posted on 2021-09-01, 14:04authored byF. Hainer, N. Alagna, A. Reddy Marri, T. J. Penfold, P. C. Gros, S. Haacke, T. Buckup
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 (3MC) 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 3MCs’ 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–1 are assigned
to the 3MC and 3MLCT states by quantum chemical
calculations. On the basis of this assignment, an ultrafast branching
directly after excitation forms not only the long-lived 3MLCT but also the 3MC as an additional loss channel.