Coordination Polymers Based on a Biphenyl Tetraphosphonate Linker: Synthesis Control and Photoluminescence

In this work, we used the rigid tetrapodal organic linker, [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid) (H₈btp), for the preparation of two lanthanide-organic framework families of compounds: layered [Ln₇(H₅btp)₄(H_5.5btp)₂(H₆btp)₂(H₂O)₁₂]∙23.5H₂O∙MeOH [where Ln³⁺ = Eu³⁺ (1Eu) and Gd³⁺ (1Gd)], prepared using microwave-irradiation followed by slow evaporation; 3D [Ln₄(H₃btp)(H₄btp)(H₅btp)(H₂O)₈]∙3H₂O [where Ln³⁺ = Ce³⁺ (2Ce), Pr³⁺ (2Pr), and Nd³⁺ (2Nd)], obtained from conventional hydro(solvo)thermal synthesis. It is shown that in this system, by carefully selecting the synthetic method and the metal centers, one can increase the dimensionality of the materials, also increasing structural robustness (particularly to the release of the various solvent molecules). Compound 1 is composed of 2D layers stacked on top of each other and maintained by weak π-π interactions, with each layer formed by discrete 1D organic cylinders stacked in a typical brick-wall-like fashion, with water molecules occupying the free space in-between cylinders. Compound 2, on the other hand, is a 3D structure with small channels filled with crystallization water molecules. A full solid-state characterization of 1 and 2 is presented (FT-IR spectroscopy, SEM microscopy, thermogravimetric studies, powder X-ray diffraction and thermodiffractometry). The photoluminescence of 1Eu was investigated.