We investigate, by means of ab initio calculations, electronic transport in molecular junctions composed of a biphenyl molecule attached to metallic carbon nanotubes. We find that the conductance is proportional to cos 2 , with the angle between phenyl rings, when the Fermi level of the contacts lies within the frontier molecular orbitals energy gap. This result, which agrees with experiments in biphenyl junctions with nonorganic contacts, suggests that the cos 2 law has a more general applicability, irrespective of the nature of the electrodes. We calculate the geometrical degree of chirality of the junction, which only depends on the atomic positions, and demonstrate that it is not only proportional to cos 2 but also is strongly correlated with the current through the system. These results indicate that molecular conformation plays the preponderant role in determining transport properties of biphenyl-carbon nanotubes molecular junctions.

We have investigated, by means of a nonequilibrium Green's function method coupled to density functional theory, the electronic transport properties of molecular junctions composed of oligo-para-phenylene (with two, three, four, and five phenyl rings) covalently bridging the gap between metallic carbon nanotubes electrodes. We have found that the current is strongly correlated to a purely geometrical chiral parameter, both on-resonance and off-resonance. The Fowler-Nordheim plot exhibits minima, V min , that occur whenever the tail of a resonant transmission peak enters in the bias window. This result corroborates the scenario in which the coherent transport model gives the correct interpretation to transition voltage spectroscopy (TVS). We have shown that V min corresponds to voltages where a negative differential resistance (NDR) occurs. The finding that V min corresponds to voltages that exhibit NDR, which can be explained only in single-molecule junctions within the coherent transport model, further confirms the applicability of such models to adequately interpret TVS. The fact that the electrodes are organic is at the origin of differences in the behavior of V min if compared to the case of molecular junctions with nonorganic contacts treated so far.