Strong Substrate–Adsorbate\nInteractions Direct\nthe Impact of Fluorinated N‑Heterocyclic Carbene Monolayers\non Au Surface Properties

Fluorinated self-assembled monolayers (SAMs) have been\nutilized\nin a variety of applications such as transistors and optoelectronic\ndevices. However, in most SAMs the fluorinated groups could not be\npositioned in high proximity to the surface due to steric effects.\nThis limitation hinders the direct analysis of the impact of the fluorination\nlevel on surface properties. Herein, fluorinated aromatic N-heterocyclic\ncarbenes (NHCs), with 1–5 fluorine atoms, were self-assembled\non a gold substrate. These NHCs enabled the positioning of fluorinated\ngroups in high proximity to the metal surface to identify the influence\nof the fluorination level on surface properties. Experimental measurements\nand theoretical calculations identified that all fluorinated NHCs\nformed SAMs and adopted a flat-lying adsorption configuration while\nanchored to the metal surface via Au adatom. A higher fluorination\nlevel induced a stronger interaction of the fluorinated side groups\nwith the Au surface. The stronger interaction and surface proximity\nof the fluorinated side groups deteriorated the overall binding energy\nof the NHC due to the less-optimized adsorption geometry of the carbene\ncarbon. Ultraviolet photoelectron spectroscopy measurements revealed\nthat fluorinated NHC monolayers lowered the surface work function\nby up to 1 eV and induced an increase of 15–20° in the\nwater contact angle. The impact on surface properties did not vary\naccording to the fluorination level of NHCs, and similar values were\nmeasured for NHC with 1–5 fluorine atoms. It is therefore identified\nthat dominant adsorbate–substrate interactions between the\nfluorinated side groups and the Au surface quenched the distinct impact\nof the fluorination level on surface functionality.