No abstract

Hybrid organic/inorganic sol−gel materials have been synthesized by carboxylic acid solvolysis of (aminopropyl)triethoxysilane or ureasil precursors. The hybrid nature of these gels is expanded with the introduction of silica esters. A main feature of the ensuing materials is the formation of hybrid organic/inorganic nanoclusters, which are founded on silica backbone but they are facilitated and assisted also by forces arising from hydrophilic/hydrophobic balance and hydrogen-bonding interactions. The silicious domains of these clusters with attached functional groups, for example, amino groups, are luminescence centers. Some precursor compounds bearing amino groups without silicious groups but with a tendency to form clusters in solution also emit luminescence. Luminescence is the result of electron−hole recombination on delocalized states so that emission wavelength depends on excitation wavelength. Luminescence can thus be emitted in almost the entire visible spectrum and it can be tuned by choosing excitation wavelength. Amino-group-containing gels give the highest photoluminescence yield. It is, generally, expected that the presence of chemical groups with electron-donating capacity within the light-generating nanoclusters will be a favorable factor for making an efficient photoluminescent gel.

Steady-state luminescence spectroscopy and luminescence decay analysis have been employed to study the association of two rare earth ions (i.e., Eu 3+ and Tb 3+ ) with poly-(ethylene glycol) in the absence and in the presence of 2,2′-bipyridine, which acted as an antenna of near-UV radiation. Three different systems have been studied at various polymer concentrations, i.e., aqueous solutions, transparent composite organic/inorganic sol-gel matrixes made by hydrolysis of tetramethoxysilane, and polymer matrixes. The photophysical behavior of the luminescent species has been studied in conjunction with the poly(ethylene glycol) content. In both aqueous solutions and silica matrixes, luminescence intensity and decay time were found to increase by increasing polymer concentration. Addition of 2,2′bipyridine resulted in complex formation between the ligand and the lanthanide ions. This complex was stabilized by association with the polymer chains. Excitation at the ligand absorption wavelength (337 nm) resulted in ligand-to-metal energy transfer and strong luminescence emission, characterized by the narrow-band emission of the metal. The complex between lanthanide ions and 2,2′-bipyridine possessed its own particular photophysical characteristics and emitted a bright broad blue luminescence with an excitation maximum around 380 nm. Freeze-drying of aqueous solutions of medium size poly(ethylene glycol) containing lanthanide ions and 2,2′-bipyridine produced an intensely luminescent solid material emitting the characteristic luminescence of the metal when excited at the ligand absorption band (337 nm) or the characteristic luminescence of the complex when excited at 380 nm.

Transparent nanocomposite gels have been synthesized by the sol−gel method using hybrid precursors composed of two triethoxysilane groups and a mid polyether chain [poly(ethylene oxide) or poly(propylene oxide)] of various chain lengths. The end silicate groups are linked with the polyether chain through urea bridges (Ureasils). These nanocomposite materials can be visualized as silica nanoparticles dispersed in the organic phase provided by polyether chains. The gels are important room-temperature luminescent materials. Luminescence is the result of delocalized electron−hole recombination processes. The emitting centers are, most probably, located on the surface of silica clusters, where there is a concentration of NH and CO groups. Larger clusters emit at longer wavelengths than smaller clusters. Precursor molecules tend to aggregate and they also emit luminescence. Gels obtained by hydrolysis in the presence of NH4F favor larger cluster formation than gels obtained by hydrolysis in the presence of HCl and tend to emit at longer wavelengths. It has been found that luminescence intensity can be increased by modifying two major parameters. Shorter polyether chains give samples with higher luminescence intensity while larger chains cause a dilution effect that acts against luminescence efficiency. Doping with divalent or trivalent elemental cations of large atomic number results in an important increase of luminescence intensity. Heavy cations are then attracted close to the silica cluster surface and enhance luminescence by eliminating surface defects.