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

The successful application of transition metal based photoactive 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 4d6 and 5d6 valence electron configuration. These photocatalysts exhibit excited state lifetimes exceeding hundreds of nanoseconds. Simple transition metal complexes with 3d6 valence electron configuration containing abundant metals exhibit lifetimes of less than 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 give access to the tetracarbene manganese(I) complex [Mn(pbmi)2]+. This purple colored complex phosphoresces at room temperature in fluid solution. Its 3MLCT excited state lifetime of 190 ns exceeds those of simple 3MLCT photoactive 3d6 complexes by more than one to two orders of magnitude. In combination with its reversible ground state MnII/I redox chemistry, this translates to a 3MLCT excited state capable of reducing an organic substrate by bimolecular quenching. The combination of manganese(I) with rigid carbene and pyridine chelate 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.