This thesis is centred around the interaction of few-molecule arrays with gold surfaces and the enhanced optical fields that occur in nanoscale junctions between gold nanoparticles (AuNPs). This behaviour is enabled by optically resonant collective electron density oscillations (and electrostatic coupling thereof) at the nanoparticle and nanojunction surfaces, which confine incident light to nanoscale volumes and boost the local electromagnetic field strength - a phenomenon known as plasmonic enhancement. This facilitates enhanced light-matter interactions with broad potential for advanced nanophotonic applications, including few-molecule sensing, reaction monitoring, quantum information systems, molecular electronic devices, nanometrology, light harvesting and catalysis.

The utility of these plasmonic nanostructures is strongly influenced by the chemical structure of the molecular spacers used for nano self-assembly. In this work, I develop bespoke spacer molecules based on a set of functional design criteria for optimisation of the nanophotonic applications described above. Three different classes of molecular spacer are investigated in this thesis; their ability to self-assemble plasmonic nanostructures is characterised, and the functionality of the resulting nanostructures is assessed. The knowledge gained from each chapter is used for iterative re-design and optimisation of the molecular spacer system, as well as inspiration for future nanophotonic devices.

The first class of molecular spacer is the bis-phthalocyanine (BPc) family: rare-earth sandwich complexes with a well-studied plethora of complex physicochemical properties and applications in sensing, optics and spin-resolved molecular electronics. BPcs were used here for their potential to create nanostructures with atypically large plasmonic enhancement; BPc@AuNP nanostructures were investigated from a plasmonic sensing and nanometrology perspective. No sensing behaviour was observed under the conditions tested, but an anomalously large shift in plasmon resonance wavelength was observed upon variation of the BPc metal centre. Several candidate hypotheses for this are discussed, and the results highlight the limitations of the approximations and assumptions often used to model tightly coupled plasmonic systems.

The second spacer class uses supramolecular chemistry to provide control over orientation, intermolecular coupling and Au affinity of aryl viologen (ArV) molecules within plasmonic nanocavities, using a macrocyclic cucurbit[n]uril (CB[n]) host. Successful self-assembly of AuNPs using various ArV⊂CB[n] inclusion complexes was confirmed using a variety of spectroscopic and theoretical techniques. This demonstrates the possibility of multi-component molecular spacers, the modular nature of which represents a supramolecular toolbox for incorporating an expanded range of optoelectronic & chemical functionality into plasmonic hotspots.

The third and final class of spacer incorporates additional optical and chemical functionality into the ArV structure via a porphyrin moiety. The design, synthesis and characterisation of the new porphyrin species and its metal complexes are reported, together with a brief investigation of the compounds’ solvochromic properties and aqueous stability. The electronic structure, excitonic coupling and internal dynamics within the discrete inclusion complexes formed between the novel porphyrins and CB[n] are then probed with various spectroscopies. Finally, the porphyrins and supramolecular CB[n] complexes thereof are used to assemble plasmonic nanostructures, and the electronic interactions between the spacer and plasmonically enhanced near field are probed.

This work uses established techniques to investigate the suitability of novel molecular junctions for advanced nanophotonic applications. In combination with cutting-edge plasmonic analysis techniques developed by others in parallel to this work, the functional spacer molecules developed here can now be used for enhanced exploration of light harvesting, electrocatalysis, charge transfer, charge transport, molecular switching and quantum cavity electrodynamics within self-assembled plasmonic nanojunctions.