Current-voltage curves for molecular junctions: the effect of substituents

We present current-voltage (I-V) curves for phenylene ethynylene oligomers between two Au surfaces computed using a Density Functional Theory/Green's Function approach. In addition to the parent molecule, two different substituents are considered: one where all the hydrogens are replaced by chlorines and a second where one H is replaced by an NO2 group. In this way, we can study the difference between electron withdrawing and pi orbital effects. For low biases, a reduced current for the derived species is consistent with a shift of HOMO to lower energy due to the electron withdrawal by Cl or NO2. At higher biases, the LUMO becomes important, and the Cl and NO2 substituted species carry more current than the parent because the LUMO is stabilized (shifted to lower energy) due to the withdrawal of electrons by the Cl and NO2. In these molecules, the C2 bridging units as well as the thiol anchor group are shown to create bottlenecks to current flow.