Multifunctional Two-Dimensional Metal–Organic Frameworks for Radionuclide Sequestration and Detection

Two lanthanide-containing porous coordination polymers, [Ln2(bpdc)6(phen)2]·nH2O (1) and [Ln2(bpdc)6(terpy)2]·3H2O (2) (Ln = Pr, Nd, or Sm–Dy; bpdc: 2,2′-bipyridine-5,5′-dicarboxylic acid; phen: 1,10-phenanthroline; and terpy: 2,2′:6′,2″-terpyridine), have been hydrothermally synthesized and structurally characterized by powder and single-crystal X-ray diffraction. Crystallographic analyses reveal that compounds 1and 2feature Ln3+-containing dimeric nodes that form a porous two-dimensional (2D) and nonporous three-dimensional (3D) framework, respectively. Each material is stable in aqueous media between pH 3 and 10 and exhibits modest thermal stability up to ∼400 °C. Notably, a portion of the phen and bpdc ligands in 1can be removed thermally, without compromising the crystal structure, causing the surface area and pore volume to increase. The optical properties of 1and 2with Gd3+, Sm3+, Tb3+, and Eu3+are explored in the solid state using absorbance, fluorescence, and lifetime spectroscopies. The analyses reveal a complex blend of metal and ligand emission in the materials containing Sm3+and Tb3+, while those featuring Eu3+are dominated by intense metal-based emission. Compound 1with Eu3+shows promise for the capture and detection of the uranyl cation (UO2)2+from aqueous media. In short, uranyl capture is observed at pH 4, and the adsorption thereof is detectable via vibrational and fluorescence spectroscopies and colorimetrically as the off-white color of 1turns yellow with uptake. Finally, both 1and 2with Eu3+produce bright red emission upon irradiation with Cu Kα X-ray radiation (8.04 keV) and are candidate materials for applications in solid-state scintillation.