Ultrafast 3MLCT quenching and vibrational coherence: excited-state dynamics of the first-discovered Fe(II)-carbene sensitiser resolved

The application of N-heterocyclic carbene (NHC) ligands represents a groundbreaking advance towards environment-friendly light-harvesting complexes, yet, even the excited-state dynamics of the first-discovered Fe-NHC photosensitiser [Fe(bmip)2]2+ (bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine) remain controversial. Using full-dimensional trajectory surface hopping (TSH) spin-vibronic dynamics simulations, we fully resolve and clarify the deactivation mechanism of [Fe(bmip)2]2+. In agreement with the most recent fs-resolved X-ray experiments, we find the excited-state population branches into triplet metal-to-ligand charge transfer (3MLCT) and metal-centered states (3MC) on early sub-ps timescales ~200 fs. The ultrafast 3MLCT quenching by 3MC is driven by multidimensional excited-state ligand motion; it is this deactivation process that ultimately weakens the photosensing efficiency of [Fe(bmip)2]2+. Crucially, the preservation of vibrational coherence along the covalent Fe-C bonds (~300 fs vibrational period) and the appearance of resulting coherent oscillations in various time-resolved experimental data unambiguously evidences the presence of the early sub-ps 3MC component.