Photoluminescence of Bridged Silsesquioxanes Containing Urea or Urethane Groups with Nanostructures Generated by the Competition between the Rates of Self-Assembly of Organic Domains and the Inorganic Polycondensation

D. P. FASCE; R. J. J. WILLIAMS; L. MATEJKA; J. PLESTIL; J. BRUS; B. SERRANO; J. C. CABANELAS; J. BASELGA

Lyon, Francia

Simposio; Second International Symposium on Nanostructured and Functional Polymer-Based Materials and Nanocomposites; 2006

INSA de Lyon

Bridged silsesquioxanes are a family of organic-inorganic hybrid materials prepared by the hydrolysis and condensation of monomers containing an organic bridging group joining two (or eventually more) trialkoxysilyl or trichlorosilyl groups. The organic group, covalently bonded to the trialkoxysilyl groups, can be varied in composition, length, rigidity and functionalization. This determines its contribution to the properties of the crosslinked material. Bridged silsesquioxanes bearing urea or urethane groups have received considerable attention in recent years due to their capacity of self-structuration through H-bonds and their white-light photoluminescence. The aim of this study was to investigate the changes produced in the nanostructures and the photoluminescence spectra of bridged silsesquioxanes  containing urea or urethane groups, by varying the relative rates between the self-assembly of organic domains and the inorganic polycondensation. Precursors of the bridged silsesquioxanes were 4,4’-[1,3 phenylenebis-(1-methylethylidene)]bis(aniline) and 4,4’-isopropylidenediphenol, end-capped with 3-isocyanatepropyltriethoxysilane. For both precursors the rate of the inorganic polycondensation was varied with the relative amount of formic acid used in the process. In this way a relative control on the ratio of rates of self-assembly of organic bridges and inorganic polycondensation could be achieved.  The generated structures were analyzed by Fourier-transformed infrared spectra (FTIR), 29Si NMR and small-angle X-ray scattering (SAXS). For silsesquioxanes with urea bridges the self-assembly rate was always higher than the rate of formation of inorganic clusters. This led to a regular arrangement of small inorganic clusters dispersed in a self-assembled organic matrix, as revealed by SAXS spectra. The self-assembled organic domains constituted a template for the formation of inorganic clusters. The position of the maximum at q = 0.23 Å-1 determines a correlation distance of 27 Å between inorganic clusters that corresponds to the length of the organic bridge. However, a higher order of the self-assembled bridges was produced using low formic acid concentrations leading to low rates of Si-O-Si formation. This was inferred from the lower final conversion attained in the inorganic polycondensation ascribed to steric restrictions for the covalent bonding of SiOH groups present at the ends of the self-assembled organic structure. The most relevant finding was the significant variation produced in the photoluminescence spectra by varying the order present in the self-assembled organic bridges. For the sample exhibiting the highest order two main bands were present in the emission spectra: a band at about 445 nm that appeared when the excitation wavelength was comprised between 330 nm and 350 nm, and a second band with a maximum in the range of 475 nm to 495 nm. Based on the literature,1 the former band was assigned to radiative recombinations that occur within the nanometer-sized inorganic clusters and the latter to a photoinduced proton-transfer generating NH2+ and N- defects and their subsequent radiative recombination. For the sample exhibiting the lowest order of self-assembled organic bridges, a third band with a maximum at about 590 nm was also present in the photoluminescence spectra that was assigned to a photoinduced proton-transfer involving H-bonds exhibiting a broad energy distribution. The maximum intensity was obtained for an excitation wavelength in the visible region (420 nm), giving a broad emission band covering practically the whole visible spectrum, a fact that might have practical applications. For silsesquioxanes with urethane bridges the rates of self-assembly of organic bridges and the rate of formation of inorganic clusters entered into competition when varying the formic acid concentration used in the synthesis. For low formic acid concentrations the self-assembly rate prevailed leading to a regular arrangement of small inorganic domains dispersed in the self-assembled organic matrix. A peak at q = 0.28 Å-1 representing a characteristic distance of 22.4 Å, was observed in SAXS spectra. This corresponds to the length of the organic bridge. Photoluminescence spectra exhibited a low intensity mainly arising from the band at about 420-430 nm. Samples synthesized using high formic acid concentrations consisted of large inorganic aggregates with radius of gyration higher than 100 Å, irregularly dispersed in the organic matrix. Under these conditions, the self assembly of organic bridges did not occur and, therefore, a regular dispersion of inorganic domains could not be generated (no peak was observed in SAXS spectra). Again, this change in the nanostructuration of the sample produced significant variations in the photoluminescence spectra. A new band that might be assigned to photoinduced proton-transfer involving H-bonds with a broad energy distribution appeared in the spectra, with a pronounced red-shift when increasing the excitation wavelength. Therefore, the photoluminescence spectra of both bridged silsesquioxanes were significantly affected by the different levels of ordering attained in the self-assembled organic domains through the variation in the rate of the inorganic polycondensation.   References (1) Carlos, L. D.; Sá Ferreira, R. A.; Pereira, R. N.; Assunção, M.; de Zea Bermudez, V. J. Phys.      Chem. B 2004, 108, 14924.   Acknowledgements. The financial support of the National Research Council (CONICET, Argentina), the National Agency for the Promotion of Science and Technology (ANPCyT, Argentina, PICT 14738-03), the University of Mar del Plata, the Grant Agency of the Czech Republic (project 203/05/2252), and Project Nanoter (Project MAT2004/01347, MEC-DGI, Spain), is gratefully acknowledged. INTEMA and the Institute of Macromolecular Chemistry acknowledge the support of the European Network of Excellence Nanofun-Poly for the diffusion of their research results.