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 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.