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.