Rational design, crystal structures and sensing properties of a series of luminescent MOFs based on a flexible tetracarboxylate ligand and N-donor ligands

Six Zn/Cd metal-organic frameworks (MOFs), {[Zn-2(dppa)(bpy)(2)(H2O)]center dot H2O}(n) (1), {[Zn-2(dppa)(phen)(2)(H2O)]center dot 3H(2)O}(n) (2), [Cd(H(2)dppa)(bpy)](n) (3), {[Cd-2(dppa)(dye)]center dot H2O}(n) (4), {[Cd-4(dppa)(2)(H2O)(9)]center dot H2O}(n) (5) and {[Cd-2(dppa)(bpe)(2)(H2O)]center dot H2O}(n) (6) (H(4)dppa = 5-(3'4'-dicarboxylphenoxy)isophthalic acid, phen = 1,10-phenanthroline, bpy = 2,2 '-bipyridine, dye = 2,2'-dipyridylamine, bpe = 1,2-bis(4-pyridy) ethane), were synthesized and structurally characterized by elemental analyses, plasma (ICP) spectrometry, IR spectroscopy, and single-crystal X-ray diffraction analyses. 1-3 are 1D ring chains. 4 shows a 2D bilayer network based on a tetranuclear cadmium cluster linked by (dppa)(4-) ligands. 5 displays a 2D bilayer network based on a trinuclear cadmium cluster, in which (dppa)(4-) ion shows two different coordination modes. In contrast to the above MOFs, 6 possesses a 3D network structure based on dinuclear cadmium units cross-linked by (dppa)(4-) and bpe ligands. For 1-6, weak hydrogen bonding and pi center dot center dot center dot pi stacking contacts link the discrete 1D chains or 2D networks to form high-dimensional supramolecular structures. Especially, in 1-6, (dppa)(4-) ligand displays seven different coordination modes and conformations, which may show natural synergy in the structural diversity of the resulting MOFs. The remarkable characteristic of these frameworks is that 2 demonstrates highly selective and sensitive bifunctional luminescent sensing towards acetone and Fe3+ ion. In addition, the thermal stabilities and luminescence properties of the MOFs were studied.