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

Two lanthanide-containing porous coordination polymers, [Ln(2) (b p d c)(6)(ph e n)(2)]center dot nH(2)O (1) and [Ln(2)(bpdc)(6)(terpy)(2)]center dot 3H(2)O (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 1 and 2 feature Ln(3+)-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 similar to 400 degrees C. Notably, a portion of the phen and bpdc ligands in 1 can be removed thermally, without compromising the crystal structure, causing the surface area and pore volume to increase. The optical properties of 1 and 2 with 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 1 with 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 1 turns yellow with uptake. Finally, both 1 and 2 with Eu3+ produce bright red emission upon irradiation with Cu Ka X-ray radiation (8.04 keV) and are candidate materials for applications in solid-state scintillation.