Mechanisms and Applications of Mono- and Biphotonic Excitations in Photoredox Catalysis

Photo(redox) catalysis has been established as a versatile synthetic method over the past decade in organic synthesis. The availability of high-power light emitting diodes as light sources as well as a growing library of reported transformations together with increasing amounts of commercially available or easily accessible catalysts facilitate light-driven reactions for small- scale synthesis. To expand the possible applications, a growing interest to merge light-driven catalysis with other concepts known from synthetic chemistry is observable in recent years. While these promising approaches indeed opened the door for numerous new applications, a mechanistic understanding of the multi-component systems becomes increasingly challenging. Joint investigations of applications and mechanisms are therefore needed to obtain insights on possible transformations as well as on the underlying mechanistic steps. The enhanced understanding of a catalytic system likely improves the rational development of new systems and can prevent misguided assumptions based on wrongly proposed mechanisms. Multi-photonic mechanisms received considerable attention within the last years to expand the (thermodynamic) boundaries of photo(redox) catalysis with one visible photon per catalytic turnover. In addition to valuable new reactivities, the growing complexity of new systems make mechanistic analysis more and more important to understand the processes. The research presented in this thesis focuses on a combination between synthetic applications and mechanistic investigations towards a holistic picture of the respective system. As main motive, the insights obtained about light-driven processes in the individual systems are further developed to provide possible concepts and guidelines towards a better control of reactivities of catalysis relying on mono- and multi-photonic excitations.
The first project presented in Chapter 2 introduces a super-photoreductant and analyses the role of the solvent on the light-induced reactivity of highly reducing excited states. Dehalogenations of aryl chlorides and aryl fluorides are achievable by excitation of a strong ground-state electron donor with blue light. Significant differences in the reaction progress over time depending on the solvent are observable and spectroscopic measurements indicate a direct substrate reduction in benzene. In acetone a solvent-mediated mechanism is proposed, resulting in a levelling effect of the achievable redox power.
The second project in Chapter 3 provides a detailed mechanistic analysis of a sensitization-initiated electron transfer mechanism. The thoughtful choice of sensitizer enables a clear-cut triplet-triplet annihilation upconversion pathway followed by reductive quenching of the annihilator. Next to the analysis of all individual elementary steps of the proposed catalytic cycle, spectroscopic evidence under upconversion conditions is provided for the formation of the pyrenyl radical anion as catalytically active species responsible for substrate reduction. This unusually detailed mechanistic study is complemented with selected synthetic applications for reduction reactions.
The third main topic discussed in Chapter 4 introduces a new concept for multi-photon excitation under red-light irradiation. The combination of a copper-based photocatalyst and 9,10-diycanoanthracene (DCA) as two individual photocatalysts in the presence of an excess of sacrificial electron donor enables dehalogenations of (activated) aryl bromides and aryl chlorides as well as detosylations of phenoles and anilines under illumination with red light. The conceptual mechanistic idea resembles the Z-scheme of natural photosynthesis. Spectroscopic investigations indicate two mechanistic pathways for the formation of DCA•− as key intermediate by sensitized triplet sate quenching or direct photoinduced electron transfer. The relative contributions between both pathways are solvent-dependent and this could potentially provide a concept for more controllable steps in light-driven transformations. The last project presented in Chapter 5 exploits the advantage of combining two (photo)- catalysts and builds on the results presented in the previous chapter to change the mechanism and the reactivity by a change of the primary photocatalyst. In the absence of any sacrificial electron donor, sensitized triplet-triplet annihilation upconversion with a osmium-based sensitizer and DCA as annihilator enables a substrate oxidation step instead of substrate reduction mentioned above. Mechanistic analyses are complemented with investigations of four different overall redox-neutral reactions.
Consequently, this thesis covers different aspects related to mono- and biphotonic excitations in photoredox catalysis and the insights on mechanisms and applications intend to contribute to a better understanding and potential further rational development of light-driven catalysis.