We have investigated charge transport in single-molecule junctions using gold nanoelectrodes at room and cryogenic (10 K) temperatures. A statistical analysis of the low-bias conductance, measured during the stretching of the molecular junctions, shows that the most probable single-molecule conductance is insensitive to the temperature as expected for off-resonant coherent transport. Low-temperature current-voltage measurements show that these junction conformations have a smooth tunnelinglike shape. While separating the electrodes further we find that, in about one-fourth of the cases, the junction switches in an abrupt way to a configuration with I-V characteristics exhibiting a gap around zero bias and resonances at finite bias. The analysis of the I-V shape and of the conductance distance dependence suggests a stretching-induced transition from the strong to the weak electronic coupling regime. The transition involves a large renormalization of the injection barrier and of the electronic coupling between the molecule and the electrodes.

Recent observations of destructive quantum interference in single-molecule junctions confirm the role of quantum effects in the electronic conductance properties of molecular systems. These effects are central to a broad range of chemical and biological processes and may be beneficial for the design of single-molecule electronic components to exploit the intrinsic quantum effects that occur at the molecular scale. Here we show that destructive interference can be turned on or off within the same molecular system by mechanically controlling its conformation. Using a combination of ab initio calculations and single-molecule conductance measurements, we demonstrate the existence of a quasiperiodic destructive quantum-interference pattern along the breaking traces of π-stacked molecular dimers. The results demonstrate that it is possible to control the molecular conductance over more than one order of magnitude and with a sub-ångström resolution by exploiting the subtle structure-property relationship of π-stacked dimers.

The single-molecule conductance of a 3-ring, conjugated azomethine was studied using the mechanically controlled breakjunction technique. Charge transport properties are found to be comparable to vinyl-based analogues; findings are supported with density functional calculations. The simple preparation and good transport properties make azomethine-based molecules an attractive class for use in polymer and single-molecule organic electronics.

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.