Conformation dependence of molecular conductance: chemistry versus geometry

Recent experiments by Venkataraman et al (2006 Nature 442 904) on a series of molecular wires with varying chemical compositions revealed a linear dependence of the conductance on cos 2 θ, where θ is the angle of twist between neighbouring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first-principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy E F lies within the HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) gap, then we reproduce the above experimental results. More generally, however, if E F is located within either the LUMO or the HOMO states, the presence of resonances destroys the linear dependence of the conductance on cos 2 θ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.