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

Abstract

The light-induced ultrafast spin and structure changes upon excitation of the singlet metal-toligand-charge-transfer (1 MLCT) 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 upconversion, 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 1 MLCT state from the singlet low spin ground state (1 GS) as the following sequence: 1,3 MLCT→ 5 T→ 1 GS, which does not involve intermediate singlet and triplet ligand field states. The population time of the HS state is found to be~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.

Computational approach to the study of thermal spin crossover phenomena

“…3 However, the interest in SCO materials increased when it was discovered that conversion between the two spin states can be controlled by light irradiation, thus opening the possibility of using such materials as optically switchable devices. [4][5][6] This phenomenon, called Light-Induced Excited Spin State Trapping (LIESST), initially found in Fe(II) complexes [7][8][9][10][11] and later also observed in systems containing Fe(III), [12][13][14][15] and Ni(II) [16][17][18] has been intensively studied in the last years in order to unravel its mechanism both with experimental techniques 11,[19][20][21][22][23][24][25] and by means of theoretical calculations. [26][27][28][29][30][31][32][33] The most numerous and most studied family of SCO systems involves octahedral Fe(II) complexes in the solid state or in solution.…”

Electron Small Polarons and Their Mobility in Iron (Oxyhydr)oxide Nanoparticles

“…Transport characteristics are most commonly assessed using temperature-dependent measurements of the bulk conductivity within complex polycrystalline materials, e.g., with pressed pellets 12,13 . [16][17][18] . UV-pump-X-ray probe studies have been used to study a single photochemical reaction of iron, ferrioxalate photolysis Synthesis methods were optimized to achieve the smallest size nanoparticles without impurity phases, as confirmed by X-ray diffraction and electron microscopy (Figs.…”

Charge transfer to solvent dynamics in iodide aqueous solution studied at ionization threshold

“…Even though this technique is designed to investigate femtosecond dynamics, the decreased photon flux of approximately 50 photons per pulse at a repetition rate of 6 kHz at the slicing beamline at BESSYII [147], which is ∼ 3 orders of magnitude lower than in normal operation, complicates experiments on diluted systems by the increased data accquisition times. Besides solid samples, the only diluted systems investigated using the slicing technique were aqueous [Fe(bpy) 3 ] 2+ in the hard X-ray regime [148] and [Fe(tren(py) 3 )]…”