Vibrational Coherence Spectroscopy Identifies Ultrafast Branching in an Iron(II) Sensitizer.

The introduction of N-heterocyclic carbene ligands has greatly increased the lifetimes of metal-to-ligand charge transfer states (MLCT) in iron(II) complexes, making them promising candidates for photocatalytic applications. However, the spectrally elusive triplet metal-centered state (MC) has been suggested to play a decisive role in the relaxation of the MLCT manifold to the ground state, shortening their lifetimes and consequently limiting the application potential. In this work, time-resolved vibrational spectroscopy and quantum chemical calculations are applied to shed light on the MCs' involvement in the deactivation of the MLCT manifold of an iron(II) sensitizer. Two distinct symmetric Fe-L breathing vibrations at frequencies below 150 cm are assigned to the MC and MLCT states by quantum chemical calculations. On the basis of this assignment, an ultrafast branching directly after excitation forms not only the long-lived MLCT but also the MC as an additional loss channel.