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

Abstract

Six Zn/Cd metal–organic frameworks (MOFs), {[Zn₂(dppa)(bpy)₂(H₂O)]·H₂O}_n (1), {[Zn₂(dppa)(phen)₂(H₂O)]·3H₂O}_n (2), [Cd(H₂dppa)(bpy)]_n (3), {[Cd₂(dppa)(dye)]·H₂O}_n (4), {[Cd₄(dppa)₂(H₂O)₉]·H₂O}_n (5) and {[Cd₂(dppa)(bpe)₂(H₂O)]·H₂O}_n (6) (H₄dppa = 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 π⋯π 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.