similar strategy was used previously. Prior to electrical characterization, the molecules were deposited using self-assembly from solution. To study the conductance of these molecules we used lithographic mechanically controllable break junctions (MCBJs) in vacuum at room temperature. The layout of an MCBJ device in a threepoint bending mechanism is shown in Figure 1d. Details concerning the synthesis of the molecules and the experimental procedures are given in the Supporting Information. Sets of 1000 consecutive breaking traces from individual junctions were analyzed numerically to construct “trace histograms” of the conductance (log10G versus the electrode displacement d). This statistical method maps the breaking dynamics of the junctions beyond the point of rupture of the last monatomic gold contact (defined as d= 0), which has a conductance of one quantum unit G0= 2e h. Areas of high counts represent the most typical breaking behavior of the molecular junctions. Figure 2 presents trace histograms as well as examples of individual breaking traces for acetone as reference, H2-TPP, H2-TPPdT, and Ru-TPPdT. For all three porphyrin molecules as well as for the reference sample several junctions were measured (see the Supporting Information). Herein, we only show a typical set of junctions. In the junction which was exposed to pure acetone (Figure 2a), the Au bridge initially gets stretched until a plateau around the conductance quantum (G G0) is observed (only visible in the individual traces shown in [*] M. L. Perrin, F. Prins, Dr. C. A. Martin, Prof. Dr. H. S. J. van der Zant, Dr. D. Dulic Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft (The Netherlands) E-mail: d.dulic@tudelft.nl