Mechanophotocatalysis : towards a generalisable methodology for solvent-minimised light-driven organic synthesis reactions

This thesis aims to combine photocatalysis with mechanochemistry in order to conduct solvent-minimised light-driven organic synthesis reactions, and to explore the benefits of this synergistic union. Chapter 1 introduces the underlying principles and synthetic value of photocatalysis, and the issues associated with using organic solvents as reaction environments, before describing the design principle of mechanophotocatalysis. Chapter 2 details the development of a mechanophotocatalysis reactor and prototype reaction vessels, and the application of mechanophotocatalysis to four archetypal photocatalytic reactions. Notably, we report the first example of a solvent-minimised photocatalysis reaction possessing an enhanced tolerance to air when compared with the solution-state analogue reaction. Chapter 3 reports an improved reaction vessel design, and applies the methodology to four industrially relevant metallaphotoredox catalysis reactions. Mechanophotocatalysis enabled significant reductions in the use of problematic organic solvents, such as N,N-dimethylacetamide, and facilitated a cross-electrophile coupling possessing an increased tolerance to aerobic conditions relative to the solution-state reaction. Chapter 4 probes the behaviour of the excited states of five archetypal photocatalysts in solution and in the solid-state using absorption and photoluminescence spectroscopies. Crucially, the excited states of the photocatalysts were quenched more significantly by oxygen in solution than as solids, which likely contributes to the increased tolerance to aerobic conditions that mechanophotocatalysis reactions can experience. Chapter 5 details the use of a poorly soluble sacrificial electron donor, sodium ascorbate, in a net reductive mechanophotocatalysis system. The solvent-minimised protocol returned more consistent results than the solution-state reference reaction, highlighting mechanophotocatalysis as a valuable tool for mediating highly heterogeneous photocatalytic reactions. Chapter 6 demonstrates proof-of-principle studies for scaling mechanophotocatalysis using extrusion technologies towards developing continuous solvent-minimised light-driven reactions.