Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)₂]⁺, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}^-, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (²LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the ²LMCT state of [Fe(phtmeimb)₂]⁺ in bimolecular quenching studies with methylviologen and diphenylamine.