A flexible Eu(iii)-based metal-organic framework: Turn-off luminescent sensor for the detection of Fe(iii) and picric acid

A metal-organic framework (MOF) {[Eu₂(MFDA)₂(HCOO) ₂(H₂O)₆]·H₂O}_n (1) (H₂MFDA = 9,9-dimethylfluorene-2,7-dicarboxylic acid) has been solvothermally synthesized and structurally characterized. 1 possesses the three-dimensional pcu type rod-packing structure with one-dimensional rhombic channels. The framework of 1 can reversibly shrink/swell along the c axis upon partial/full release of the water molecules. Correspondingly, the rhombic channels become narrow/large and 1 transforms to narrow-pore 1a/large-pore 1b. 1, 1a and 1b have almost the same excitation and emission spectra with the strong characteristic red-light-emission of Eu(iii). A high photoluminescence quantum yield of 77% and long luminescence lifetime of around 1.1 ms was observed for 1. The potential of 1b for Fe³⁺ ions and PA sensing was studied in DMF through the luminescence quenching experiments, which show 1b is a potential turn-off luminescent sensory material for the selective detection of Fe³⁺ ions and PA with detection limits of around 10^-7 M for both of them. The fluorescence quenching mechanism for Fe³⁺ ions and PA was also investigated.