Current-voltage curves for molecular junctions: Effect of substitutients

We present current-voltage $(I\text{\ensuremath{-}}V)$ curves for phenylene ethynylene oligomers between two Au surfaces computed using a density-functional-theory\char21{}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 $\mathrm{N}{\mathrm{O}}_{2}$ group. In this way, we can study the difference between electron withdrawing and $\ensuremath{\pi}$ 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 $\mathrm{N}{\mathrm{O}}_{2}$. At higher biases, the lowest unccupied molecular orbital (LUMO) becomes important, and the Cl and $\mathrm{N}{\mathrm{O}}_{2}$ 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 $\mathrm{N}{\mathrm{O}}_{2}$. In these molecules, the ${\mathrm{C}}_{2}$ bridging units as well as the thiol anchor group are shown to create bottlenecks to current flow.