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 Fe^II NHC 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 ³ MLCT state, from the initially excited ¹ MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the ³ MC state, in competition with vibrational relaxation and cooling to the relaxed ³ MLCT state. The relaxed ³ MLCT state then decays much more slowly (7.6 ps) to the ³ MC state. The ³ MC state is rapidly (2.2 ps) deactivated to the ground state. The ⁵ MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the ³ MLCT 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.