D-d and charge transfer photochemistry of 3d metal complexes

Abstract

Listen

Translate article

The photophysics and photochemistry of 3d transition metal complexes strongly differ from their heavier 4d and 5d homologs. The distinct excited state dynamics are a direct consequence of the weaker ligand field splitting and the smaller spin-orbit coupling in 3d transition metal complexes. The often very fast non-radiative relaxation of photoexcited 3d transition metal complexes to the ground state has prevented a widespread use of these much more abundant transition metals in photophysical and photochemical applications so far. Recent exciting advancements in ligand design, synthesis, ultrafast spectroscopy, computational chemistry and understanding of excited state dynamics as well as the awareness for a more sustainable photochemistry led to a paradigm change in the reputation and emerging importance of 3d transition metals. The introduction chapter contrasts the photophysical background of 3d transition metal complexes with that of their heavier homologs. This is followed by overviews of the excited state reactivity of 3d transition metal complexes, namely luminescence, bi- and unimolecular reactivity. Luminescence from excited states is divided into spin-flip and charge transfer luminescence. The charge transfer excited states and the resulting luminescence can be of ligand-to-metal, metal-to-ligand, ligand-to-ligand charge transfer and charge transfer to solvent character. Bimolecular reactivity covers photoinduced electron and energy transfer reactions. In solution, the lifetimes of the reacting excited states play a particularly important and challenging role. Dissociative unimolecular reactivity comprises dissociation of ligands such as CO and NO, CO2 dissociation from carboxylato complexes, N2 dissociation from azido complexes, MC bond homolysis and other MX dissociations as well as photoisomerizations. These unimolecular reactions can occur on ultrafast timescales.