Metalloporphyrins 4.2 Vapochromism as a result of indirect analyte-metal interactions 4.2.1 Vapochromism as a consequence of a change in spin state 4.2.2 Hydrogen-bonded proton transfer assemblies 4.2.3 Metallophilic, π-π, and donor-acceptor interactions 4.2.4 Vapochromism in coordination polymers and metal-organic frameworks 4.2.5 Vapochromism in cyanometallates

In this invited Perspective recent developments and possible future directions of research on photoactive coordination compounds made from non-precious transition metal elements will be discussed. The focus is on conceptually new, structurally well-characterized complexes with excited-state lifetimes between 10 ps and 1 ms in fluid solution for possible applications in photosensitizing, light-harvesting, luminescence, and catalysis. The key metal elements considered herein are Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, W, and Ce in various oxidation states equipped with diverse ligands, giving access to long-lived excited states via a range of fundamentally different types of electronic transitions. Research performed in this area over the past five years demonstrated that a much broader spectrum of metal complexes than what was long believed relevant exhibits useful photophysics and photochemistry.

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible‐light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near‐IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low‐energy photons can address both challenges simultaneously, and whilst multi‐photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi‐photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

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Ruthenium complexes with polypyridine ligands are very popularc hoices for applications in photophysics and photochemistry,f or example, in lighting, sensing, solar cells, and photoredox catalysis. There is al ong-standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap.H owever,i ti sv ery difficult to obtain iron complexes with an electronic structure similart ot hat of ruthenium(II) polypyridines. The latter typicallyh ave al onglived excited state with pronounced charge-transferc haracter between the ruthenium metal and ligands. These metalto-ligand charge-transfer( MLCT) excited states can be luminescent, with typical lifetimesi nt he range of 100 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 [a] Prof.