Current-voltage curves for molecular junctions: Effect of substitutients

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 substitutients are considered: one where all the hydrogens are replaced by chlorines and a second where one H is replaced by an NO(2) 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 the highest occupied molecular orbital (HOMO) to lower energy due to the electron withdrawal by Cl or NO(2). At higher biases, the lowest unccupied molecular orbital (LUMO) becomes important, and the Cl and NO(2) 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 NO(2). In these molecules, the C(2) bridging units as well as the thiol anchor group are shown to create bottlenecks to current flow.