The Quest for Luminescent Iron Complexes

The photochemistry of transition metal complexes has been crucial for the development of many fundamental topics, as well as to pave the way for several important applications. However, in most cases, photoactive transition metal complexes involved precious metals, with luminescent ruthenium polypyridine complexes playing the dominant role. Developing photoactive species based on earth-abundant metals is highly important for fundamental and applicative reasons. Iron is one of the most abundant metals on Earth’s crust, so luminescent iron complexes are highly desired. The recent search for iron complexes with long-lived and luminescent excited states is here presented, including Fe(II) species with metal-to-ligand charge transfer (MLCT) excited states and Fe(III) species with luminescent ligand-to-metal charge transfer (LMCT) states. The excited-state equilibration approach to prolong the luminescence lifetimes of Fe(III) compounds in multichromophoric species is also discussed. This latter approach can increase the possibility of luminescent iron complexes being involved in bimolecular processes as well as in photoinduced electron and energy transfer at interfaces, which is relevant for many applications.