Light-Induced Spin Crossover Probed by Ultrafast Optical and X-ray Spectroscopies

Gawelda, Wojciech; Cannizzo, Andrea; Pham, Van-Thai; El Nahhas, Amal; Milne, Christopher J.; van der Veen, Renske; Bressler, Christian; Chergui, Majed (2007). Light-Induced Spin Crossover Probed by Ultrafast Optical and X-ray Spectroscopies. CHIMIA, 61(4), pp. 179-183. Schweizerische Chemische Gesellschaft 10.2533/chimia.2007.179

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The photoinduced energy and structural relaxation of aqueous iron(II)-tris-bipyridine ([FeII(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 5T2 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.

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