Synthesis, structures and properties of two lead halide coordination polymers based on pyridinedicarboxylate ligand

Two new lead halide coordination polymers, [Pb2(pydc)I2]n (1), [Pb2(pydc)ClI]n (2), (H2pydc 1/4 2,5-pyridinedicarboxylic acid) have been synthesized under hydrothermal condition, and structurally characterized by single-crystal X-ray diffraction. The results of X-ray crystallographic analysis reveal that both compounds are three-dimensional (3D) frameworks. For 1, 1D inorganic cation [Pb2I2]n2n thorn ribbons are fused by the carboxylate oxygen atoms to generate 2D layers. Then the 2D layers are sustained by pydc2- ligands and result in a 3D pillared framework. Remarkably, a novel 2D mixed-halide cation [Pb2ClI]n2n thorn layer is found in 2. The mixed-halide cation layers are further connected by pydc2- ligands and create a 3D network. In addition, solid-state properties of thermal stability and fluorescence for these crystalline materials are also presented. Time-dependent density functional theory (TDDFT) calculations reveal that the mechanism of the fluorescence emissions dominantly results from the ligand-to-metal charge transfer (LMCT) and partial the intraligand pi -> pi* and/or n ->pi* transitions for 1 and 2, which is in good agreement with their fluorescence spectra and crystal structures.