Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

There is a long-standing interest in iron­(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium­(II) polypyridines. The 3d6 electrons of iron­(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature. Here, we present a concept that boosts the luminescence quantum yield of a chromium(0) isocyanide complex by nearly 2 orders of magnitude, accompanied by a significant increase of the MLCT lifetime. Pyrene units in the diisocyanide ligand backbone lead to an enlarged π-conjugation system and to a strongly delocalized MLCT state, from which nonradiative relaxation is less dominant despite a sizable redshift of the emission. While the pyrene moiety is electronically coupled to the core of the chromium(0) complex in the excited state, UV–vis absorption and 2D NMR spectroscopy show that this is not the case in the ground state. Luminescence lifetimes and quantum yields for our pyrenyl-decorated chromium(0) complex exhibit an unusual bell-shaped dependence on solvent polarity, indicative of two counteracting effects governing the MLCT deactivation. These two effects are identified as predominant deactivation either through an energetically nearby lying metal-centered state in the most apolar solvents, or alternatively via direct nonradiative relaxation to the ground state following the energy gap law in more polar solvents. This is the first example of a 3d6 MLCT emitter to benefit from an increased π-conjugation network.

The reactivity of [Fe (tpena)] (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H O and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H O disproportionation, 2) selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3) oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2 ) [(tpenaH)Fe O-OH] . The resultant iron(IV) oxo and hydroxyl radical both participate in controllable hydrogen-atom transfer (HAT) reactions. Consistent with the presence of a weaker σ-donor carboxylate ligand, the most pronounced difference in the spectroscopic properties of [Fe(OOH)(tpenaH)] and its conjugate base, [Fe(OO)(tpenaH)] , compared to non-heme iron(III) peroxide analogues supported by neutral multidentate N-only ligands, are slightly blue-shifted maxima of the visible absorption band assigned to ligand-to-metal charge-transfer (LMCT) transitions and, corroborating this, lower Fe /Fe redox potentials for the pro-catalysts.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d 6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L CNC ), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L CNC ) 2 ](PF 6 ), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(L CNC ) 2 ](PF 6 ) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(L CNC ) 2 ](PF 6 ), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

The iron(III) complex of hexadentate N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate (tpena(-) ) is a more effective homogenous catalyst for selective sulfoxidation and epoxidation with insoluble iodosylbenzene, [PhIO]n , compared with soluble methyl-morpholine-N-oxide (NMO). We propose that two molecules of [Fe(tpena)](2+) cooperate to solubilize PhIO, extracting two equivalents to form the halogen-bonded dimeric {[Fe(tpena)OIPh]2}(4+). The closest intradimeric I⋅⋅⋅O distance, 2.56 Å, is nearly 1 Å less than the sum of the van de Waals radii of these atoms. A correlation of the rates of the reaction of {[Fe(tpena)OIPh]2}(4+) with para-substituted thioanisoles indicate that this species is a direct metal-based oxidant rather than a derived ferryl or perferryl complex. A study of gas-phase reactions indicate that an ion at m/z=231.06100 originates from solution-state {[Fe(tpena)OIPh]2}(4+) and is ascribed to [Fe(III) (tpenaO)](2+), derived from an intramolecular O atom insertion into an Fe-tpena donor bond. Proposed ion pairs, {[Fe(tpena)OIPh]Cl}(+) and {[Fe(tpena)OIPh]ClO4}(+), are more stable than native [Fe(tpena)OIPh](2+) ions, suggesting that halogen-bonding, as for the solution and solid states, operates also in the gas phase.

In this Frontier article, recently discovered chromium(0) and manganese(i) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states are highlighted.

[FeIII(OH)­(tpena)]+ (tpena– = N,N,N′-tris­(2-pyridylmethyl)­ethylenediamine-N′-acetate) catalytically activates H2O2 with the concomitant formation of the active oxidants [FeIV(O)­(tpena)]+ and HO• in aqueous solutions at pH 8. A kinetic model is used to demonstrate that the activation of [FeIII(OH)­(tpena)]+ by H2O2 proceeds by the formation of [FeIII(OOH)­(tpena)]+. Two previously unreported reactions of [FeIII(OOH)­(tpena)]+, the first with another H2O2 molecule to afford [FeIII(OH)­(tpena)]+, O2 •–, and HO• and the second, and dominant, with [FeIII(OH)­(tpena)]+ to yield 2 equiv of [FeIV(O)­(tpena)]+ and H2O, are found to be the major pathways for the formation of HO• and [FeIV(O)­(tpena)]+, respectively. The production of HO• was quantified by a chemiluminescence method showing that [FeIV(O)­(tpena)]+ is produced in much larger yields than HO•. The generation of HO• compromises the stability of [FeIII(OH)­(tpena)]+ unless an external substrate is present that can outcompete [FeIII(OH)­(tpena)]+ for HO•. Significantly, we demonstrate that the reaction commonly assumed to occur in the decay of nonheme iron­(III)­hydroperoxides, homolytic O–O bond cleavage, is of minor significance for the generation of HO• and the iron­(IV)­oxo complex. The production of both a reactive high-valent iron–oxo species and HO• under mild, aqueous ambient conditions represents a significant contribution to the current state of the art for biomimetic nonheme chemistry in water.