Design of 3-D europium(iii)–organic frameworks based on pyridinecarboxylate N-oxide and acyclic binary carboxylate: syntheses, structures, and luminescence properties

Abstract

Four europium(III) complexes of 3-D open frameworks [Eu₂(NNO)₄(ox)(H₂O)₂]_n (1), [Eu₂(INO)₂(ox)₂(H₂O)₂]_n (2), {[Eu₂(INO)₂(suc)₂]·2.99H₂O}_n (3) and {[Eu₂(suc)₃(H₂O)₂]·H₂O}_n (4) (HNNO = nicotinic acid N-oxide, H₂ox = oxalic acid, HINO = isonicotinic acid, H₂suc = succinic acid) have been synthesized and characterized by IR, elemental analysis, TG/DTA and single-crystal X-ray diffraction analysis. Each of the four complexes has 1-D channels, and in 3 and 4 these are filled with solvent water molecules, making them bigger than in 1 and 2, where no lattice water molecules can reside. The formation of the four 3-D europium(III) frameworks of different molecular and crystal structures reflects the influence of the ligands’ coordination modes, the length of the acyclic binary carboxylates, the ligand conformation as well as the molar ratio of reactants. At room temperature, all the complexes in the solid state exhibit typical red luminescence from Eu³⁺ ions, indicating effective energy transfer from the ligand to Eu³⁺ ion; however, their luminescence intensities are clearly different. The luminescence of 3 is the most intense due to the absence of coordinated water, while the complexes 1, 2 and 4 exhibit weaker luminescence due to oscillation of coordinated water molecules, which partially quenches their luminescence.