N‐Heterocyclic Olefins on a Silicon Surface

Abstract

The adsorption of N‐heterocyclic olefins (NHOs) on silicon is investigated in a combined scanning tunneling microscopy, X‐ray photoelectron spectroscopy, and density functional theory study. We find that both of the studied NHOs bind covalently, with ylidic character, to the silicon adatoms of the substrate and exhibit good thermal stability. The adsorption geometry strongly depends on the N‐substituents: for large N‐substituents, an upright adsorption geometry is favored, while a flat‐lying geometry is found for the NHO with smaller wingtips. These different geometries strongly influence the quality and properties of the obtained monolayers. The upright geometry leads to the formation of ordered monolayers, whereas the flat‐lying NHOs yield a mostly disordered, but denser, monolayer. The obtained monolayers both show large work function reductions, as the higher density of the flat‐lying monolayer is found to compensate for the smaller vertical dipole moments. Our findings offer new prospects in the design of tailor‐made ligand structures in organic electronics and optoelectronics, catalysis, and material science.

The Electron-Rich and Nucleophilic N-Heterocyclic Imines on Metal Surfaces: Binding Modes and Interfacial Charge Transfer

“…N-Heterocyclic imines (NHIs) are a class of imino-ligands which is based on the Arduengo type N-heterocyclic carbene (NHC) scaffold (Scheme ) and also related to the N-heterocyclic olefins (NHOs). − The highly polarized exocyclic CN bond makes these ligands very electron-rich and nucleophilic. The cationic charge density is efficiently delocalized into the five-membered imidazoline ring, enhancing the electron-donating character of exocyclic nitrogen atoms, therefore, NHIs are usually treated as 2σ, 2π, or 4π donors (Scheme b). − In 2004, Tamm and co-workers successfully demonstrated an easy synthetic route to NHI ligands and reported several metal complexes with these ligands .…”

Elucidating the Lability and Diversity of Self-Assembled Structures of N-Heterocyclic Carbenes at the Liquid/Metal Interface

“…While the properties of NHC-functionalized metals are to a large extent associated with the binding and organization of NHCs on the surface, − a molecular-level characterization of NHCs on surfaces is necessary. − Scanning tunneling microscopy (STM) studies at the vacuum/solid interface have made great progresses in getting in-depth insights into the unique characteristics of NHCs on metal surfaces, , for instance, the complexation between multiple NHC ligands and a metal adatom, the ballbot-type motion of and the unidirectional rotation of the NHC molecule on the surface. − Also, regular self-assembled monolayers (SAMs) of NHCs at the solid surface of the metal, − metal oxide, and silicon − have been obtained through the sublimation of NHC–CO 2 precursors in vacuum chambers. However, ordered monolayers of NHCs were usually obtained in the absence of a liquid phase.…”

A Rare Earth Modified Silicon Surface as a Template for Ordered Organic Growth

“…Organic layers on silicon surfaces are a fascinating research field because of the promising combination of silicon technology with the particular and widely tunable properties of organic molecules. , However, an ordered formation is often hindered by the high dangling bond density of the clean silicon surfaces preventing sufficient molecular diffusion, − in contrast to the situation on metal surfaces. − An interesting strategy is the modification of the silicon surfaces by different elements leading to an at least partial saturation of the dangling bonds and therewith promoting molecular ordering. − …”