Crystal Structures and Effect of Temperature on the Luminescence of Two Lanthanide Coordination Polymers with Twofold Interpenetrating pcu Topology

Two new lanthanide coordination polymers formulated as [Ln(BDC)1.5(DMF)(H2O)]n [Ln = Tb (1), Gd (2)] [H2BDC = 1,4-benzenedicarboxylic acid, DMF = N,N′-dimethylformamide] were synthesized under solvothermal conditions. The coordination polymers were characterized by IR spectroscopy, elemental analysis and X-ray single-crystal diffraction. The two coordination polymers are isostructural and exhibit twofold interpenetrating pcu three-dimensional open frameworks constructed by tetradentate ligand of H2BDC. The fluorescence properties of 1 and 2 were investigated at 298 or 77 K both in solid state and in CH2Cl2 solvent dispersed as suspensions. Coordination polymer 1 exhibits characteristic Tb3+ ions emission transitions of 5D4 → 7FJ (J = 6–2) at 77 or 298 K both in the solid-state and in CH2Cl2 solvent. Moreover, the vibrational structure of 1 is more defined at 77 K, which exhibits another two 5D4 → 7F1 and 5D4 → 7F2 transitions more than that at 298 K. The lifetimes of 1 are longer at 77 K (915.2 μs in the solid state and 874.11 μs in the CH2Cl2 solvent) than that at 298 K (866.31 μs in the solid state and 801.04 μs in the CH2Cl2 solvent), which may be caused by the increase of radiative rate and decrease of non-radiative rate at low temperature. The singlet excited state (28,653 cm−1) and the lowest triplet energy level (23,641 cm−1) of H2BDC ligand were calculated based on the UV–Vis absorbance edges of ligand and the phosphorescence spectrum of Gd3+ coordination polymer (2) at 77 K, showing that the effective extent of energy transfer from H2BDC ligand to Tb3+ ion. Finally, thermal behaviors of the two coordination polymers were studied by thermogravimetric analysis, which exhibit thermal stability to 250 °C.