Lanthanide metal-organic frameworks assembled from a fluorene-based ligand: Selective sensing of Pb²⁺ and Fe³⁺ ions

Fast and selective detection of heavy metal ions in the aqueous phase plays a key role in meeting human health and environmental concerns. Herein we report a series of fluorene-based lanthanide metal-organic frameworks ([Ln₂(FDC)₃DMA(H₂O)₃]·DMA·4.5H₂O, Ln = Sm (1), Eu (2), Gd (3) and Tb (4), H₂FDC = 9,9-dimethyl-2,7-fluorenedicarboxylic acid, DMA = dimethylacetamide). Single crystal X-ray diffraction reveals that 1-4 are isostructural and display a 3D neutral framework. 2 exhibits intense characteristic red emission of Eu³⁺ ions in the solid state and high selectivity for Pb²⁺ and Fe³⁺ ions through fluorescence enhancement and the quenching effect in aqueous solutions, respectively. Interestingly, the fluorescence intensity of 2 shows a good linear relationship with Pb²⁺ concentration in the range of 0.02-0.1 mM. Furthermore, the dynamic and static quenching constants are calculated to be 320 M^-1 and 10 680 M^-1 by the fluorescence lifetime and titration experiments in low concentration of Fe³⁺. In addition, 4 exhibits different fluorescence response behaviors in the presence of Sm³⁺ or Eu³⁺ in aqueous solution.