Light-induced spin crossover in Fe(II)-based complexes: The full photocycle unraveled by ultrafast optical and X-ray spectroscopies

The light-induced spin and structure changes upon excitation of the singlet metal-to-ligand charge transfer ((MLCT)-M-1) state of Fe(II)-polypyridine complexes are investigated in detail in the case of aqueous iron(II)-tris-bipyridine ([Fe-II(bpy)(3)](2+)) by a combination of ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion, transient absorption studies in the 290-600 nm region and femtosecond X-ray absorption spectroscopy allow us to retrieve the entire photocycle upon excitation of the (MLCT)-M-1 state from the singlet low-spin ground state ((1)GS) as the following sequence: (MLCT)-M-1,3 -> T-5 ->(1)GS, which does not involve intermediate singlet and triplet ligand-field states. The population time of the HS state is found to be similar to 150 fs, leaving it in a vibrationally hot state that relaxes in 2-3 ps, before decaying to the ground state in 650 Ps. We also determine the structure of the high-spin quintet excited state by picosecond X-ray absorption spectroscopy at the K-edge of Fe. We argue that given the many common electronic (ordering of electronic states) and structural (Fe-N bond elongation in the high-spin state, Fe-N mode frequencies, etc.) similarities between all Fe(II)-polypyridine complexes, the results on the electronic relaxation processes reported in the case of [Fe-II(bpy)(3)](2+) are of general validity to the entire family of Fe(II)-polypyridine complexes. (C) 2009 Elsevier B.V. All rights reserved.