Structure and Photoluminescent Properties of Lanthanide Coordination Polymers Formed by the Interweaving of Bis(triple-stranded) Helical Chains



Seven novel lanthanide coordination polymers (CPs) with the general formula [Ln2(cpp)3(H2O)·(H2O)]n [Ln = Pr (cpp-Pr), Nd (cpp-Nd), Sm (cpp-Sm), Eu (cpp-Eu), Tb (cpp-Tb), Er (cpp-Er), and Yb (cpp-Yb)] have been synthesized by hydrothermal reactions of the corresponding lanthanide nitrates and the ligand 3-(4-chlorophenyl)pentanedioic acid (H2cpp). All of the lanthanide CPs were structurally characterized by elemental analysis, FTIR spectroscopy, TG analysis, single-crystal X-ray diffraction, and X-ray powder diffraction (XRPD), and their photoluminescent properties were also investigated. X-ray crystallographic analysis revealed that all of the lanthanide CPs are isostructural. The high coordination numbers and flexible coordination modes of the lanthanide ions, together with the flexible nature of the ligand H2cpp and the steric hindrance effects of the bulky chlorophenyl groups, bestow on the novel lanthanide CPs interesting structural features. Their overall structures can be described as 2D tubular meso-helices, which result from the interweaving of bis(triple-stranded) helical chains. As a result of efficient energy transfer from the ligands to metal ions, cpp-Eu and cpp-Tb are highly luminescent and exhibit intense emission bands in the visible region characteristic of the corresponding lanthanide ions. The solid-state quantum yields and average lifetimes were determined to be 20 ± 3 % and τa(5D0) = 0.43 ± 0.02 ms for cpp-Eu and 39 ± 3 % and τa(5D4) = 0.85 ± 0.01 ms for cpp-Tb. In contrast, cpp-Pr, cpp-Nd, cpp-Sm, cpp-Er, and cpp-Yb all display similar emission spectra originating from the ligand-centered (LC) or ligand-to-ligand charge-transfer (LLCT) excited state in the visible region. In addition, the near-infrared (NIR) emission behavior of cpp-Nd, cpp-Er, and cpp-Yb is also reported.