Photocatalysis from a mechanistic perspective

The first part of this talk will focus on the mechanism of sensitization-initiated electron transfer, a process that has received substantial attention from the organic-synthetic photoredox community for the visible light-driven generation of aryl radicals. Our own mechanistic study with an iridium(III) photosensitizer and a pyrene energy acceptor shows that triplet-triplet annihilation upconversion can play a key role in sensitization-initiated electron transfer, enabling challenging reductions after the formation of pyrenyl radical anions in the presence of excess sacrificial electron donors. The second part of the presentation will concentrate on red light-based dual photocatalysis, in which two photosensitizers each absorb one photon separately. The pooled energy of the two absorbed red photons can then drive a variety of reductive dehalogenation and detosylation reactions requiring very negative reduction potentials. Based on UV-Vis transient absorption spectroscopy, the solvent determines the dominant reaction mechanism, involving either photoinduced electron transfer or triplet-triplet energy transfer in the initial elementary reaction step. The last part of the talk will concentrate on recent advances in photocatalyst development, with particular focus on metal complexes acting as strong photoreductants or as effective triplet-energy donors. This includes classical iridium(III)- and ruthenium(II)-based compounds, as well as conceptually new types of complexes made from Earth-abundant transition metals such as molybdenum or manganese.