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), [Ln2(BPDC)(BDC)2(H2O)2]n (Ln = Eu (1) and Tb (2)), have been successfully synthesized via a mixed ligand approach using 2,2'-bipyridine-3,3'-dicarboxylic acid (H2BPDC) and 1,4-benzenedicarboxylic acid (H2BDC) under hydrothermal conditions. Structural analysis shows that two lanthanide ions are 4-fold linked by two κ(1)-κ(1)-μ2 carboxylates from BDC(2-) and the other two κ(2)-κ(1)-μ2 carboxylates from BPDC(2-) to form a binuclear core. The binuclear units are further connected by BDC(2-) and BPDC(2-) to build a three-dimensional framework possessing tfz-d topology with the short (Schläfli) vertex symbol {4(3)}2{4(6)·6(18)·8(4)}. Moreover, isostructural doped Ln-MOFs [Eu(2x)Tb2(1-x)(BPDC)(BDC)2(H2O)2]n (x = 0.1 (1a), 0.3 (1b), 0.5 (1c), 0.7 (1d), and 0.9 (1e)) 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(3+) and Tb(3+) ions are simultaneously observed in 1a-e. Further luminescent studies reveal that 1, 2, and 1a 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.