PHOTOLUMINESCENCE OF NANOSTRUCTURED BRIDGED SILSESQUIOXANES CONTAINING UREA GROUPS DOPED WITH SAFRANIN-O

M. L. GÓMEZ; D. P. FASCE; R. J. J. WILLIAMS; H. A. MONTEJANO; C. M. PREVITALI

Corfu, Grecia

Simposio; 3rd International Symposium on Nanostructured and Functional Polymer-Based Materials and Nanocomposites; 2007

European Center for Nanostructured Polymers

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 groups have received considerable attention in recent years due to their capacity of self-structuration through H-bonds and their white-light photoluminescence.1,2 The aim of this study was to extend our previous research on the photoluminescence of a bridged silsesquioxanes synthesized by the hydrolysis and condensation of a precursor based on the reaction of 4,4’-[1,3 phenylenebis-(1-methylethylidene)]bis(aniline) (BSA) end-capped with 3-isocyanatepropyltriethoxysilane (IPTES).2 Nanostructuration was confirmed by the presence of a correlation peak in SAXS spectra at q = 0.23 Å-1 determining a correlation distance of 27 Å between inorganic clusters that corresponds to the length of the organic bridge.2 Safranin-O was selected as a convenient photoluminescent molecule to dope the bridged silsesquioxane. Similarly to BSA it has 2 aromatic amine groups that may be end-capped with IPTES leading to an urea type precursor. Structures of urea bridges based on BSA and Safranin-O are shown in Figure 1.   Figure 1. Structures of organic bridges based on BSA and Saf-O.   The synthesis of the precursor was carried out in THF at 50 ºC for 44 h. Molar concentrations of BSA and Saf-O were 0.1 M and 7x10-5 M and IPTES was used in a stoichiometric proportion. After 44 h reaction the isocyanate groups were completely reacted as confirmed by the disappearance of the absorption peak at 2272 cm-1 in FTIR spectra. The hydrolysis and condensation of the precursor was carried out in the THF solution at room temperature employing either formic acid or acetic acid solutions in the following molar ratios: Si / acid / water = 1 / 0.1 / 3. Solutions were cast in polyacetal recipients of 5 cm diameter with an initial height of liquid close to 5 mm and a glass cover that enabled the control of the solvent evaporation rate. Hydrolysis and condensation reactions took place together with solvent evaporation. After about 2 weeks THF was completely evaporated, and the resulting pink-colored films could be easily detached from the plastic recipient. In order to eliminate any trace of Saf-O that was not covalently bonded in the hybrid material, films were immersed in ethanol for 1 week, the solvent was eliminated and films were dried at 60 ºC for 24 h. Films retained their intense pink color meaning that Saf-O was covalently bonded to the structure. Films prepared without Saf-O were also synthesized for comparison purposes.     Excitation and emission photoluminescence (PL) spectra of the different films were recorded using excitation wavelengths equal to or higher than 350 nm avoiding the UV region where the aromatic structure of the UBSA precursor exhibits an intrinsic PL. Films synthesized without Saf-O exhibited similar PL spectra independently of the use of formic or acetic acid as catalysts. Broad emission PL spectra were observed that could be deconvoluted into at least three bands: a minor band at about 440-450 nm (purplish-blue band) and two main bands located in the region of 480-500 nm (blue band) and in the region of 550-570 nm. The purplish-blue band was assigned to the PL of defects located in the inorganic domains,1 the blue band was assigned to a photoinduced proton-transfer in the self-assembled organic bridges generating NH2+ and N- defects and their subsequent radiative recombination,1,2 and the band at 550-570 nm was assigned to a similar mechanism taking place in less-ordered organic regions.2 An absolute maxiumum of the emission intensity was observed at about 500 nm for an excitation wavelength of 425 nm.   Doping the bridged silsesquioxanes with Saf-O produced a significant change in the PL spectra. Of the bands present in the undoped material only the purplish-blue band was still present. But the two main bands arising from the PL arising by proton transfer in organic bridges completely disappeared. A strong PL band with a maximum at 610 nm, for materials synthesized with formic acid, and at about 650 nm, for hybrids obtained with acetic acid, that is characteristic of Saf-O, was present in PL spectra. An absolute maximum of the emission spectra was obtained for an excitation wavelength of 550 nm. The excitation spectra were very broad and could be deconvoluted into one band covering the wavelength region where emission from the undoped material was expected and one peak with a maximum at 550 nm, characteristic of Saf-O. Therefore, it can be inferred that the excited state of self-assembled UBSA bridges is involved in an energy transfer process to Saf-O bridges, exciting its intrinsic PL. This mechanism broadens the wavelength region where Saf-O can be effectively excited.          In conclusion, it was shown that a PL molecule (Saf-O) bearing two amine groups in the chemical structure could be covalently incorporated as a dopant of a bridged silsesquioxane containing urea groups. The intrinsic PL arising from the proton transfer in self-assembled organic bridges was eliminated and the main PL peak was the intrinsic one of Saf-O. From the excitation spectra it was inferred that the excited state of self-assembled domains was involved in an energy transfer mechanism to Saf-O. This extended the wavelength region where the dopant could be effectively excited.     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. (2) Fasce, D. P.; Williams, R. J. J.; Matějka, L.; Pleštil, J.; Brus, J.; Serrano, B.; Cabanelas, J. C.;      Baselga, J. Macromolecules 2006, 39, 3794.   Acknowledgements. The financial support of the National Research Council (CONICET), the National Agency for the Promotion of Science and Technology (ANPCyT, PICT 14738-03), and the University of Mar del Plata, is gratefully acknowledged.