Construction of ZnII/CdII-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, [Zn3(3-BABA)2(bpa)2(H2O)2]n (1), [Cd3(3-BABA)2(bpa)2]n (2), [Cd3(3-BABA)2(bpe)2]n (3), and {[Cd3(3-BABA)2(bib)(DMA)2]·H2O}n (4) (where 3-H3BABA = 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 [Zn3(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 [Cd3(CO2)6] units are bridged by six 3-BABA ligands into a three-dimensional structure. However, the adjacent [Cd3(CO2)6] units in 2 and 3 are doubly-bridged simultaneously by corresponding pyridyl ligands, while the adjacent [Cd3(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 μM, and compounds 2–4 show extremely sensitive and selective detection of Fe3+ with detection limits of 0.539 μM, 0.265 μM, and 0.748 μM, respectively. Competitive experiments in the presence of interfering cations with 1–4 revealed no major change in the quenching efficiency.