The Interaction between TEMPO Radicals and Gold Surfaces

Organic radical molecules are, due to their relevance for spintronics and fundamental interest, studied in mechanically controlled break junctions. It is often assumed that organic radicals are anchored to the gold electrodes by designated linker thiol groups, with the radical substituents far from the electrodes. However, the interaction between a radical substituent and gold, in addition to the functional groups designed for anchoring the molecule, could influence the interaction between the whole molecule and the surface. Although some relevant experiments have been reported, the findings are inconsistent. To elucidate a possible influence, we discuss the interaction between a commonly used nitroxyl radical, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), with gold electrode surfaces by density functional theory methods, combined with empirical dispersion corrections. Within the uncertainty inherent in this methodology, our results suggest that the interaction between TEMPO and gold is made up of two competitive and complementary contributions: direct binding between the nitroxyl and gold adatoms, and dispersion interactions between bulky methyl groups and the surface (which is more pronounced for clean Au(111) surfaces but also prevents direct binding in case). Importantly, the overall interaction is not negligible and is even comparable to some commonly used anchoring groups (e.g., amino groups). This may have consequences for interpreting conductance and magnetoresistance data on organic radicals in molecular junctions.