Stable Self-Assembled Monolayers on Gold for Modulating Interfacial Properties

Surface engineering to modulate the interfacial properties of materials at nanoscopic scales is an important scientific matter. Studies of fluorocarbon materials have shown hydrophobicity, rigidity, and inertness for lubricants and anticorrosive applications; while the use of polar functional groups (e.g., ethylene glycol moieties) has been investigated for surface resistance against biomaterials adsorption. Furthermore, materials with amphiphilicity, containing both hydrophilicity and hydrophobicity, have recently been an essential consideration for future antifouling coatings.
Self-assembled monolayers (SAMs) offer a widespread approach to controlling the interfacial properties of the surfaces of interest due to their ease of formation and densely packed molecular coatings. However, SAMs based on monodentate alkanethiol adsorbates generate films with only moderate stability. Alternatively, the emergence of bidentate headgroup structures offer enhanced bonding of the adsorbates on the substrates by the "chelate effect". Recently, N-heterocyclic carbenes (NHCs) have also exhibited strong carbon-gold bonding, which can overcome the stability issues that plague thiol-based thin films on gold. However, relatively low packing densities of the thin films generated from the current bidentate adsorbates and relatively harsh conditions required to form NHC SAMs on gold limit their utilization in various practical applications.
The purposes of this dissertation are twofold: (1) to design and synthesize new aromatic bidentate adsorbates and to characterize their corresponding SAMs on gold to generate stable and dense organic thin films possessing a variety of surface functionalities and (2) to develop a unique azo-terminated NHC monolayer on gold for surface-initiated polymerizations. While Chapter 1 provides an overview of the use of SAMs as antifouling coatings, Chapter 2 describes a series of stable SAMs on gold generated from aromatic bidentate adsorbates having two hydrophobic chains in a single molecule, which generated no discernible interfacial properties compared to the analogous monodentate SAMs. Chapter 3 describes the development of a new class of double-chained aromatic bidentate adsorbates possessing oligo(ethylene glycol) (OEG)-terminated alkyl tailgroups for providing stable SAMs on gold with excellent protein-resistance properties. Chapter 4 highlights the generation of stable phase-incompatible SAMs on gold derived from an aromatic bidentate adsorbate that contains both fluorocarbon and OEG chains within a single molecule. Finally, Chapter 5 describes a new class of NHC-based SAMs for initiating surface polymerizations and growing stable polymer brushes on gold for surface functionalization and protection.