Molecular design of luminescent organic-inorganic hybrid materials activated by europium (III) ions

Luminescent hybrid materials consisting in rare-earth (Eu3+, Gd3+) organic complexes covalently attached to a silica-based network have been obtained by a sol-gel process. Four dicarboxylic acids with different aromatic subun its (dipicolinic acid, 4-phenyl-2,6-pyridinedicarboxylic acid, 4-(phenyleth ynyl)-2,6-pyridinedicarboxylic acid and 2,6-Bis(3-carboxy-1-pyrazolyl)pyrid ine) have been chosen as ligands for Ln(3+) ions. They were grafted to 3-am inopropyltriethoxysilane (APTES) to give organically modified alkoxysilanes that were used as molecular precursors for the preparation of hybrid mater ials. Ln(3+) first coordination sphere, composition of the siloxane matrix and connection between the organic and inorganic parts have been characteri zed by infrared spectroscopy, by (CSi)-C-13-Si-29 solid-state NMR as well a s by elemental analyses. UV excitation in the organic component resulted in strong emission from Eu3+ ions due to an efficient ligand-to-metal energy transfer. As compared to reference organic molecules, hybrid samples exhibi ted similar emission properties under UV excitation in addition to mainly u nchanged excited states lifetimes. However, by direct excitation of the Eu3 +-D-5(0) energy level the presence of two different site distributions were evidenced in the four hybrid compounds. Emission features related to each of these site distributions and their respective attribution were investiga ted. Variations in the relative emission intensities were observed accordin g to the nature of the organic chromophore. These variations were discussed in relation to the ATE (Absorption-Transfer-Emission) mechanism and to the relative energy positions of the ligand and the rare-earth ions respective ly. (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights r eserved.