Four isostructural lanthanide metal–organic frameworks: luminescence properties and fluorescence sensing for Fe3+ and Cr2O72− ions

Abstract

Four novel isostructural lanthanide metal–organic frameworks (Ln-MOFs), [Ln₂(L)₃(DMF)₂(H₂O)₄]·2DMF (Ln = Eu(1), Tb(2), Gd(3) and Y(4); H₂L = 4,4′-([2,4′-bipyridine]-4,6-diyl)dibenzoic acid; DMF = N,N-dimethylformamide), were solvothermally synthesized. These Ln-MOFs were characterized by IR spectroscopy, thermogravimetry, and powder X-ray diffraction methods. The single-crystal analysis reveals that in 1, three Eu³⁺ centres are linked together by two bidentate bridging carboxylate groups from two L²⁻ ligands to form trinuclear secondary building units, which further construct a three-dimensional framework. These Ln-MOFs exhibit selective sensing of Fe³⁺ ions and Cr₂O₇²⁻ anions with better quenching constant values and lower detection limits. According to the tests, 3 and 4 show higher sensitivity and selectivity towards Fe³⁺ ions, but 2 reveals lower sensitivity and selectivity towards Fe³⁺ ions. Moreover, 1–4 display higher sensitivity and selectivity towards Cr₂O₇²⁻ anions. Notably, 3 is a rare Gd-MOF sensor for Fe³⁺ ions, and 4 is a rare Y-MOF sensor for Cr₂O₇²⁻ anions. Besides, the four Ln-MOFs can remain stable during the tests. The fluorescence quenching can be attributed to the competitive light absorption and the energy transformation between Ln-MOFs and Fe³⁺/Cr₂O₇²⁻ ions. Furthermore, the luminescence properties of Ln-MOFs have also been investigated.