Luminescence behavior of sol-gel-derived hybrid materials resulting from covalent grafting of a chromophore unit to different organically modified alkoxysilanes

The compounds studied in this work are sol-gel-derived organic-inorganic hybrid materials in which the two components are covalently linked via Si-C bonds. The organic part is a chromophore group derived from dipicolinic acid that is functionalized with tri-alkoxysilyl groups; the as-obtained silylated monomers are afterward submitted to complexation with rare-earth ions (Eu3+, Gd3+) and are used as the siloxane network precursors. The preparation of hybrid materials including covalent grafting and the sol-gel process is described, as well as their luminescence properties. Modifications of the ligand structure (mono- or disubstituted amides) lead to different coordinating properties and to variable absorption edges. As a result, the absorption efficiency or the ability of the chelates to transfer the absorbed energy to Ln(3+) and consequently the quantum yield of the emission are changed. The major effect of silica is a broadening of the emission peaks, whereas spectral repartitions and lifetimes are mainly unchanged as compared with the corresponding organic molecules.