Self-Structuring of Lamellar Bridged Silsesquioxanes with Long Side Spacers


Diurea cross-linked bridged silsesquioxanes (BSs) C10C11C10 derived from organosilane precursors, including decylene chains as side spacers and alkylene chains with variable length as central spacers (EtO)(3)Si- (CH2)(10)-Y(CH2)(n)-Y-(CH2)(10)-Si(OEt)(3) (n = 7, 9-12; Y = urea group and Et = ethyl), have been synthesized through the combination of self-directed assembly and an acid-catalyzed sol gel route involving the addition of dimethylsulfoxide (DMSO) and a large excess of water. This new family of hybrids has enabled us to conclude that the length of the side spacers plays a unique role in the structuring of alkylene-based BSs, although their morphology remains unaffected. All the samples adopt a lamellar structure. While the alkylene chains are totally disordered in the case of the C10C7C10 sample, a variable proportion of all-trans and gauche conformers exists in the materials with longer central spacers. The highest degree of structuring occurs for n = 9. The inclusion of decylene instead of propylene chains as side spacers leads to the formation of a stronger hydrogen-bonded urea-urea array as evidenced by two dimensional correlation Fourier transform infrared spectroscopic analysis. The emission spectra and emission quantum yields of the C10CnC10 Cm materials are similar to those reported for diurea cross-linked alkylene-based BSs incorporating propylene chains as side spacers and prepared under different experimental conditions. The emission of the C10CnC10 hybrids is ascribed to the overlap of two distinct components that occur within the urea cross-linkages and within the siliceous nanodomains. Time-resolved photoluminescence spectroscopy has provided evidence that the average distance between the siliceous domains and the urea cross-links is similar in the C10CnC10 BSs and in oxyethylene-based hybrid analogues incorporating propylene chains as side spacers (diureasils), an indication that the longer side chains in the former materials adopt gauche conformations. It has also allowed us to demonstrate for the first time that the emission features of the urea-related component of the emission of alkylene-based BSs depend critically on the length of the side spacers.



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Fernandes, M; Nobre, SS; Xu, QH; Carcel, C; Cachia, JN; Cattoen, X; Sousa, JM; Ferreira, RAS; Carlos, LD; Santilli, CV; Man, MWC; Bermudez, VD

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This work was financially supported by the Fundacao para a Ciencia e a Tecnologia (FCT)/Ministere des Affaires Etrangeres et Europeennes (Programme Pessoa/Hubert Curien, Contract 441.00 2009/2010) and CNRS. M. Fernandes thanks FCT for a grant (SFRH/BD/38530/2007) and University Montpellier 2 for financial support. LNLS is thanked for providing beamtime at the SAXS beamline and for financial support. The authors are grateful to Professor Isao Noda (The Proctor and Gamble Company, Cincinnati, OH, USA) for enlightening discussions regarding the 2D FT-IR data and acknowledge Andreia G. Macedo for the AFM measurements.

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