Scanning Probe Study of N-Heterocyclic Carbene Self-Assembly on Coinage Metal Surfaces
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The autonomous process of molecular level organization into self-assembled monolayers (SAMs) as a method to functionalize surfaces is suitable to a wide range of applications. Since the inception of SAMs, alkanethiols have been the standard surface anchor. More recently, N-heterocyclic carbenes (NHCs) have gained interest due to their increased oxidative and thermal robustness relative to the thiol. Since the emergence of NHCs in surface science, several key insights into how their structure relates to their adsorption motif have become apparent. These investigations have centred around the exo-cyclic nitrogen groups called 'wingtips’. However, several areas remain unaddressed, particularly how backbone modifications of the NHC or differing substrates impact the self-assembly of NHCs. By modifying NHC structures and depositing NHCs onto different surfaces, this thesis addresses these gaps in the cause of forming a general theory of NHC self-assembly. Using a low-temperature ultra-high vacuum scanning tunnelling microscope (STM), this work provides a molecular-level study of NHC self-assembly on several surfaces. The work contained in this thesis begins with a study of NHC-IiPr on Au(111), the imidazole analogue of NHC-iPr. It was found that the removal of the benzene backbone from NHC-iPr led to a tighter packed structure and inhibits bis-complexation which may be indicative of stable upright self-assembly on platinum group metals. Next, an investigation of pro-chiral self-assemblies of NHC-Bz and NHC-iPr on Cu(111); where it was found that a trimer configuration of the NHC was the preferred coordination motif. The next chapter demonstrates the structural isomorphism of NHC-iPr on Cu(100) to Au(111). A theory as to why structural isomorphism arises in this case is presented. More generally, the concept of structural isomorphism as it relates to NHC self-assembly is discussed. Finally, the results of an attempt to drive upright self-assembly via a gain in entropy are presented. Modifications of NHC-Et by alkylating the backbone introduced geometric isomerization into the system, which precipitated a decrease in order relative to the non-alkylated NHC-Et. Gaining insight into the transferability of self-assembly principles and outcomes is impactful for the continued development of NHCs as anchors for SAMs.