Understanding Valence and Core d-d Correlations on Ground and Excited States of Solvated Ruthenium Complexes with Novel X-ray Spectroscopies

Abstract

X-ray spectroscopy has become a widely used technique for investigating the electronic structure,oxidation state, and metal-ligand bonding of transition metal complexes with element specificity. Second-row transition metals, such as ruthenium, have been the target of relatively fewer X-ray studies than first-row transition metal studies even though second-row transition metal complexes are being increasingly used in photocatalysis, solar energy conversion, and medical applications. This thesis demonstrates that X-ray spectroscopy of solvated ruthenium complexes can provide important insights into the properties that drive their functionality. X-ray absorption spectroscopy (XAS) is used to probe the unoccupied valence orbitals of a series of Ru complexes ranging from the model complex [RuII(bpy)3]2+, a prototypical example of a Ru photosensitizer, to a Ru-Ru dimer that can be used as a chromophorecatalyst assembly. From the spectra of these complexes, their oxidation states, the strength of certain metal-ligand bonds, and the ligand field splitting for RuIII complexes is determined. Next, the model complexes [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4−are investigated with 2p3d resonant inelastic X-ray scattering (RIXS) which is a second-order process that involves an initial X-ray absorption followed by an X-ray emission. This technique provides insight into the unoccupied valence orbitals, the occupied 3d core orbitals, and their couplings. From the RIXS spectra, the spin–orbit coupling of the 3d orbitals can be directly measured, and show slight deviations from atomic values which suggests that the molecule nature of these complexes can affect orbitals that reside ≈300 eV below the valence. Additionally, ligand field multiplet RIXS calculations of [RuIII(NH3)6]3+show that the inclusion of spin–orbit coupling of the 2p, 3d, and 4d orbitals is necessary for accurate modeling of its RIXS spectrum. To gain insight into the interactions of valence and core electrons, this thesis extends the work on Ru model complexes through the use of 2p4d RIXS and non-resonant valence-tocore (VtC) X-ray emission spectroscopy. Both non-resonant VtC XES and 2p4d RIXS probe unoccupied valence orbitals, though 2p4d RIXS offers higher spectral resolution and intensity. In addition, as 2p4d RIXS probes both the unoccupied and occupied valence orbitals, it is a more powerful technique, and the RIXS final states correspond to valence excited states. This allows for the determinations of metal-ligand bond strength, and the development of a spectro-chemical series for 4d transition metals as the ligand field splitting of closed-shell complexes is measured by 2p4d RIXS. Complementing to the novel experimental results, a theoretical framework using TDDFT-based methods was developed to simulate the 2p4d RIXS signals which allowed for reliable characterization of both ground and valence excited states in ruthenium complexes. Finally, this theses explores the ultrafast time regime by investigating the Ru-Ru dimer, [RuII(tpy)(bpy)(μ-CN)RuII(bpy)2(CH3CN)]3+, by both transient infrared and 2p3d RIXS. Upon photoexcitation, a mixed valence excited state is formed which presents signatures of a high degree of electronic delocalization. This was determined as the cyanide bridging mode red shifts upon photoexcitation and its intensity is enhanced, additionally a broad excitedstate absorption (ESA) feature that spans over 200 cm−1 was identified which matches well with low-energy electronic transitions found from TDDFT calculations. The time evolution of the infrared features show an initial ≈250 fs decay of this broad ESA which could correspond to relaxation through a manifold of triplet excited states. The CN bridging mode feature blue shifts by ≈15 cm−1 in around 5 ps which corresponds to vibrational relaxation and provides insight into the anharmonicity of this bridging mode. The 2p3d RIXS identifies the transient formation of a hole in the t2g orbitals corresponding to oxidation, the splitting between this t2g feature and the eg feature is 2.9 eV which is significantly lower than other ruthenium complexes, and has been found to be a measure of delocalization in iron-ruthenium mixed valence complexes.