One-dimensional Zn(II) oligo(phenyleneethynylene)dicarboxylate coordination polymers: Synthesis, crystal structures, thermal and photoluminescent properties

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The rigid, π-conjugated dicarboxylic acid 1,4-bis-[2-(4-carboxyphenyl)ethynyl]-2,5-dihexylbenzene {HO 2C[PEP(hexyl) 2EP]CO 2H} has been used to synthesise the new crystalline coordination polymers {Zn(O 2C[PEP(hexyl) 2EP]CO 2)(DMF) 2} ( 1) and {Zn(O 2C[PEP(hexyl) 2EP]CO 2)(DEF) 2} ( 2) in N,N-dimethylformamide (DMF) and N,N-diethylformamide (DEF), respectively, under mild conditions. Single-crystal X-ray crystallography revealed that 1 and 2 are isostructural and consist of uncharged zigzag coordination chains in which [Zn(formamide) 2] 2+ fragments are bridged by (O 2C[PEP(hexyl) 2EP]CO 2) 2− ligands. The zigzag chains possess different intra-chain Zn⋯Zn⋯Zn angles due to the different volumes of the coordinating formamide molecules and subtle differences in the hydrophobic inter-chain interactions. Upon heating 1 and 2 to 200 °C, removal of the coordinating formamide molecules occurs, yielding the formamide-free compounds 1-DMF and 2-DEF of composition {Zn(O 2C[PEP(hexyl) 2EP]CO 2)}. According to powder X-ray diffraction and FT-IR spectroscopy studies, these materials are not crystalline but still possess partial ordering of intact, yet modified coordination chains in a structural arrangement which appears to be related to the respective parent compounds. Compounds 1, 2, 1-DMF and 2-DEF exhibit blue photoluminescence. The emission maxima of 1-DMF and 2-DEF are red-shifted by ca. 25 nm with respect to λ max of 1 and 2, respectively.