Hot branching dynamics in a light‐harvesting iron carbene complex revealed by ultrafast x‐ray emission spectroscopy

Iron nitrogen heterocyclic carbenes (NHC) have received a great deal of attention recently, due to their growing potential as e.g light sensitizers and photocatalysts. We present a sub-ps x-ray spectroscopy study of a Fe^IINHC complex allowing us to identify and quantify the states involved in the deactivation cascade after light absorption. We find that excited molecules relax back to the ground state populating first the ³MLCT and then along two pathways the ³MC state. One of these pathways is ultrafast (~150 fs) for ~30% of the excited molecules, in competition with vibrational relaxation and cooling, followed by a much slower (7.6 ps) decay of the relaxed ³MLCT state. The ³MC state then rapidly (2.2 ps) decays to the ground state. The ultrafast deactivation of the ³MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen other FeNHC complexes (and perhaps other transition metal complexes) for this ultrafast decay of ³MLCT population, in order to optimize photochemical performance.