Time-resolved x-ray absorption spectroscopy of transition metal complexes

Electronic structure changes are at the origin of the making, breaking and transformation of bonds. These changes can be visualized by measuring the geometric structure in "real-time" during the course of a chemical reaction, a biological function or a physical process. Time-resolved X-ray Absorption Spectroscopy (XAS) delivers information about both the electronic (via XANES) and geometric (via EXAFS) transient structural changes, when interfaced with an ultrafast laser in a pump-probe scheme. Moreover, XAS offers unique flexibility, since it is both element-selective and it can be applied to any kind of disordered or ordered systems. In this thesis, we successfully investigated the excited state electronic and geometric structures of two different transition metal complexes. In both cases, for the first time, their excited state molecular geometries were characterized "on the fly", without any a priori assumptions about its excited state structure. More importantly, it has been shown that time-resolved XAS is the only method capable of delivering the transient molecular structures of their short-lived excited states. First, we investigated ruthenium(II)-tris(2,2'-bipyridine), [RuII(bpy)3]2+. This molecule has served as a prototype and a model system of intramolecular electron and energy transfer reactions, due to its unique excited state properties. Our studies focused on the energy and structural relaxation process of the short-lived excited states of this molecule. By using the combined ultrafast laser and x-ray spectroscopies, we have determined various relaxation pathways of its excited states down to 15 fs lifetimes. The geometrical distortion of its lowest-lying excited state (3MLCT state) has also been determined by picosecond XAS, delivering a Ru-N bond contraction of ∼ -0.04 Å. Second, our study focused on iron(II)-tris(2,2'-bipyridine) [FeII(bpy)3]2+. This class of compounds is being extensively studied in relation to the phenomenon of spin crossover, where a spin transition takes place, involving the low-spin (LS) ground state and the high-spin (HS) excited state. Here, we have characterized its excited states by means of both ultrafast optical and x-ray spectroscopy. The optical studies have revealed several new aspects concerning the relaxation pathways of its charge transfer and ligand-field states, including their corresponding lifetimes. The structural analysis has determined the geometric distortions taking place in the lowest-lying excited HS state of [FeII(bpy)3]2+.The extracted Fe-N bond elongation of 0.2 Å agrees well with previously predicted values and it is for the first time that the room-temperature solvated structure of the HS short-lived excited state of a ferrous transition metal is obtained.

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