New Ln-MOFs based on mixed organic ligands: synthesis, structure and efficient luminescence sensing of the Hg ions in aqueous solutions.

In view of Hg ion sensing by luminescence, a series of new, phenanthroline-decorated 3D lanthanide metal organic frameworks (Ln-MOFs) valorising an original combination of four different lanthanides and two organic ligands, thiobis(4-methylene-benzoic acid) (Htmba) and 1,10-phenanthroline (phen), have been successfully synthesized, namely {[Ln(tmba)(phen)]·(HO)(phen)} [Ln = Ce, = 3 (1); Pr, = 1 (2); Eu, = 3 (3); and Tb, = 3 (4)]. Compounds 1-4 were characterised by single-crystal X-ray diffraction, elemental and thermogravimetric analyses, and powder X-ray diffraction. The luminescence properties of complexes 3 and 4 were thoroughly investigated. It is herein proved that compound 3 sensitively and selectively acts as an excellent luminescent probe for the detection of Hg ions in waters, with a detection limit of 1.00 μM. As additional assets, 3 displays superb stability over a wide pH range (3-12) of the aqueous media, as well as convenient recycling after completion of the detection experiments. The rationale for the observed luminescence quenching effect of mercury might be a strong interaction arising between Hg ions and the carboxylate oxygen atoms of the tmba ligand. The results open new perspectives for applications in environmental remediation.