Comprehensively understanding the steric hindrance effect on the coordination sphere of Pb2+ ions and photophysical nature of two luminescent Pb(ii)-coordination polymers.

New luminescent Pb(ii)-coordination polymers (Pb(ii)-CPs), [Pb(2-anc)2]n (1) and [Pb(3-qlc)2(H2O)2]n (2) (2-anc = 2-aminonicotinate, 3-qlc = 3-quinolinecarboxylate), were synthesized by a solvothermal reaction and characterized using microanalysis, IR spectrometry and thermogravimetric analysis. Single crystal structure analysis revealed that the Pb2+ ion displays a hemi-directed coordination geometry in 1 and a holo-directed coordination geometry in 2. The difference between the coordination spheres of 1 and 2 is related to the steric hindrance effect of the ligands. A two-dimensional (2D) corrugated layer structure is formed in 1, and the neighboring layers are further extended into a three-dimensional (3D) van der Waals crystal by ππ interactions between the pyridyl rings in the neighboring layers. In contrast, a one-dimensional (1D) coordination polymeric chain is formed in 2, and the adjacent chains are connected in a 2D network by hydrogen bonds. The photophysical properties of 1 and 2 were studied at ambient conditions, disclosing that 1 emits phosphorescence at 548 nm with a millisecond-scale emission lifetime and an absolute quantum yield of 2.2%; 2 emits only blue fluorescence with an absolute quantum yield of 3.5% and nanosecond-scale emission lifetime. The reasons for the difference in the photophysical nature of 1 and 2 are discussed in regards to the electron band structure and density of states analysis.

Crystal Structures, Photoluminescence, and Magnetism of Two Novel Transition-Metal Complex Cocrystals with Three-Dimensional H-Bonding Organic Framework or Alternating Noncovalent Anionic and Cationic Layers.

Cocrystallization may alter material physicochemical properties; thus, the strategy of forming a cocrystal is generally used to improve the material performance for practical applications. In this study, two transition-metal complex cocrystals [Zn(bpy)3]H0.5BDC·H1.5BDC·0.5bpy·3H2O (1) and [Cu2(BDC)(bpy)4]BDC·bpy·2H2O (2) have been achieved using a hydrothermal reaction, where bpy and H2BDC represent 2,2'-bipyridine and benzene-1,3-dicarboxylic acid, respectively. Cocrystals were characterized by microanalysis, infrared spectroscopy, and UV-visible spectroscopy. Cocrystal 1 contains five components and crystallizes in a monoclinic space group P21/n. The H0.5BDC1.5-, H1.5BDC0.5-, and H2O molecules construct three-dimensional H-bonding organic framework; the [Zn(bpy)3]2+ coordination cations and uncoordinated bpy molecules reside in channels, where two coordinated bpy ligands in [Zn(bpy)3]2+ and one uncoordinated bpy adopt sandwich-type alignment via π···π stacking interactions. Cocrystal 2 with four components crystallizes in a triclinic space group P-1 to form alternating layers; the binuclear [Cu2(bpy)4(BDC)]2+ cations and uncoordinated bpy molecules build the cationic layers, and the BDC2- species with disordered lattice water molecules form the anionic layers. Cocrystal 1 shows intense photoluminescence at an ambient condition with a quantum yield of 14.96% and decay time of 0.48 ns, attributed to the π* → π electron transition within phenyl/pyridyl rings, and 2 exhibits magnetic behavior of an almost isolated spin system with rather weak antiferromagnetic coupling in the [Cu2(bpy)4(BDC)]2+ cation.