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)(H₂btc)₂]_n (bipy=2,2′-bipyridine, H₃btc=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(ΙΙ) 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 π-π interactions. Moreover, 1 emits fluorescence at 350.1 nm (λ_ex = 310.0 nm) and 2 exhibits fluorescent property at about 351.4 nm (λ_ex = 302.0 nm).