Hanna Oher PhD defense

Thesis title:

Development of innovative spectral modelling methods for the investigation of actinide-ligand interactions using time-resolved laser-induced fluorescence spectroscopy

Summary of the thesis:

Uranyl complexes have been the subject of many research works for fundamental chemistry of actinides, environmental issues, or nuclear fuel cycle processes. The formation of various uranium(VI) complexes, with ligands in solution must be characterized for a better understanding of U(VI) speciation. Uranyl-ligand interactions and symmetry of the complexes both affect the electronic structure of U(VI), and thus its luminescence properties. Time-resolved laser induced fluorescence spectroscopy (TRLFS) is one of the widely used techniques to get insights on the closest chemical environment of the uranyl ion in samples, owing to its high sensitivity and selectivity. However, the luminescence spectra fingerprints hold information within and beyond the first-coordination sphere of uranyl(VI), that needs to be more deeply investigated by supplementary techniques. A promising route for data interpretation consists in creating a synergy between TRLFS and ab initio-based interpretations. Luminescence spectra of uranyl complexes in solution typically show well-spaced vibronic progressions that overlap with the pure electronic transition from the excited state to the ground state. This has driven the theoretical methodology implementation. In the frame of this thesis, time-dependent density functional theory (TD-DFT) with hybrid and range-separated functionals is used to model the electronic structure of uranium(VI) complexes. This represents an effective theoretical approach with a reasonable computational cost and accuracy, compared with computationally expensive wave-function based methods, in a relativistic context. It enabled to characterize the main spectral parameters and the first low-lying excited state of uranyl compounds with different ligands and counterions after the photo-excitation, and to compute with a high accuracy the vibronic progression in order to guide the interpretation of experimental results. In particular we focused our efforts on characterizing the influence of the organic or inorganic closest chemical environment of the uranium(VI)-based complexes. We studied 1) the influence of the extracting agent such as Aliquate 336 and solvent effect on uranyl tetrahalides; 2) inorganic Ca²⁺ and Mg²⁺ counterions on uranyl triscarbonates; and 3) monoamide ligands (di-2-ethylhexyl-isobutyramide) on uranyl binitrate complexes. Their electronic structures and main spectroscopic properties have been estimated by both TRLFS and ab initio techniques. The theoretical approach enabled to calculate the main luminescence emissions of the complexes with the corresponding assignment of the electronic transitions and vibronic modes involved. For all the studied complexes, a good agreement between theory and experiment was found, allowing to build a full picture about the capabilities of the methods.