On-surface chemistry: directing and controlling chemical interactions

On-surface synthesis, where molecular building blocks react together to produce covalently bonded products, offers the potential for the generation of structures via alternative routes to those performed within traditional solution-phase chemistry. However, a better understanding of the mechanistic process of on-surface reactions, obtained through surface analysis techniques such as scanning probe microscopy, is first required in order to develop these reactions. Within this thesis, investigations into on-surface synthesis are primarily performed using scanning tunnelling microscopy techniques under ultra high vacuum conditions. These techniques are used to examine molecular structures formed on coinage metal surfaces, obtained via the on-surface Ullmann-like coupling reaction. The effects of surface topography upon the synthesis of surface-confined structures are initially investigated. Ullmann coupling reactions of 4,4''-diiodo-m-terphenyl (DITP) on Ag(111) and Ag(110) are performed and the resulting structures characterised at each phase of the reaction. The dimensions of these structures and their adsorption geometries relative to the surface are then compared, finding that intermediate and product structure geometries are driven by their alignment to the surface. Utilising a combination of normal incidence X-ray standing wave (NIXSW) technique and STM imaging, the adsorption of 1,3,5-tris(4-iodophenyl)benzene (TIPB) and DITP on Ag(111) is studied. NIXSW measurements of molecular structures in the metal organic and covalent phases of the Ullmann coupling reaction are used to determine vertical positioning of molecules relative to the bulk surface. Additionally, these results are used to determine the presence of metal adatoms within metal-organic structures. Further NIXSW measurements combined with STM imaging are then used to generate lateral adsorption models for molecular structures. Building upon the observations of reactions upon 'atomically flat' surfaces, the concept of a molecular template is introduced and used to demonstrate a methodology for confining on-surface reactions. Large porphyrin macrocycles, consisting of 40 linked porphyrin units (c-P40) are used to confine the reaction of TIPB on Au(111). Covalently bonded structures are successfully produced within templates while exploring the limitations of this system, including thermal stability and the presence of contaminant species. Additionally, the electronic properties of c-P40 nanorings are characterised. Quantum confinement effects are observed in stacks of porphyrin rings, including quantised energy levels and localisation of electron states. Measurements are supported by density functional theory (DFT) and extended tight binding (xTB) calculations. These results act to demonstrate the potential for stacks of c-P40 to act as molecular quantum rings. The findings presented within this thesis offer additional understanding on the role certain factors play within on-surface reactions and provide details of potential methods which may be used to influence and control these reactions