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

MACROMOLECULES

American Chemical Society

Lugar: Washington DC; Año: 2006 vol. 39 p. 3794 - 3801

0024-9297

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. The inorganic polycondensation was produced using either high or low formic acid concentrations, leading to transparent films with different nanostructures as revealed by SAXS and 29 Si NMR spectra. For the bridged silsesquioxanes containing urea groups the self-assembly of organic domains through H-bonds was much faster than the polycondensation rate for both formic acid concentrations. A regular dispersion of nanosized inorganic clusters was generated, as revealed by SAXS spectra. The order of the organic self-assembled structures was higher when using low formic acid concentrations as inferred from the lower final conversion attained in the inorganic polycondensation. The less-ordered structures exhibited a new band in the photoluminescence spectra with a maximum at about 590 nm that was assigned to a photoinduced proton-transfer involving H-bonds with a broad energy distribution. This sample converted blue light (excitation wavelength of 420 nm) into white light (a broad emission spectrum with a maximum at 535 nm), a fact that has potential technological applications. Bridged silsesquioxanes containing urethane groups also exhibited different nanostructures and photoluminescence spectra depending on the conditions of the synthesis. The use of high formic acid concentrations led to large inorganic aggregates with radius of gyration higher than 100 Å irregularly dispersed in the organic matrix. The photoluminescence spectra of this sample showed a broad band with a pronounced red-shift when increasing the excitation wavelength.