Tuning the ²LMCT Deactivation of Cyclometalated Iron Carbene Complexes with Electronic Substituent Effects

Fe^III complexes based on the [Fe^III(ImP)₂]⁺ motif (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)), where the ligand contains both carbene and cyclometalated moieties, are a promising class of photoactive materials made from this abundant metal. In this work, it is shown that bromo or furanyl substituents attached to the cyclometalating moiety of the ImP ligands stabilize the ²LMCT excited state to very different extent resulting in opposing effects on the ²LMCT lifetime. For [Fe^III(ImPBr)₂]⁺, the lifetime (255 ps) of its moderately stabilized ²LMCT state (1.85 eV) is slightly increased compared to the parent complex (1.90 eV, 240 ps) pointing to an increased barrier for deactivation via the ⁴MC state and enabling applications as photoredox catalyst. In contrast, the ²LMCT energy of [Fe^III(ImPFur)₂]⁺ is lowered substantially to a value of 1.63 eV due to the extended π-system of the ligands and the reduced energy gap favors internal conversion directly to the ground state resulting in a considerably reduced ²LMCT lifetime of 59 ps. These findings have general implications for design of ligand modifications aiming at extended LMCT lifetimes and/or modified ground and excited state potentials.