Fragmentation and Distortion of Terpyridine-Based Spin-Crossover Complexes on Au(111)

Spin-crossover complexes are attractive for their spin-switching functionality. However, only a few compounds have been found to remain intact in direct contact to metal surfaces. For the design of new spin-crossover complexes, it is important to understand the mechanisms leading to fragmentation. Here, we investigate, using low-temperature scanning tunneling microscopy along with density functional theory calculations, two Fe(terpyridine)2 complexes deposited on Au(111) by electrospray ionization with in-line mass selection. Only fragments of the first compound are observed on the surface, while the second compound is strongly flattened. On the basis of a detailed analysis of the adsorbates on the surface, possible mechanisms for the fragmentation and molecular distortion are proposed

Stretching-Induced Conductance Increase in a Spin-Crossover Molecule

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of FeII-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic FeII-complexes can be mechanically controlled. Mechanical pulling may thus distort the FeII coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current–voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active FeII-complex displays a conductance increase for increasing electrode separation and this increase can reach 1–2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the FeII-complex and a larger transmission for the high-spin configuration

Enhanced Separation Concept (ESC): Removing the Functional Subunit from the Electrode by Molecular Design

A new concept to improve the reliability of functional single molecule junctions is presented using the E-field triggered switching of FeIIbis-terpyridine complexes in a mechanically controlled break junction experiment as model system. The complexes comprise a push-pull ligand sensing the applied E-field and the resulting distortion of the FeII ligand field is expected to trigger a spin-crossover event reflected in a sudden jump of the transport current. By molecular engineering, the active centre of the complex is separated from the gold electrodes in order to eliminate undesired side-effects. Two aspects are considered to isolate the central metal ion, namely the spacing by introducing additional alkynes, and the steric shielding achieved by bulky isopropyl groups. With this small series of model complexes, a pronounced correlation is observed between the occurrence of bistable junctions and the extent of separation of the central metal ion, affirming the hypothesized Enhanced Separation Concept (ESC)

Einzelmolekül-Spinschalter auf Basis spannungsinduzierter Verzerrung der Koordinationssphäre

Wir berichten hier über ein neuartiges Einzelmolekül-Schaltkonzept, das auf dem Koordinationssphären-abhängigen Spinzustand von FeII-Komplexen beruht. Die lotrechte Anordnung zweier Terpyridin(tpy)-Liganden eines heteroleptischen Komplexes wird durch das angelegte elektrische Feld verzerrt. Während der eine Ligand den Komplex zwischen beiden Elektroden fixiert, erfühlt der zweite Ligand durch ein intrinsisches Dipolmoment das angelegte elektrische Feld, was zur Verzerrung der Koordinationssphäre und dadurch zum Spinübergang führt. Eine Serie von Komplexen mit unterschiedlichen Dipolmomenten wurde hergestellt und auf ihre Transporteigenschaften in mechanisch kontrollierten Bruchkontakten untersucht. Die statistische Auswertung stützt die Hypothese des Schaltmechanismus, bei dem mit zunehmender Stärke des Dipolmomentes des Eisenkomplexes die Anzahl der Einzelmolekülkontakte mit spannungsabhängigen Bistabilitäten zunimmt. Ein konstanter Schwellwert der E-Feldstärke, die zum Schalten eines Kontaktes erforderlich ist, spricht ebenfalls für den postulierten Schaltmechanismus

Single-Molecule Spin Switch Based on Voltage-Triggered Distortion of the Coordination Sphere

Here, we report on a new single-molecule-switching concept based on the coordination-sphere-dependent spin state of FeII species. The perpendicular arrangement of two terpyridine (tpy) ligands within heteroleptic complexes is distorted by the applied electric field. Whereas one ligand fixes the complex in the junction, the second one exhibits an intrinsic dipole moment which senses the E field and causes the distortion of the FeII coordination sphere triggering the alteration of its spin state. A series of complexes with different dipole moments have been synthesized and their transport features were investigated via mechanically controlled break-junctions. Statistical analyses support the hypothesized switching mechanism with increasing numbers of junctions displaying voltage-dependent bistabilities upon increasing the FeII complexes' intrinsic dipole moments. A constant threshold value of the E field required for switching corroborates the mechanism

Enhanced Separation Concept (ESC): Removing the Functional Subunit from the Electrode by Molecular Design

A new concept to improve the reliability of functional single molecule junctions is presented using the E-field triggered switching of FeIIbis-terpyridine complexes in a mechanically controlled break junction experiment as model system. The complexes comprise a push-pull ligand sensing the applied E-field and the resulting distortion of the FeII ligand field is expected to trigger a spin-crossover event reflected in a sudden jump of the transport current. By molecular engineering, the active centre of the complex is separated from the gold electrodes in order to eliminate undesired side-effects. Two aspects are considered to isolate the central metal ion, namely the spacing by introducing additional alkynes, and the steric shielding achieved by bulky isopropyl groups. With this small series of model complexes, a pronounced correlation is observed between the occurrence of bistable junctions and the extent of separation of the central metal ion, affirming the hypothesized Enhanced Separation Concept (ESC).QN/van der Zant La