Thesis of Hanna Oher

Amphithéâtre Pierre Glorieux (CERLA) Thesis defense of Hanna Oher - laboratory Phlam Approches combinées ab initio et par spectroscopie de luminescence résolue en temps pour l'étude de l'interaction uranium-ligand Abstract : 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 uraniuml(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.