abstract
The bridged silsesquioxane precursor (EtO)(3)Si(CH2)(3)NH(C=O)NH-(CH2)(12)-N1-1(C=O)NH-(CH2)(3) Si(OEt)(3), combining polymerizable silylated groups, urea functionalities, and alkyl chains, undergoes fluoride (F-)-catalyzed sol-gel reactions in the presence or absence of EuCl3 center dot 6H(2)O. Supramolecular self-assembly of the growing structure relies primarily on the establishment of strong and ordered hydrogen bonding interactions. In the case of the Eu3+-containing hybrids the lanthanide ions play a totally unparallel dual-role acting simultaneously as structure directing agents and structural probes to sense locally morphological alterations. In the early stages of the synthesis, while a fraction of the Eu3+ ions promotes the formation of a unidirectional urea-urea hydrogen-bonded array, the remaining ions coordinate to silanol (Si-OH) groups inhibiting the growth of the siloxane network and yielding small anisotropic flakes (similar to 200 rim lone). These are subsequently assembled on the micrometer scale in a brick-like tile-to-tile arrangement that ends up with the formation of fibers or twisted bundles (3.0-4.0 mu m long and 0.5-1.0 mu m wide). At higher Eu3+ concentrations, Eu3+-based ionic interfiber cross-links hinder the solvent flow and force adjacent fibers to adopt a bow-tie form (3.0-4.0 mu m long and 3.0 mu m wide at the tips). The hybrids are room temperature multiwavelength emitters because of the convolution of the hybrids' intrinsic emission and the Eu3+ intra-4f(6) transitions. The photoluminescence features (D-5(0)-> F-7(0) energy, D-5(0) quantum efficiency, number of coordinated water molecules, and experimental intensity parameters) as a function of the Eu3+ content and acidic- and F--catalyzed conditions used in the synthesis are compared to address the effect of the morphology in the photoluminescence features of the hybrid materials.
keywords
ORGANIC-INORGANIC HYBRIDS; EMISSION QUANTUM YIELDS; FULL-COLOR PHOSPHORS; SOL-GEL PROCESS; ENERGY-TRANSFER; LANTHANIDE COMPOUNDS; BETA-DIKETONE; SILICA; DESIGN; NANOCOMPOSITES
subject category
Chemistry; Materials Science
authors
Nobre, SS; Cattoen, X; Ferreira, RAS; Carcel, C; Bermudez, VD; Man, MWC; Carlos, LD
our authors
Luís Carlos
Full professor
Maria Rute de Amorim e Sá Ferreira André
Full professoracknowledgements
The support of NoE FAME, Fundacao para a Ciencia e Tecnologia (FCT), FEDER, LUSO and PHC-PESSOA (Ministere des Affaires Etrangeres et Europeennes) is gratefully acknowledged. The authors would like to thank the collaboration of A. G. Macedo and M. C. Ferro, Aveiro University, for the AFM and SEM measurements, respectively. S.S.N. thanks FCT grant (SFRH/BD/28739/06).