Ionothermal syntheses, crystal structures and luminescence of three three-dimensional lanthanide-1,4-benzenedicarboxylate frameworks

Ionothermal reactions of LnCl3 (Ln = Sm(1), Eu(2) and Tb(3)) with 1,4-benzenedicarboxylic acid (H2BDC) in 1-ethyl-3-methylimidazolium bromide (EMIM-Br) ionic liquid (IL) afforded three compounds, namely, {(EMIM)2[Sm2(BDC)3(H2-BDC)Cl2]}n (1) and {(EMIM)[Ln2(μ2-Cl)(BDC)3]}n (Ln = Eu(2) and Tb(3)). These compounds consist of anionic three-dimensional frameworks and extraframework charge-balancing species of imidazolium [EMIM]+ cations. Compound 1 has a 8-connected (36·418·53·6) topology based on the dinuclear samarium(III) secondary building units. Compounds Eu(2) and Tb(3) are isomorphous and are constructed from the one-dimensional rod-shaped lanthanide-carboxylate secondary building units. Each lanthanide-carboxylate rod connects to six neighboring rods through the benzene rings of BDC2− ligands to form a parallel packing of hex type topology. It is noted that the chloride ion deriving from lanthanide salt as the co-ligand is incorporated into the structures under the ionothermal reactions. Detailed photoluminescence studies showed all compounds exhibit typical lanthanide luminescent emissions and the Eu(III) and Tb(III) luminescence was efficiently sensitized by the organic ligand. Additionally, the use of an IL solvent for preparation of the lanthanide compounds can effectively prevent the coordination of water and organic solvent molecules from lanthanide centers. Thus the luminescence quenching resulting from the coordinated solvent molecules is avoided.