Two new dysprosium-organic frameworks contaning rigid dicarboxylate ligands: Synthesis and effect of solvents on the luminescent properties

Two new two-dimensional (2D) dysprosium coordination polymers [Dy(2,4'-bpdc)(DMF)(2)(NO3)](n) (1) and {[Dy(2,4'-bpdc)(1,4-BDC)(0.5)(DMF)(H2O)]center dot 1.5H(2)O}(n) (2) (2,4'-H(2)bpdc=2,4'-biphenyldicarboxylic acid, 1,4-H2BDC=1,4-benzenedicarboxylic acid, DMF=N,N'-dimethylformamide) were synthesized under solvothermal condition and stucturally characterized by means of single-crystal X-ray diffraction, IR spectroscopy, and elemental analysis. Single-crystal X-ray analysis revealed that the two coordination polymers possess two types of 2D layered structures. From the viewpoint of network topology, the structures of I and 2 can be simplified as (4,4) network. We discuss the effect of solvents and temperature on luminescence properties. The fluorescence spectra of 1 at room temperature and 77 K in the solid-state are almost the same, except the stronger emission intensities derived from ligand-centered at 77 K. It is because the quenching by O-H oscillators was protected at low temperature. Coordination polymer 2 displays characteristic Dy3+ ion yellow-green luminescence under 290 nm excitation in DMSO (dimethyl sulfoxide), CH3CN, and CH3OH solvents. The fluorescence intensities of 2 increased in the order of DMSO > CH3CN > CH3OH. We also studied the fluorescence lifetimes of 1 and 2, and the results revealed that the lifetime in DMSO solvent at room temperature reached to 9.53 mu s. Compared with the dysprosium coordination polymers, coordination polymer 2 presents a longer lifetime. Additionally, we calculate the triplet state T-1 datum from the emission spectrum of the Gd3+ coordination polymer and discuss the energy transfer mechanisms. The energy transfer process from the lowest triplet state energy level of 2,4'-H(2)bpdc ligand to the F-4(9/2) state energy level of Dy3+ ion is inefficient for both 1 and 2. The energy transfer process is effective after we introducing the chelating ligand 1,4-H2BDC to the construct of coordination polymer 2, which is ascribe to 1,4-H2BDC may participate in the process of energy transfer. (C) 2013 Elsevier B.V. All rights reserved.