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

Abstract

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.