Ligand design strategies for photoactive first-row transition metal complexes

Photochemistry attracts increasing attention, but still most of the used photoactive transition metal complexes are based on metals of the second and third row. The scarcity of the popular metals like ruthenium and iridium prohibits their widespread use in applications. The transition towards more sustainable photochemistry using photoactive complexes based on abundant first-row transition metals is an attractive research goal nowadays. This thesis aims at introducing different strategies towards this goal and highlights two projects, where new first-row transition metal complexes have been investigated. In the first project complexes of 3d10 electronic configuration have been used for photochemical applications (Chapter 3) and in the second project new 3d6 and 3d5 transition metal complexes were synthesized and studied (Chapter 4).
The careful design of a charge transfer ligand bearing an electron-donating bis(4-methoxyphenyl)amine unit and an electron-accepting benzoxazole backbone leads to two constitutionally isomeric zinc(II) complexes with fundamentally different photophysical properties in the first project (Chapter 3). Their favorable properties enabled their use in different photocatalytic reactions, the transfer of an electron to a suitable substrate and as sensitizer in a sensitized triplet triplet annihilation upconversion. Photostability studies provided insights into the main decomposition pathways of the two complexes.
In the second project (Chaper 4), new synthetic approaches towards geometrically optimized N-heterocyclic carbene (NHC) ligands were investigated. Additionally, in the course of this project, two tridentate ligands with a central cyclometalating phenyl ring framed by two NHC side arms and a bidentate ligand with a pyridine and a NHC binding site were successfully synthesized. The complexation of the tridentate cyclometalating ligand to Co(III) and Fe(III) resulted in two homoleptic complexes that were investigated. Preliminary results of photophysical measurements allowed a first insight into the excited state properties of these complexes.