A new concept to improve the reliability of functional single molecule junctions is presented using the E-field triggered switching of Fe^IIbis-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 Fe^II 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).

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe^II-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe^II-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe^II 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 Fe^II-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 Fe^II-complex and a larger transmission for the high-spin configuration.