Beyond wingtips: backbone alkylation affects the orientation of N-heterocyclic carbenes on gold nanoparticles

The effect of wingtip groups on the orientation of N-heterocyclic carbene (NHC)-based self-assembled monolayers (SAMs) on a variety of metal surfaces has received considerable attention. However, the influence of backbone substituents on orientation has received virtually no attention, despite the fact that backbone interactions are critical for upright orientation of thiolate-based SAMs and that backbone functionalization is important for many applications. To address this question, a series of gold nanoparticles (NPs) supported by NHCs featuring symmetrical or asymmetrical long alkyl backbone substituents and ethyl and isopropyl wingtips were synthesized. The gold NPs were characterized using UV-vis spectroscopy, electron microscopy, mass spectrometry, and surface-enhanced Raman spectroscopy (SERS). Experimental SER spectra were compared to simulated spectra, illustrating that both ethyl and isopropyl NHCs with symmetrical dodecyl long chains in the backbone adopt a primarily vertical configuration on the gold surface. However, the ethyl NHC with a single hexyloxy backbone substituent adopts mainly a flat configuration on the gold NP surface based on combined SERS and scanning tunneling microscopy (STM) results. This is attributed to on-surface interactions between long alkyl chains, which provide an unanticipated source of stability favoring the flat-lying orientation. Lastly, the thermal stability of the NHC-functionalized gold NPs at elevated temperatures was investigated. The dodecyloxy-functionalized NHC AuNPs remain thermally stable for 72 hours at 100 °C, representing a significant improvement over state-of-the-art NHC-AuNPs. NHCs containing isopropyl wingtip groups provide NPs with higher levels of stability than diethyl-substituted NHCs, regardless of backbone substituents. Taken together, our results highlight critical synthetic considerations for NHC ligand design, enabling control of ligand orientation and nanomaterial stability by tuning NHC backbone substituents.