Photoredox catalysts based on earth-abundant metal complexes

Abstract

Photoredox catalysis have been developed since at least 1968 when the term of photoredox was first used. During the past 50 years or so, the scope of reactions catalysed by a photocatalyst has experienced a rapid expansion. Photocatalysis is now employed to synthesize a huge amount of different types of organic compounds. At the same time, studies about the reaction mechanism have also become more and more detailed and comprehensive. This thesis discussed design, synthesis and photophysical properties of earth-abundant metal complexes. It also explores photoredox catalytic reactions using these earth-abundant metal complexes.

In Chapter 1, an overview of the development of photoredox catalysis is given followed by an overview of the basics of photophysics of 1ˢᵗ row transition metal complexes. Moreover, the different types of photocatalytic reactions and the corresponding reaction mechanism behind them were discussed. The importance of mechanistic studies for photocatalytic reactions and the methodology used for mechanistic studies are also addressed in Chapter 1. In particular, the optoelectronic properties of photoactive earth-abundant complexes and the photocatalysis using photocatalysts based on Copper(I) complexes are reviewed at the end of the Chapter 1.

In Chapter 2, the photophysics and electrochemistry of six sulfur-bridged luminescent copper(I) complexes are explored. This chapter discusses the influence of sulfur oxidation state and substituents effect on the optoelectronic properties of these copper(I) complexes. Furthermore, these complexes were identified as showing thermally activated delayed fluorescence (TADF) and these complexes showed potential as solid-state emitters.

In Chapter 3, the synthesis and the photophysics of two dinuclear copper(I) complexes bearing pyrimidylimidazole bridging ligand are described. The tetrahedral coordination sphere of each copper centre is completed through the use of a bulky bisphosphine ligand, either POP or Xantphos. Temperature-dependant photophysical studies demonstrated emission through a combination of phosphorescence and TADF from both complexes, and an intense emission (Φ[sub](PL) = 46%) was observed for a crystalline sample of one of the complexes reported. The photophysics of these two complexes is very sensitive to the environment. Two polymorphs of one of the dinuclear complex were isolated, and their photophysics is distinct. Upon grinding, the higher emission energy crystal could be converted to the lower energy crystal.

In Chapter 4, the optoelectronic properties of a family of six structurally related heteroleptic copper(I) complexes of the form of [Cu(N^N)(P^P)]⁺ bearing a 2,9-dimethyl-1,10-phenanthroline diimine (N^N) ligand and a series of electronically tunable Xantphos (P^P) ligands were characterized. The reactivity of these complexes in the copperphotocatalyzed Aza-Henry reaction of N-phenyltetrahydroisoquinoline was evaluated, while the related excited state

kinetics

were comprehensively studied. A combined study of structural modulation of copper(I) photocatalysts, optoelectronic properties and photocatalytic reactivity resulted in a better understanding of the rational design of a photocatalyst targeting complicated photocatalytic reactions.

In Chapter 5, two room-temperature luminescent cobalt(III) complexes were found to be powerful photo-oxidants and were used as inexpensive photoredox catalysts for the regioselective mono-(trifluoromethylation) of polycyclic aromatic hydrocarbons (PAHs) in good yields (ca. 40–58%). The reaction mechanism was comprehensively studied, and an origin of the chemoselectivity was proposed.