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 FeIINHC 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.