Polarizability as a molecular descriptor for conductance in organic molecular circuits

We explore a connection between the static molecular polarizability and the molecular conductance that arises naturally in the description of electrified molecular interfaces and that has recently been explored experimentally. We have tested this idea by using measured conductance of few different experimental design motifs for molecular junctions and relating them to the molecular polarizability. Our results show that for a family of structurally connected molecules the conductance decreases as the molecular polarizability increases. Within the limitations of our model, this striking result is consistent with the physically intuitive picture that a molecule in a junction behaves as a dielectric that is polarized by the applied bias, hence creating an interfacial barrier that hinders tunneling. The use of the polarizability as a descriptor of molecular conductance offers significant conceptual and practical advantages over a picture based on molecular orbitals. To further illustrate the plausibility of this idea, we have used Simmons' tunneling model that incorporates image charge and dielectric effects to describe transport through a barrier that represents the molecular junction. In such a model, the barrier height depends on the effective dielectric constant of the electrode-molecule-electrode junction, which in turn can be approximately expressed in terms of the molecular polarizability via the classical Clausius-Mossotti relation. Despite the simplicity of our model, it sheds light on a hitherto neglected connection between molecular polarizability and conductance and paves the way for further experimental, conceptual, and theoretical developments.