Analysis of N-Heterocyclic Carbenes and Their Monolayers by X-ray Photoelectron Spectroscopy: Peak-Fitting, Effects of Molecular Architecture and Impact of Possible Impurities

Abstract

X-ray photoelectron spectroscopy (XPS) is a frequently employed technique for surface characterization, particularly useful in studying self-assembled monolayers (SAMs). Herein, we examine precursors and SAMs of N-heterocyclic carbenes (NHCs) on gold. We establish a reliable and reproducible XPS peak-fitting model for 1,3-diisopropyl-1H-benzo[d]imidazol-3-ium hydrogen carbonate and its CF₃-tagged derivative to enable accurate detection, quantification, and chemical analysis of NHCs by XPS. The effects of molecular architecture variations on the N 1s peak were investigated and found to be insignificant across a wide range of derivatives. The characteristics of the N 1s peak after deposition on a gold surface were examined. All samples show a symmetric N 1s peak centered around 400.5 eV, with a FWHM ranging from 1.2 to 1.6 eV, which is a 0.6 eV shift compared to the starting hydrogen carbonate precursors, or other salts, which appear at 401.1 eV. Examination of hydrogen carbonate salts during extended irradiation under ultrahigh vacuum illustrates that dehydration and eventually decarboxylation likely take place during the process of measuring the XP spectra. More substantial degradation of the precursors can be observed after 4 h of exposure to the conditions of measurement for simple NHC precursors, with the CF₃-tagged NHC salts degrading more rapidly within 2 h due to C–F bond cleavage. Common impurities that may result from synthetic conditions and can complicate the N 1s spectra are also presented.