Is Iron the New Ruthenium?

Abstract

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example lighting, sensing, solar cells, and photoredox catalysis. There is a long‐standing interest in replacing ruthenium by iron, because ruthenium is rare and expensive whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long‐lived excited state with pronounced charge transfer character between the ruthenium metal and the ligands. These metal‐to‐ligand charge transfer (MLCT) excited states can be luminescent with typical lifetimes in the range of 100 ns to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal‐centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Very recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have just been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than the commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research on photoactive earth‐abundant metal complexes. After outlining some of the fundamental challenges, this invited Concept article discusses the key strategies used so far to enhance the photophysical and photochemical properties of iron complexes, and it discusses recent conceptual breakthroughs.