Conformation dependence of molecular conductance: chemistry versus geometry

TL;DR

This study uses first principles calculations to analyze how molecular conductance depends on conformation and chemical composition, revealing conditions under which conductance varies linearly with cos^2θ and how resonances affect this behavior.

Contribution

It demonstrates that the conductance dependence on molecular conformation varies with the Fermi energy position, extending previous experimental findings through theoretical analysis.

Findings

Conductance shows linear cos^2θ dependence when Fermi energy is within the HOMO-LUMO gap.

Resonances within LUMO or HOMO levels cause non-monotonic conductance behavior.

The results offer a new spectroscopic method for analyzing molecule-electrode contacts.

Abstract

Recent experiments by Venkatamaran {\em et al.} [Nature (London) {\bf 442}, 904 (2006)] on a series of molecular wires with varying chemical compositions, revealed a linear dependence of the conductance on $\mathrm{cos}^2\theta$, where $\theta$ is the angle of twist between neighboring 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 $E_\mathrm{F}$ lies within the HOMO-LUMO gap, then we reproduce the above experimental results. More generally, however, if $E_\mathrm{F}$ is located within either the LUMO or HOMO states, the presence of resonances destroys the linear dependence of the conductance on $\mathrm{cos}^2\theta$ 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.