Studying charge transport across metal-organic interfaces informs the rational design of single molecule devices. Halides have been demonstrated as linkers to bind organic molecules to metal electrodes, but they are infrequently used and their binding is poorly understood. The gold-iodine interaction is usually considered dative, though some studies suggest that iodide dissociates, yielding a direct gold-carbon (Au-C) linkage. Au-C bonds are particularly attractive for single molecule electronics, as covalent interactions lead to an increase in conductance compared to analogous molecules comprising other linkers. Using the scanning tunneling microscopy break junction technique, we demonstrate through a series of asymmetric model compounds that aryl iodides undergo oxidative addition on a gold surface to form a covalent Au-C bond with increased conductance. The asymmetry of the linkers (one iodide and one thiomethyl) allows us to specifically probe the behavior of the iodide. When measured in the junction, two distinct binding modes are present, therefore we posit that the molecules bind with and without the iodide attached. To verify this, a parallel series of gold-terminated complexes are compared. These molecules, with an intrinsic Au-C bond, have conductance values that align with the those of the high-conducting mode of the iodides, confirming our hypothesis. Moreover, the <i>in situ </i>oxidative addition can be modulated by applying an electric field in a polar solvent, wherein one electrode is polarized to increase the chemical potential, selectively promoting the process. The gold, which is electron rich, donates electron to the aryl carbon, forming the Au-C bond after iodide dissociation. Therefore, applying a negative tip bias increases the electron density on the gold, encouraging electron donation (oxidation of the gold) and formation of Au-C junctions.

Gold−carbon contacts from oxidative addition of aryl iodides

ACS Spring Meeting 2021

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