Theoretical evidence for the direct 3MLCT-HS deactivation in the light-induced spin crossover of Fe(ii)–polypyridyl complexes

Abstract

Spin–orbit couplings have been calculated in twenty snapshots of a molecular dynamics trajectory of [Fe(bpy)₃]²⁺ to address the importance of geometrical distortions and second-order spin–orbit coupling on the intersystem crossing rate constants in the light-induced spin crossover process. It was found that the effective spin–orbit coupling between the ³MLCT and ⁵T₂ state is much larger than the direct coupling in the symmetric structure, which opens the possibility of a direct ³MLCT–⁵T₂ deactivation without the intervention of triplet metal-centered states. Based on the calculated deactivation times, we conclude that both the direct pathway and the one involving intermediate triplet states are active in the ultrafast population of the metastable HS state, bringing in agreement two experimental observations that advocate for either deactivation mechanism. This resolves a long-standing dispute about the deactivation mechanism of Fe(II)–polypyridyl complexes in particular, and about light-induced magnetism in transition metal complexes in general.