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abstracts
Previous work from our group has focused on uranyl hybrid materials with para-substituted halogen benzoic acids.1 Applying a `cap and link' strategy we have successfully controlled the nuclearity of UO22+ in a series of coordination polymers and Ln3+ in a family of molecular materials.2,3 Here we have expanded on this strategy and produced a series of 12 unique uranyl hybrid materials. The hydrothermally synthesized complexes all feature bromo-substituted benzoic acid linkers and the UO22+ metal centers are capped with a chelating N-donor. Single crystal X-ray diffraction analyses of these materials allowed for the exploration of the structural relationship between the benzoic acid group and the chelating N-donor as well as the influence of pH on uranyl speciation. At an unadjusted pH (~3) a mix of molecular monomers and dimers are observed while at higher pH (5-6) molecular uranyl dimers are exclusively produced. A systematic study of the supramolecular interactions that governed extended solid-state assembly was investigated by varying the bromine position on the benzoic acid groups along with the chelating N-donor with pH. Assembly via halogen-halogen, halogen-π, π-π, and halogen-oxygen interactions was observed and the influence of these interactions on uranyl emission was also investigated. Solid-state luminescence and lifetime measurements were used to explore the effects of speciation and chelating N-donor ligands on the spectroscopic properties of the uranyl cation.
abstracts
Three dimensional lanthanide (Ln) framework compounds are renowned for their excellent photoluminescence properties, and there is growing interest in the development of this class of metal-organic framework (MOFs) materials for a diverse range of applications ranging from size-selective sensor technology to bio-imaging. Yet, although the physical and structural properties of Ln-MOFs under ambient conditions are well documented, there remains a distinct lack of information pertaining to the behaviour of these materials under non-ambient conditions. In this contribution we present both variable pressure (0-4 GPa) and temperature (100-300 K) single-crystal X-ray diffraction (XRD) studies of several Nd and Pr oxalate MOFs with different topologies (Fig. 1). Furthermore, these extensive XRD investigations have been complemented by variable pressure spectroscopic measurements that allow for evaluation of the influence of pressure on the photoluminescent emissions of these Ln-MOF compounds. This combined diffraction and spectroscopy study has enabled the structure-property relationships, which are so critical to the development of Ln-MOFs for practical usage, to be evaluated comprehensively. We will also show how the framework topology influences the structural behaviour of the Ln-MOF in response to pressure, resulting in the occurrence of unusual phenomena such as negative linear compression (NLC) in which one of the crystallographic axes expands, rather than contracts, with increasing pressure. Analysis of the high-pressure single-crystal XRD data has enabled the NLC mechanism to be elucidated and this will be presented.
