Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy

Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an (FeNHC)-N-II complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot (MLCT)-M-3 state, from the initially excited (MLCT)-M-1 state, 30% of the molecules undergo ultrafast (150 fs) relaxation to the (MC)-M-3 state, in competition with vibrational relaxation and cooling to the relaxed (MLCT)-M-3 state. The relaxed (MLCT)-M-3 state then decays much more slowly (7.6 ps) to the (MC)-M-3 state. The (MC)-M-3 state is rapidly (2.2 ps) deactivated to the ground state. The (MC)-M-5 state is not involved in the deactivation pathway. The ultrafast partial deactivation of the (MLCT)-M-3 state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.