Synthesis, Structure, and Luminescent Properties of Lanthanide-Based Two-Dimensional and Three-Dimensional Metal–Organic Frameworks with 2,4′-Biphenyldicarboxylic Acid

A series of novel lanthanide coordination polymers have been successfully synthesized under hydrothermal or solvothermal conditions, namely, [Eu2(2,4′-bpdc)2(OH)­(NO3)­(H2O)3]n (1), [Ln­(2,4′-bpdc)­(1,4-bdc)0.5(DMF)-(H2O)·H2O]n [Ln = Eu (2); Ln = Pr (3); Ln = Nd (4)], [Ln­(2,4′-bpdc)­(DMF)2NO3]n [Ln = Eu (5); Ln = Nd (6); Ln = Gd (7)], and [Eu­(2,4′-bpdc)­(2,4′-Hbpdc)­(H2O)]n (8), (2,4′-H2bpdc = 2,4′-biphenyldicarboxylic acid, 1,4-H2bdc = 1,4-benzenedicarboxylic acid, and DMF = N,N′-dimethylformamide). X-ray crystallographic studies reveal that 1 features a three-dimensional (3D) framework with 45 topology. 2–4, which are isostructural and synthesized by using a mixture of 2,4′-H2bpdc and 1,4-H2bdc, exhibit two-dimensional (2D) (3,4)-connected net structures. 5–7 show 2D layered structures containing double helix chains (Δ and Λ) with 44 topology. 8 possesses a 2D layered structure containing (4,6)-connected networks, which are connected by means of infinite zigzag chains linked by 2,4′-bpdc2– and 2,4′-Hbpdc– anions. Furthermore, steady-state and time-resolved spectroscopic measurements reveal that the four Eu­(III) complexes, 1, 2, 5, and 8, exhibit remarkable red luminescence emissions with lifetimes at the millisecond scale. Complexes 4 and 6 display characteristic luminescent properties for Nd­(III) in the near-infrared region. The singlet state and the lowest triplet level of ligand 2,4′-H2bpdc were calculated on the basis of the UV–vis absorbance edges of ligand and the phosphorescence spectrum of Gd­(III) complex 7. The energy transfer mechanisms in the lanthanide polymers are described and discussed.