Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe^II(ImP)₂] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its ³MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (²MLCT) state of its Fe(iii) analogue [Fe^III(ImP)₂]⁺, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe^III(ImP)₂]⁺, we confirmed however that higher (²MLCT) states of [Fe^III(ImP)₂]⁺ are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe^III(ImP)₂]⁺ prepared by us only shows red emission from its lower ²LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe^III(ImP)₂]⁺ is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe^III(ImP)₂]⁺ itself.