resumo
Mono-amide cross-linked alkyl/siloxane hybrids (classed as mono-amidosils and represented by the notation m-A(x), where m and A stand for mono and amide, respectively, and x is the number of methylene repeat units) with water-mediated tunable structures have been prepared by means of sol-gel chemistry and self-direct assembly routes from the organosilanes CH3(CH2)(x)-C(=O)NH-(CH2)(3)-Si-(OCH2CH3)(3). The amorphous sample m-A(8) has been produced under the stoichiometric conditions (molar ratio Si : ethanol : water = 1 : 4 : 2) used previously to obtain the lamellar bilayer highly organized m-A(14) hybrid material, showing that the length of the pendant alkyl chains affects the degree of order of the materials. Three structurally ordered samples identified as AC-m-A(8) (1 : y) (where AC represents acid catalysis and y is the number of moles of water permole of Si) have been obtained using the acid catalyzed hydrolytic route and different water contents (y = 600, 300 and 100). Water plays a unique role in the organized mono-amidosils: it not only reverses the natural tendency of the precursor molecule with x = 8 to yield a disordered material, but it also allows the induction of order, leading to the formation of a lamellar structure exclusively in AC-m-A(8) (1 : 100). The presence of a higher water content promotes extra ordering in AC-m-A(8) (1 : 300) and AC-m-A(8) (1 : 600), where the lamellar structure coexists with a bilamellar superstructure. Upon heating the AC-mA(8) (1 : 600) sample to 120 degrees C and then cooling it to room temperature, the lamellar structure remained unaffected, while the superstructure was destroyed. The occurrence of the superstructure, although basically associated with the preferential entrapment of water molecules every two lamellae at the siliceous nanodomains, is also correlated with the amide-amide hydrogen bonded array. The m-A(8) and AC-m-A(8) (1 : 600) hybrids are multi-wavelength emitters under UV/VIS excitation. The emission spectra exhibit a broad band (300-650 nm) resulting from two contributions: a "blue'' band, due to electron-hole recombinations in the NH/C=O groups of the amide cross-links, and a "purplish-blue'' band, due to oxygen-related defects O-O-Si equivalent to(CO2) in the siliceous nanoclusters. The absolute emission quantum yields determined for m-A(8) and AC-m-A(8) (1 : 600) in the 360-380 nm excitation wavelength range were 0.10 +/- 0.01 and 0.15 +/- 0.01, respectively. While the lifetime values of the high and low wavelength components of the siliceous-related emission are practically the same for both samples (similar to 3.5 ms), the lifetime value of the NH/C=O component for m-A(8) is higher than that of AC-m-A(8) (1 : 600) (similar to 292 and similar to 156 ms, respectively), suggesting a lower non-radiative transition probability for the former hybrid.
palavras-chave
SOLID-STATE NMR; NORMAL-ALKYL CHAINS; H STRETCHING MODES; BRIDGED SILSESQUIOXANES; INFRARED TEMPERATURE; AUTO-ORGANIZATION; LIGHT EMISSION; SILICA-GELS; LONG; PHOTOLUMINESCENCE
categoria
Chemistry
autores
Nunes, SC; Silva, NJO; Hummer, J; Ferreira, RAS; Almeida, P; Carlos, LD; Bermudez, VD
nossos autores
Luís Carlos
Professor Catedrático
Maria Rute de Amorim e Sá Ferreira André
Professor Catedrático
Nuno João Silva
Investigador Principalagradecimentos
This work was supported by Fundacao para a Ciencia e a Tecnologia (FCT) and FEDER (contracts PTDC/CTM/101324/2008 and PTDC/QUI-QUI/100896/2008). S. C. Nunes acknowledges FCT for a grant (SFRH/BPD/63152/2009). The authors are grateful to Denis Ostrovskii, of the Department of Applied Physics, Chalmers University of Technology, Goteborg (Sweden), for recording the FT-IR and FT-Raman spectra, and to Sidney J. L. Ribeiro, of the Chemistry Institute of Araraquara, UNESP (Brazil), for recording the SAXS patterns at the LNLS, Campinas (Brazil). J. Hummer, a student of Technology of Functional Materials at the Julius-Maximilian-University in Wurzburg (Germany), was involved in this work in the framework of an internship at the Department of Chemistry of UTAD, Vila Real, from September to October 2010.