Construction of Zn-II/Cd-II-based MOFs containing a tripodal aromatic carboxylate ligand with unequal arms and their fluorescence detection for Cu2+ and Fe3+ cations

Four MOFs with mixed ligands, [Zn-3(3-BABA)(2)(bpa)(2)(H2O)(2)](n) (1), [Cd-3(3-BABA)(2)(bpa)(2)](n) (2), [Cd-3(3-BABA)(2)(bpe)(2)](n) (3), and {[Cd-3(3-BABA)(2)(bib)(DMA)(2)]& BULL;H2O}(n) (4) (where 3-H(3)BABA = 4' ,4''' -(((2-carboxyphenyl)azanediyl)bis(methylene))bis(([1,1' -biphenyl]-2-carboxylic acid)), bpa = 4,4' -azopyridine, bpe = 1,2-bis(pyridin-4-yl)ethane, and bib = 1,4-bis(imidazole)butane), have been synthesized. They are characterized by single crystal X-ray diffraction, X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy. In 1, two 3-BABA and two bpa bridge three Zn2+ ions to form a bicyclic [Zn-3(3-BABA)(2)(bpa)(2)] unit which is further bridged by carboxylate groups with its adjacent units, resulting in a one-dimensional chain. In 2, 3 and 4, the [Cd-3(CO2)(6)] units are bridged by six 3-BABA ligands into a three-dimensional structure. However, the adjacent [Cd-3(CO2)(6)] units in 2 and 3 are doubly-bridged simultaneously by corresponding pyridyl ligands, while the adjacent [Cd-3(CO2)(6)] units in 4 are singly-bridged simultaneously by corresponding pyridyl ligands. The luminescence sensing investigation indicates that compound 1 exhibits extremely sensitive and selective detection of Cu2+ with a detection limit of 0.363 mu M, and compounds 2-4 show extremely sensitive and selective detection of Fe3+ with detection limits of 0.539 mu M, 0.265 mu M, and 0.748 mu M, respectively. Competitive experiments in the presence of interfering cations with 1-4 revealed no major change in the quenching efficiency.