Highly Selective Luminescent Sensing of Fluoride and Organic Small-Molecule Pollutants Based on Novel Lanthanide Metal–Organic Frameworks

Two novel isostructural lanthanide metal–organic frameworks (Ln-MOFs), [Ln<sub>2</sub>(BPDC)­(BDC)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub><i>n</i></sub> (Ln = Eu (<b>1</b>) and Tb (<b>2</b>)), have been successfully synthesized via a mixed ligand approach using 2,2′-bipyridine-3,3′-dicarboxylic acid (H<sub>2</sub>BPDC) and 1,4-benzenedicarboxylic acid (H<sub>2</sub>BDC) under hydrothermal conditions. Structural analysis shows that two lanthanide ions are 4-fold linked by two κ<sup>1</sup>-κ<sup>1</sup>-μ<sub>2</sub> carboxylates from BDC<sup>2–</sup> and the other two κ<sup>2</sup>-κ<sup>1</sup>-μ<sub>2</sub> carboxylates from BPDC<sup>2–</sup> to form a binuclear core. The binuclear units are further connected by BDC<sup>2–</sup> and BPDC<sup>2–</sup> to build a three-dimensional framework possessing <b>tfz-d</b> topology with the short (Schläfli) vertex symbol {4<sup>3</sup>}<sub>2</sub>{4<sup>6</sup>·6<sup>18</sup>·8<sup>4</sup>}. Moreover, isostructural doped Ln-MOFs [Eu<sub>2<i>x</i></sub>Tb<sub>2(1–<i>x</i>)</sub>(BPDC)­(BDC)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sub><i>n</i></sub> (<i>x</i> = 0.1 (<b>1a</b>), 0.3 (<b>1b</b>), 0.5 (<b>1c</b>), 0.7 (<b>1d</b>), and 0.9 (<b>1e</b>)) were also successfully synthesized. Thermal gravimetric analyses (TGA) reveal high thermal stability of these Ln-MOFs. Luminescent measurements indicate that the characteristic sharp emission bands of Eu<sup>3+</sup> and Tb<sup>3+</sup> ions are simultaneously observed in <b>1a</b>–<b>e</b>. Further luminescent studies reveal that <b>1</b>, <b>2</b>, and <b>1a</b> not only display a high-sensitivity sensing function with respect to fluoride but also exhibit significant solvent-dependent luminescent response to small-molecule pollutants, such as formaldehyde, acetonitrile, and acetone.