Molecular-level study of N-heterocyclic carbenes: Binding modes and self-assembly on Au(111)
Self-assembled monolayers (SAMs) of organic molecules are extensively used to functionalize surfaces for a wide range of applications from medicine to nanophotonics. However, creating a SAM that is sufficiently stable has been a persistent problem. Although thiols have been the gold-standard for thirty years, N-heterocyclic carbenes (NHCs) have recently been used to create SAMs that are more thermally and chemically stable. For example, NHCs based on diisopropyl benzimidazolylidene (NHCiPr) have been integrated into biosensors. The field of NHC-based material science is very new and the self-assembly mechanisms are still not well understood. To provide a molecular-level perspective, density functional theory was combined with scanning tunneling microscopy to study the self-assembly of NHCiPr on Au(111). The isopropyl wing-tip groups of NHCiPr play an important role in the self-assembly process. They are responsible for steric interactions with the surface and for stabilizing an ordered zig-zag lattice via intermolecular CH-pi non-covalent interactions. These NHCs are bound to adatoms extracted from terraces. However, unlike other systems that have been reported, they have a relatively low diffusion rate at 77 K, and their surface transport mechanism is largely dominated by a second phase. Thermal annealing induces an irreversible conversion of the zig-zag lattice into flat-lying (NHCiPr)2Au complexes. These complexes interact weakly with the surface and diffuse with a high hopping frequency, yet they self-assemble into an ordered lattice when spatially constrained. Below a critical coverage, NHCiPr self-assembles into: an ordered phase bound directly to surface atoms, and an exotic mixed phase of surface-bound NHCiPr and (NHCiPr)2Au complexes. The self-assembly of five other NHCs, with structurally different wing-tip groups, have also been studied in contrast to NHCiPr. In general, NHCs with sterically bulky wing-tip groups prefer to bind upright to adatoms, whereas NHCs with smaller wingtip groups prefer to form flat-lying complexes. Through careful control of molecular coverage and surface temperature, NHCiPr forms an upright zig-zag phase, which can be readily functionalized and used in biosensors. The results presented herein provide a major step forward in understanding the mechanism for self-assembly of NHCs, a process which for thiols has taken thirty years to understand.