Photo-induced spin-state conversion in solvated transition metal complexes probed via time-resolved soft X-ray spectroscopy

Solution-phase photoinduced low-spin to high-spin conversion in the Fe ^II polypyridyl complex [Fe(tren(py)₃)]²⁺ (where tren(py)₃ is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following ¹A₁ → ¹MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L₂- and L₃-edges were observed concomitant with formation of the transient high-spin ⁵T₂ state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of ∼1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 Å increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in δ-donation is compensated by significant attenuation of φ-back-bonding in the metal-ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations.