Vibronic and Spin-Orbit Interplay Governs Ultrafast Spin Crossover in an Iron(II) Carbene Complex

Iron(II) complexes are increasingly recognized as sustainable, earth-abundant alternatives to precious metal chromophores, though their use has been constrained by ultrafast excited-state deactivation through low-lying metal-centered (MC) states. Here, we present a comprehensive theoretical investigation of the iron carbene complex [Fe(bmip)₂]²⁺ (bmip = 2,6-bis(3-methyl-imidazole-1-ylidene)pyridine), elucidating how vibronic coupling, spin-orbit coupling (SOC), and environmental reorganization collectively modulate spin-crossover (SCO) dynamics following metal-to-ligand charge transfer (MLCT) excitation. Using ab initio electronic structure calculations and hierarchical equations of motion simulations, we trace population transfer from the initially photoexcited singlet MLCT to triplet states. The strong σ-donor character of the carbene ligands raises the MC state energies, thereby stabilizing the MLCT manifold and extending its lifetime to several picoseconds, around two orders of magnitude longer than in conventional Fe(II) polypyridines. The results reveal sub-100 fs intersystem crossing (ISC) from singlet to triplet MLCT states, followed by a slower decay that avoids rapid relaxation to MC levels. Importantly, ISC proceeds efficiently only via specific vibrational modes, identifying vibronic coupling as the principal gating mechanism. These findings provide a clear framework for tuning ligand fields and coupling strengths to optimize excited-state lifetimes and spin dynamics in next-generation Fe-based photosensitisers.