Luminescence tuning and white-light emission of co-doped Ln-Cd-organic frameworks

Four new three-dimensional isostructural lanthanide-cadmium metal-organic frameworks (Ln-Cd MOFs), [LnCd(2)(imdc)(2)(Ac)(H(2)O)(2) ]·H(2)O (Ln=Pr (1), Eu (2), Gd (3), and Tb (4); H(3)imdc=4,5-imidazoledicarboxylic acid; Ac=acetate), have been synthesized under hydrothermal conditions and characterized by IR, elemental analyses, inductively coupled plasma (ICP) analysis, and X-ray diffraction. Single-crystal X-ray diffraction shows that two Ln(III) ions are surrounded by four Cd(II) ions to form a heteronuclear building block. The blocks are further linked to form 3D Ln-Cd MOFs by the bridging imdc(3-) ligand. Furthermore, the left- and right-handed helices array alternatively in the lattice. Eu-Cd and Tb-Cd MOFs can emit characteristic red light with the Eu(III) ion and green light with the Tb(III) ion, respectively, while both Gd-Cd and Pr-Cd MOFs generate blue emission when they are excited. Different concentrations of Eu(3+) and Tb(3+) ions were co-doped into Gd-Cd/Pr-Cd MOFs, and tunable luminescence from yellow to white was achieved. White-light emission was obtained successfully by adjusting the excitation wavelength or the co-doping ratio of the co-doped Gd-Cd and Pr-Cd MOFs. These results show that the relative emission intensity of white light for Gd-Cd:Eu(3+),Tb(3+) MOFs is stronger than that of Pr-Cd:Eu(3+),Tb(3+) MOFs, which implies that the Gd complex is a better matrix than the Pr complex to obtain white-light emission materials.