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)-(H2O)(2)]center dot H2O (Ln=Pr (1), Eu (2), Gd (3), and Tb (4); H(3)imdc=4,5-imidazole-dicarboxylic 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 Eu3+ and Tb3+ 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: Eu3+, Tb3+ MOFs is stronger than that of Pr-Cd: Eu3+, Tb3+ MOFs, which implies that the Gd complex is a better matrix than the Pr complex to obtain white-light emission materials.