Five new metal-organic frameworks, namely, [Cd(HIDC)(bix)] n (1), [Cd 2 (BDC) 2 (mbix)H 2 O] n (2), [Cd(HBTEC)-(H 2 bix) 0.5 ] n (3), [Cd(BTEC)(H 2 mbix)] n (4), and [Zn(BTEC) 0.5 (mbix)] n (5) (H 3 IDC ) imidazole 4,5-dicarboxylic acid, H 2 BDC )1,4-benzenedicarboxylate, H 4 BTEC ) 1,2,4,5-benzenetetracarboxylate, bix ) 1,4-bis(imidazol-1-ylmethyl)-benzene, and mbix ) 1,3-bis(imidazol-1-ylmethyl)-benzene), have been synthesized under hydrothermal conditions by employing mixed ligands of various carboxylic acids with bix (or mbix). Coordination polymer 1 is a 3-fold-interpenetrated diamondoid network. Particularly, in 1 there occur triple-stranded helices based on Cd ions and bix ligands when viewed along b axis. Coordination polymer 2 is a 2D infinite layer framework, which is further interconnected by hydrogen-bond interactions leading to a 3D supramolecular architecture. Both 3 and 4 feature a 3D open framework with protonated bix (or mbix) locating in the channels, and the 3D open frameworks can be rationalized as a zeolite ABW topology for 3 and pts net for 4. Coordination polymer 5 also has a 3D architecture, and the topological study shows that its framework features a 3D four-connected (6 4 8 2 )(6 6 ) net. The photoluminescent properties of the coordination polymers were investigated. All of these coordination polymers exhibited intense fluorescent emissions in the solid state at room temperature.

The detection of hypochlorite (ClO–) content in tap water is extremely important because excess amounts of hypochlorite can convert into highly toxic species and inadequate amounts of hypochlorite cannot fully kill bacteria and viruses. Although several metal–organic frameworks (MOFs) have been successfully employed as fluorescent sensors for hypochlorite detection, all these sensors are based on single emission that responds to the dose of hypochlorite. Ratiometric sensors are highly desirable, which can improve the sensitivity, accuracy, and reliability via self-calibration. Herein, a nanoscale dual-emission multivariate 5-5-Eu/BPyDC@MOF-253-NH2 was synthesized by sequential mixed-ligand self-assembly and postsynthesis method. Among the two emission bands of 5-5-Eu/BPyDC@MOF-253-NH2, the strong blue emitting derived from ligands is sensitive to hypochlorite, while the red emitting derived from Eu­(III) almost keeps invariable. Therefore, 5-5-Eu/BPyDC@MOF-253-NH2 was exploited as a fluorescent ratiometric nanosensor for “on–off” sensing of hypochlorite. Notably, the proposed sensing system showed an excellent performance including fast response (within 15 s), relative high specificity, wide linear range (0.1–30 μM), and low detection limit (0.094 μM). Besides, the suppressed blue emitting was recovered after the addition of ascorbic acid (AA) that consumes ClO– via the redox reaction. Therefore, 5-5-Eu/BPyDC@MOF-253-NH2 was further employed as a fluorescent ratiometric nanosensor for the “on–off–on” sensing of AA. This work represents the first MOF-based fluorescent “switch” for the ratiometric sensing of hypochlorite and the second for ratiometric sensing of AA.

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