The photofunctionality of the cobalt−hexacarbene complex [Co(III)(PhB(MeIm) 3 ) 2 ] + (PhB(MeIm) 3 = tris(3methylimidazolin-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 3d 6 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.

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-methylimidazoline-2-ylidine)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazoline-2-ylidine)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.

Steady state and ultrafast spectroscopy on [FeIII(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer (2LMCT) state...

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