Self-assembled monolayers of mesoionic triazolylidene dimers on Au(111)

Mesoionic carbenes (MICs) hold great promise as surface ligands, due to their electronic properties and charge distribution, yet their self-assembly rules remain essentially unexplored. Here we combine synchrotron X-ray photoelectron and absorption spectroscopies, scanning-tunnelling microscopy, and density-functional theory to map, atom by atom, the self-assembly of 1,2,3-triazolylidene MICs on Au(111). We discover that the molecules adsorb flat, pair via a shared Au adatom, and form two highly ordered phases whose lattice constants differ by ∼5%. The resulting monolayers reach high coverages (1.4–1.5 molecules per nm²) while retaining long-range order. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure reveal pronounced charge transfer into the metal and a molecule–adatom–molecule motif that lifts the Au adatom by ∼0.8 Å, in excellent agreement with theory. The molecules exhibit thermal stability up to 200 °C, after which they desorb from the surface without detectable decomposition. By elucidating how the mesoionic electronic structure directs adatom extraction, dimer formation, and high-density packing, this work establishes MICs as a versatile platform for stable, strongly coupled organic-metal interfaces.