Photoinduced Low-Spin → High-Spin Mechanism of an Octahedral Fe(II) Complex Revealed by Synergistic Spin-Vibronic Dynamics

[Image: see text] The Fe(II) low-spin (LS; (1)A(1g), t(2g)(6)e(g)(0)) → high-spin (HS; (5)T(2g), t(2g)(4)e(g)(2)) light-induced excited spin state trapping (LIESST) mechanism solely involving metal-centered states is revealed by synergistic spin-vibronic dynamics simulations. For the octahedral [Fe(NCH)(6)](2+) complex, we identify an initial ∼100 fs (1)T(1g) → (3)T(2g) intersystem crossing, controlled by vibronic coupling to antisymmetric Fe–N stretching motion. Subsequently, population branching into (3)T(1g), (5)T(2g) (HS), and (1)A(1g) (LS) is observed on a subpicosecond time scale, with the dynamics dominated by coherent Fe–N breathing wavepackets. These findings are consistent with ultrafast experiments, methodologically establish a new state of the art, and will give a strong impetus for further intriguing dynamical studies on LS → HS photoswitching.