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 (CT) states of most Fe complexes are limited by picosecond or sub-picosecond deactivation through low-lying metal centered (MC) states, resulting in inefficient electron transfer reactivity and complete lack of photoluminescence. Here we show that octahedral coordination of Fe(III) by two mono-anionic facial tris-carbene ligands can suppress such deactivation dramatically. 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 ns lifetime and 2% quantum yield via spin-allowed transition from a ligand-to-metal charge-transfer (² LMCT) state to the ground state (² GS). Reductive and oxidative electron transfer reactions were observed for the² LMCT state of [Fe(phtmeimb)₂]⁺ in bimolecular quenching studies with methylviologen and diphenylamine.