Effective luminescence sensing of Fe3+, Cr2O72−, MnO4− and 4-nitrophenol by lanthanide metal–organic frameworks with a new topology type

Abstract

Lanthanide MOFs (Ln-MOFs), {[Ln₂(L)₂(H₂O)₂]·5H₂O·6DMAC}_n, [Ln^||| = Eu(1) and Tb(2); H₃L = 4,4′-(((5-carboxy-1,3-phenylene)bis(azanediyl))bis(carbonyl)) dibenzoic acid, DMAC = N,N′-dimethylacetamide], with a new topology type have been isolated. Single crystal X-ray diffraction indicates that complexes 1 and 2 are isostructural with binuclear [Eu₂(COO)₇]_n secondary building units as 7-connected nodes and H₃L ligands as 3-connected nodes and can be viewed as a (5,7)-connected 3D framework with a new topological point symbol of {3²·4⁴·5⁴} {3⁴·4⁶·5⁶·6⁵}. Complexes 1 and 2 exhibit an excellent luminescence sensing response to inorganic ions Fe³⁺, Cr₂O₇²⁻, MnO₄⁻ and 4-nitrophenol, with a low detection limit and high K_sv value. Interestingly, when the MnO₄⁻ ions are detected, the color of the solid sample is observed to change from yellow to brown, visually indicating luminescence induction, which makes the process of detecting MnO₄⁻ ions simpler and more practical. Moreover, by using time-resolved photoluminescence techniques, complex 1 can effectively eliminate background fluorescence interference during detection and improve detection accuracy. Solvent luminescence studies, pH stability and PXRD data indicate that complexes 1 and 2 can be used as excellent water-stable multi-response luminescent sensors for detecting a wide variety of toxic substances. In addition, the mechanism of selective detection is explained by the energy competition between the excitation of complexes 1 and 2 and the ultraviolet absorption of the responsive substance.