Three new Zn^II coordination polymers constructed from a semi-rigid tricarboxylic acid: structural changes caused by...

Coordination polymers (CPs) with specific structures and functional luminescence have been widely designed as sensors for detecting small molecules and ions. In this study, with or without the help of an N-donor auxiliary linker, three new Zn^II CPs, namely, three-dimensional (3D) poly[[pentaaquabis[μ₃-5-(4-carboxybenzyloxy)isophthalato]bis[μ₆-5-(4-carboxylatobenzyloxy)isophthalato]di-μ₃-hydroxido-hexazinc(II)] trihydrate], {[Zn₆(C₁₆H₁₀O₇)₂(C₁₆H₉O₇)₂(OH)₂(H₂O)₅]·3H₂O}_n or {[Zn₆(μ₃-HL)₂(μ₆-L)₂(μ₃-OH)₂(H₂O)₅]·3H₂O}_n, (I), one-dimensional (1D) catena-poly[[[aqua(1,10-phenanthroline)zinc(II)]-μ₂-5-(4-carboxybenzyloxy)isophthalato] dihydrate], {[Zn(C₁₆H₁₀O₇)(C₁₂H₈N₂)(H₂O)]·2H₂O}_n or {[Zn(μ₂-HL)(phen)(H₂O)]·2H₂O}_n (phen is 1,10-phenanthroline), (II), and 3D poly[diaquatetrakis(4,4'-bipyridine)bis[μ₆-5-(4-carboxylatobenzyloxy)isophthalato]di-μ₃-formato-di-μ₃-hydroxido-pentazinc(II)], [Zn₅(C₁₆H₉O₇)₂(HCOO)₂(OH)₂(C₁₀H₈N₂)₄(H₂O)₂]_n or [Zn₅(μ₄-L)₂(bpy)₄(μ₂-OH)₂(μ₃-HCOO)₂(H₂O)₂]_n (bpy is 4,4'-bipyridine), (III), have been constructed from the semi-rigid tricarboxylic acid 5-(4-carboxybenzyloxy)isophthalic acid (H₃L) under hydrothermal conditions. CP (I) exhibits a twofold interpenetrated 3D+3D→3D skeleton with a 3,5-conn topology constructed from triangular trinuclear [Zn₃(COO)₄(μ₃-OH)] clusters, in which the H₃L ligand adopts three different coordination modes. CP (II) exhibits a 1D infinite chain and stacking that gives a 3D structure mediated by hydrogen bonds and weak interactions. CP (III) is an interesting 3D 3,4,8-conn network including linear tetranuclear [Zn₄(μ₂-OH)₂(HCOO)₂(COO)₂] clusters with a new {4·6²}₂{4·6⁴·8}{4⁶·6¹⁹·8³} topological symbol. The influences of the flexible -CH₂-O- linker of the H₃L ligand and subtle environmental factors, such as solvent, pH value and auxiliary ligands, on the formation of the final structures are also discussed. The solid-state fluorescence spectra of CPs (I)-(III) were recorded at room temperature and all show better fluorescence performances than H₃L. In particular, (II) can act as a potential multifunctional fluorescent material for sensing hexavalent chromium ions in aqueous solution with high stability, selectivity and sensitivity. Under ultraviolet light of 365 nm from a UV lamp, a signal response of fluorescence from turning on to off can be observed with the naked eye. It was found that the detection for hexavalent chromium (i.e. Cr₂O₇^2-) by (II) has a high selectivity [K_SV = 1.61 × 10⁴ M^-1 and limit of detection (LOD) = 0.434 µM] in aqueous solution. Quenching mechanisms were also studied in detail.