Light-induced spin crossover probed by ultrafast optical and X-ray spectroscopies

The photoinduced energy and structural relaxation of aqueous iron(II)-tris-bipyridine ([Fe^II(bpy)₃]²⁺), upon excitation into its singlet metal-to-ligand charge transfer (¹MLCT) band, and the population of its short-lived (≤0.6 ns) high-spin excited state have been characterized by combined ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion reveals a very short lived ¹MLCT emission band, which decays in ≤20 fs by intersystem crossing to a ³MLCT, whose very weak emission is also observed. Transient absorption studies show that the latter decays in <150 fs, while the population of the lowest excited high spin quintet state ⁵T ₂ occurs in ∼1 ps. Subsequently it decays in ∼665 ps to the ground state. Therefore, we determined the structure of the high-spin excited state by picosecond X-ray absorption spectroscopy at the K edge of Fe. The structural analysis of both the transient difference and excited state X-ray spectra delivered an Fe-N bond elongation of 0.2 Å in the quintet state compared to the singlet ground state.