Molecular rectification: Why is it so rare?

Although conductance measurements of single molecule and few molecules junctions are currently being reported, there is a striking rarity of molecular rectification in these reports. Molecular rectification can be defined as the absence of inversion symmetry, I(V) = -I(-V), where I and V are the measured current and applied voltage. In molecular junctions of the form metal/molecule/metal, there is generally an absence of structural mirror symmetry. One might then expect rectification arising from this asymmetrical structure. We suggest here that molecular rectification in tunneling junctions is generally difficult to achieve, essentially because deformation of the structure in the presence of finite voltage will result in effectively symmetric voltage profiles for forward and reverse biases.