N-Heterocyclic Carbene vs. Thiophene - Chiral Adsorption and Unidirectional Rotation on Au(111).

N-Heterocyclic carbenes are highly effective ligands for anchoring functional organic molecules to metal surfaces and nanoparticles, facilitating the formation of self-assembled monolayers. However, their adsorption on surface is difficult to predict and control, and there is an ongoing debate on the geometry of NHC derivatives on gold surfaces and on the role of gold adatoms. We present two single molecules based on a benzimidazole NHC, one equipped with a thiophene substituent, and the other ending with a Br atom. By low temperature scanning tunneling microscopy we show that both molecules adsorb planar on Au(111) and are chiral on the surface. Our results indicate that in both cases a complex between NHC and a gold adatom is formed. Upon voltage pulses with the STM tip, both complexes move excited by inelastic tunneling electrons. For the derivative with thiophene, we observe a stepwise 60° unidirectional rotation around the S atom. The direction of rotation is determined by both the chirality and the position of the applied pulse. On the contrary, the NHC derivative without thiophene moves laterally on the surface. Adsorption, binding to gold atoms, and motion are discussed with the support of density functional theory calculations and image simulations.