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 1 and 2 feature 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 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 Kα X-ray radiation (8.04 keV) and are candidate materials for applications in solid-state scintillation.