DOI: 10.7274/25527784
title: N-Heterocyclic Carbenes: From Laboratory Curiosities to Versatile Self-Assembled Monolayers

description: This dissertation focuses on the synthesis, characterization, and application of N-heterocyclic carbene (NHC) monolayers on noble metal surfaces. The discovery that NHC ligands form more robust monolayers than benchmark thiols motivated intense research into surface science applications of NHCs. Recent research efforts focus on acquiring a fundamental understanding of monolayer formation, surface structure, and stability on noble metal surfaces so that NHCs may be rationally designed for surface science applications previously limited by thiol chemistry. To this end, this research focuses on designing facile synthetic routes to form NHC monolayers and developing new analytical chemistry tools that provide a window into the monolayer surface structure. The results of this research set the stage for emerging NHC applications in biosensors and mass spectrometry. 
To broaden the scope of analytical methods available for NHC monolayer characterization, this work created a new suite of spectroscopic and spectrometric methods. Prior to this research, the dominant characterization techniques were low throughput, lacked biocompatibility, and could not easily distinguish complex NHC surface structures. To address these limitations, laser desorption/ionization mass spectrometry (LDI-MS) methods were adapted for the characterization of NHC monolayers. Initial studies revealed that NHC ligands readily ionize in LDI-MS to form small clusters composed of NHC ligands and gold atoms. The nature of these clusters was explored using two NHC isotopologues to form a mixed monolayer, which revealed that these ions encode information about the local monolayer chemistry on the nanoparticle surface. Additionally, a model reduction of a nitro-NHC to an amine-NHC monolayer was monitored via LDI-MS, illustrating the utility of this technique for chemical reaction monitoring. 
These initial studies illustrated that NHC ligands ionize in high yield and resist fragmentation in mass spectrometry. This discovery motivated a follow-up study investigating the use of NHC monolayers on gold nanoparticles for mass spectrometry applications. In this study, the LDI-MS performance of NHC ligands was compared to benchmark thiol ligands. These results illustrate that NHCs fragment less and produce more ions in LDI-MS than thiol monolayers. Then, the utility of NHC mass tags was explored for data storage, bioconjugation monitoring, and security applications. When taken together, LDI-MS is a useful method to probe monolayer composition and reactivity, and NHC ligands have a bright future as mass spectrometry tags. 
To further expand the scope of analytical methods for NHC monolayer characterization, electrochemical surface-enhanced Raman spectroscopy (EC-SERS) methods were adapted for in situ measurements of NHC monolayers. Then, EC-SERS was leveraged to explore the operational voltage window of benchmark NHC monolayers. This study revealed that the monolayers remain stable across a wide voltage window and during continuous voltammetric cycling, but the monolayers degrade after applying strongly reducing or oxidizing potentials. Therefore, NHC monolayers are well-suited for electrochemical sensing applications, but further optimization of the monolayer chemistry is required for applications employing extreme potentials. Because EC-SERS provides an in situ probe of NHC integrity during electrochemistry experiments, this technique will be especially useful for the rapid evaluation of NHC monolayer chemistries for electrocatalysis and sensor applications. 
In addition to analytical method development, this work compares the leading monolayer deposition methods and establishes new deposition protocols to form NHC monolayers. SERS, LDI-MS, and XPS were used to compare the surface structures formed from the most widely used NHC monolayer deposition methods, which were thought to produce equivalent surface structures. These results illustrate that only surfaces treated with a free-carbene or that undergo vacuum annealing show evidence of an NHC bound to gold, whereas solution phase deposition methods produce fundamentally different monolayers. Additionally, the use of transmetalation reactions was explored as a new deposition protocol to functionalize gold surfaces with NHCs. This study illustrates that transmetalation reactions are an effective tool to synthesize monolayers by transferring an NHC ligand from a copper or silver complex to a gold nanoparticle, but the transfer depends heavily on the ligand structure and whether a NHC silver or copper complex is used as a precursor.