Orange-glowing iron at room temperature: Many photoactive coordination compounds contain precious metals. Replacing ruthenium with more–earth-abundant iron has been a long-sought goal, but iron compounds generally relax too rapidly after light absorption to channel the energy productively. Kjær et al. prepared an iron compound with an excited state stable enough to emit light for nanoseconds, or that could engage in bimolecular electron transfer (see the Perspective by Young and Oldacre). Targeting a ligand-to-metal rather than metal-to-ligand charge-transfer state was key to the achievement, as was the octahedral coordination environment rigidly enforced by two tridentate carbene ligands. Science , this issue p. 249 ; see also p. 225

The photofunctionality of the cobalt–hexacarbene complex [Co(III)(PhB(MeIm)₃)₂]⁺ (PhB(MeIm)₃ = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ∼ 10^–4) but remarkably long-lived (τ ∼ 1 μs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d⁶ complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Photoinduced bimolecular charge transfer processes involving an iron(III) N-heterocyclic carbene (FeNHC) photosensitizer [Fe(phtmeimb)2]+ (with phtmeimb being [phenyl(tris(3-methylimidazol-1-ylidene))borate])- and triethylamine as well as N,N-dimethylaniline donors have been studied using optical spec-troscopy. The full photocycle of charge separation and recombina-tion down to ultrafast timescales were was studied by investigating the excited state dynamics up to high quencher concentrations. The unconventional doublet ligand-to-metal charge transfer (2LMCT) photoactive excited state exhibits donor-dependent charge separa-tion rates up to 1.25 ps-1 that exceed rates found for typical rutheni-um based systems and are instead more similar to results reported for organic sensitizers. The ultrafast charge transfer rates probed at high electron donor concentrations outpace solvent dynamics and go beyond the classical Marcus electron transfer regime. Poor photoproduct yields are explained by donor-independent, fast charge recombination with rates of ~0.2 ps-1, thus inhibiting cage escape and photoproduct formation. This study thus shows that the ultimate bottlenecks for bimolecular photoredox processes involv-ing these FeNHC photosensitizers can only be determined from the ultrafast dynamics of the full photocycle; of particular importance when the bimolecular charge transfer processes are not limited by the intrinsic excited state lifetime of the photosensitizer.

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)₂]PF₆ (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]^-, [Fe(meophtmeimb)₂]PF₆ (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-, and [Fe(coohphtmeimb)₂]PF₆ (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-. These were derived from the parent complex [Fe(phtmeimb)₂]PF₆ (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]^- by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (²LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (²GS), completely in line with the parent complex [Fe(phtmeimb)₂]PF₆ (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.