Construction of a series of lanthanide metal–organic frameworks: synthesis, structure, luminescence and white light emission

Abstract

A series of new lanthanide metal–organic frameworks (Ln-MOFs) [Ln₂(L)₃(H₂O)₄]·2H₂O (1) (Ln = Eu, Tb, Gd), and [Gd₂(H₂O)₄(L)(C₂O₄)₂]·3H₂O (2) were synthesized based on 4-hydroxypyridine-2,6-dicarboxylic acid (H₂L) under hydrothermal conditions. 1 exhibits isomorphous 2D wave-like networks constructed by paddle-wheel [Ln(L)₃] units and LnO₄(H₂O)₄ polyhedra. 2 shows a (4,4)-connected 3D framework with a Schlafli symbol of 6⁶. Oxalic acid in the resulting framework of 2 is formed in situ under hydrothermal conditions. Luminescence studies indicate that 1-Eu and 1-Tb show characteristic red and green emissions of the corresponding Ln³⁺ ions, respectively, while 1-Gd and 2 exhibit blue emission arising from the H₂L ligand. Then, by adjusting the co-doping ratio of three different Ln³⁺ ions into the same framework as that of 1, a novel doped Ln-MOF, [(Eu_0.0073Tb_0.0007Gd_0.992)₂(H₂O)₄(L)₃]·2H₂O (3), is successfully designed and synthesized, which shows white light emission upon excitation at 340 nm and its emission can also be switched between different colors. In addition, 1-Gd shows weak ferromagnetic interactions.