Evaluation of MR-TADF compounds as photocatalysts and their application in NHC/photoredox catalysis

Abstract

This thesis concerns the study of TADF and MR-TADF compounds for use as photocatalysts, with a focus on their application in Lewis base/photoredox dual catalysis. Chapter 1 introduces the important concepts that are key to understanding both TADF photocatalysis and Lewis base catalysis as well as a literature survey of these areas of research. Chapter 2 describes the synthesis and investigation of a bifunctional material that contains distinct isothiourea (ITU) and TADF moieties. Disappointingly, when incorporated into the same molecule, the two components proved to be incompatible due to oxidation of the Lewis basic ITU. Further investigations into ITU/photoredox dual catalysis using distinct ITU catalysts and photocatalysts also pointed to an issue of incompatibility due to the low oxidation potential of ITU catalysts. However, subsequent investigations by others¹,² showed that this catalytic system can be utilised effectively if a suitable radical precursor is identified that can be oxidised more easily than the ITU catalyst. Chapter 3 investigates the benefits of using MR-TADF compounds DiKTa and Mes₃DiKTa as photocatalysts in a series of standard photocatalytic reactions using 4CzIPN as a benchmark for comparison. These reactions included reductive quenching reactions, oxidative quenching reactions, Dexter energy transfer (DET) reactions and dual catalytic reactions. Further comparisons were made through the analysis of the rates of reaction using the three different catalysts in a standard oxidative quenching reaction using an in-situ NMR technique. Chapter 4 applies the knowledge gained in chapter 3 to a new NHC/photoredox catalysed synthesis of 1,4-diketones via a three-component radical relay reaction. The optimized conditions combined aroyl fluorides, α-ketoacids and styrenes in the presence of DiKTa and an NHC catalyst. Subsequent investigation of the scope was achieved by varying the substituents of each starting material. A mechanism is proposed and supported through Stern-Volmer analysis. Chapter 5 explores the application of benzophenone as a DET photocatalyst in the key [2+2] photochemical cycloaddition step in the synthesis of dimethyl cubane-1,4-dicarboxylate. This allowed for the use of significantly lower energy light (λexc = 390 nm) than previously reported (λexc = 311 nm).³

References

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