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

Abstract

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.