A manganese(I) complex with a 190 ns metal-to-ligand charge transfer lifetime

Application of photoactive transition metal complexes in lighting, imaging, sensing, and photocatalysis is usually based on the triplet metal-to-ligand charge transfer (³MLCT) excited state of precious metal complexes with 4/5d⁶ valence electron configurations. These photocatalysts exhibit excited state lifetimes exceeding hundreds of nanoseconds. Simple 3d⁶ transition metal complexes containing abundant metals exhibit lifetimes below 1-2 nanoseconds, and they require multistep ligand syntheses mitigating large-scale implementation. We report that a commercially available bis(imidazolium) pyridine pro-ligand [H₂pbmi]²⁺ and a manganese(II) salt yield the tetracarbene manganese(I) complex [Mn(pbmi)₂]⁺. This complex phosphoresces at room temperature in fluid solution from a ³MLCT state with a lifetime of 190 ns. In combination with the reversible [Mn(pbmi)₂]^2+/+ process, this translates to an excited state capable of reducing benzophenone. Combination of manganese(I) with rigid carbene/pyridine ligands expands key strategies for photoactive 3d⁶ metal complexes of earth-abundant metals with ³MLCT lifetimes rivalling those of precious metals and providing a conceptual starting point for a sustainable photochemistry.