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

The Fe(II) low-spin (LS; 1A1g, t2g6eg0) → high-spin (HS; 5T2g, t2g4eg2) 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 1T1g → 3T2g intersystem crossing, controlled by vibronic coupling to antisymmetric Fe-N stretching motion. Subsequently, population branching into 3T1g, 5T2g (HS), and 1A1g (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.