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

Abstract

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 that 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 photosensitising 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 evidence the presence of the early sub-ps ³MC component.