Eu³⁺- and Tb³⁺-Dipicolinate Complexes Covalently Grafted into Kaolinite as Luminescence-Functionalized Clay Hybrid Materials

The luminescence properties of Eu³⁺- and Tb³⁺-dipicolinate (pyridine-dicarboxylate) complexes covalently grafted into a kaolinite matrix were studied. The stability of the grafted lanthanide complexes as a function of the thermal treatment was also investigated. Kaolinite intercalated with dimethyl sulfoxide was heated in the presence of melted dipicolinic acid to form dipicolinate-intercalated kaolinite. The luminescent hybrid solids were obtained by complexation of Eu³⁺ or Tb³⁺ cations with this intercalated solid at cation/ligand molar ratios of 1:1, 1:2, or 1:3. The resulting materials were characterized by thermal analysis, CHN element analysis, powder X-ray diffraction, infrared absorption spectroscopy, and photoluminescence. The lanthanide complexes covalently grafted into kaolinite were thermally more stable than the isolated lanthanide complexes. The hybrid materials exhibited more intense Eu³⁺ and Tb³⁺ emissions than the isolated complexes. The excitation spectra of the hybrid materials showed a broad band at 277 nm, assigned to a ligand-to-metal charge transfer, whereas the emission spectra showed bands related to the typical electronic transitions of Eu³⁺ and Tb³⁺ ions from the excited states ⁵D₀ and ⁵D₄ to the ⁷F_J fundamental states. The (4 → 5 and 4 → 4) and (0 → 2) transitions were the most intense ones and corresponded to green and red emissions, respectively.