System-pH-dependent supramolecular isomers of puckered three-dimensional layered hydrogen-bonded networks: Syntheses, characterization and fluorescent properties

Two true supramolecular isomers (1 and 2) formulated as [Zn(bipy)(H(2)btc)(2)](n) (bipy = 2,2-bipyridine, H(3)btc = benzene-1,3,5-tricarboxylic acid) have been hydrothermally prepared through systematically changing the pH value of reaction mixture, and characterized by single-crystal X-ray diffraction, element analysis, infrared spectra (IR), and differential scanning calorimetry (DSC). Isomer 1 crystallizes in triclinic space group P-1, tetrahedral Zn(II) ions possess four-coordinated environment, while isomer 2 crystallizes in monoclinic space group C2/c and the distorted octahedral Zn(II) ions adopt six-coordinated fashion. 1 and 2 can be regarded as supramolecular structural isomers, the formation of which should be undoubtedly attributed to the preference for pH-dependent crystallization leading to multiple connectivity (monodentate and chelating bidentate) of organic ligand despite the same stoichiometry employed. The supramolecular chemistry, which organizes the coordination complexes into three-dimensional (3D) layered open structure, is driven by a combination of hydrogen-bond and pi-pi interactions. Moreover, 1 emits fluorescence at 350.1 nm (lambda(ex) = 310.0 nm) and 2 exhibits fluorescent property at about 351.4 nm (lambda(ex) = 302.0 nm). (c) 2006 Elsevier Inc. All rights reserved.