Structural Variations and Luminescence of UO22+ Hybrid Materials Containing N-donor Chelating Ligands and/or O-donor Assembly Linkers

• Main Author: Thangavelu, Sonia G.
• Language: EN
• Published: The George Washington University 2015
• Subjects: Inorganic chemistry
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3717926

<p> This dissertation is based on the hydro(solvo)thermal syntheses and characterization of uranyl (UO₂²⁺) coordination polymers (CPs) via single-crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), and luminescence spectroscopy. The rich structural portfolio of uranyl CPs arises from the UO₂²⁺ cation&rsquo;s tendency to undergo hydrolysis, and form uranyl oligomeric species (or secondary building units, SBUs). Because of hydrolysis, synthetic control of SBUs is difficult and more often then not, their solid-state crystallization is random and unpredictable. Thus, it is challenging to know what building unit will be observed in a uranyl CP. </p><p> Our strategy to address such challenges and potentially thwart hydrolysis is to use N-donor chelating ligands. These ligands offer potential chelating sites that may allow for direct coordination to UO₂²⁺ and thus essentially promote specific uranyl building units. The N-donor ligands chosen in our study are 2,2&rsquo;:6,2&rdquo;-terpyridine (TPY), analogs of 2,2&rsquo;-bipyridine (BPY), 2,4,6-tripyridyl-s-triazine (TPTZ), and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ). By restricting UO₂²⁺ speciation, assembly of aromatic or aliphatic O-donor linkers to available coordination sites on UO₂²⁺ allowed us to synthesize a series of uranyl CPs containing N-donor (TPY, TPTZ, BPY analogs) and O-donor co-ligands. These coordination polymers resulted in extended structures with unique structural topologies and luminescent features. </p><p> Depending on the choice of N- and O-donors, structural variations in the local UO₂²⁺ coordination sphere and global structure within a uranyl CP were observed. N-donor chelating ligands were also explored as guest molecules, in which a series of CPs containing TPTZ, BPY analogs, or TPPZ and different O-donor aliphatic or aromatic linkers were synthesized. These guests were found to stabilize the structure through non-covalent interactions or participate as charge balancing species. Beyond structural manipulation of our materials, we also studied UO₂²⁺ luminescence and lifetimes within our uranyl CPs. We observe that modifications on either the N- or O-donor (i.e. sterics, functional groups, and/or non-covalent interactions) or a change in the local and global structure of a CP influences UO₂²⁺ luminescence thus resulting in unique spectral signatures. </p><p> Given the influence of N-donors and O-donors on the structure and luminescence of an uranyl CP, we also explored the synthesis of uranyl complexes using N-donor BPY, 5,5&rsquo;-dimethyl-2,2&rsquo;-bipyridine (MeBPY), and TPY ligands exclusively via self assembly conditions in the presence of sunlight and ambient light. Unexpectedly, we observe the presence of peroxo ligands in our crystal structures. To explore the origin of the peroxo ligand, rigorous synthetic experiments were performed in which the presence of peroxo most likely arises from a mechanism consistent to photo-excitation of UO₂²⁺.</p>