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

Fundamental characterization, photophysics and photocatalysis of a base metal iron(II)-cobalt(III) dyad

A new base metal iron-cobalt dyad has been obtained by connection between a heteroleptic tetra-NHC iron(II) photosensitizer combining a 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine with 2,6-bis(3-methyl-imidazol-2-ylidene)-4,4′-bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)-cobalt(III) assembly has been extensively characterized by ground- and excited-state methods like X-ray crystallography, X-ray absorption spectroscopy, (spectro-)electrochemistry, and steady-state and time-resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited-state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal-to-ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the $^{3}$MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a $^{3}$MC state. Attachment of cobaloxime to the iron photosensitizer increases the $^{3}$MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two-component system, consisting of the iron photosensitizer and Co(dmgH)$_2$(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems

Unveiling the origin of photo-induced enhancement of oxidation catalysis at Mo(VI) centres of Ru(II)–Mo(VI) dyads

Photo-induced oxidation-enhancement in biomimetic bridged Ru(II)–Mo(VI) photo-catalyst is unexpectedly photo-activated in ps timescales. One-photon absorption generates an excited state where both photo-oxidized and photo-reduced catalytic centres are activated simultaneously and independently

Spin-state studies with XES and RIXS: From static to ultrafast

We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) Fe-II complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS-LS difference spectra measured at thermal spin crossover, and reference HS-LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe Is pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales. (C) 2012 Elsevier B.V. All rights reserved

Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins

In haemoglobin (consisting of four globular myoglobin-like subunits), the change from the low-spin (LS) hexacoordinated haem to the high spin (HS) pentacoordinated domed form upon ligand detachment and the reverse process upon ligand binding, represent the transition states that ultimately drive the respiratory function. Visible-ultraviolet light has long been used to mimic the ligand release from the haem by photodissociation, while its recombination was monitored using time-resolved infrared to ultraviolet spectroscopic tools. However, these are neither element- nor spin-sensitive. Here we investigate the transition state in the case of Myoglobin-NO (MbNO) using femtosecond Fe Kalpha and Kbeta non-resonant X-ray emission spectroscopy (XES) at an X-ray free-electron laser upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) haem occurs in ca. 800 fs, and that it proceeds via an intermediate (S = 1) spin state. The XES observables also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ca. 30 ps. Thus, the entire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process

Site-Selective Real-Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L-Edge X-Ray Absorption Spectroscopy

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [IrIII (ppy)2 (bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance