Photochromic organic-inorganic hybrid materials have attracted considerable attention owing to their potential application in photoactive devices, such as optical memories, windows, photochromic decorations, optical switches, filters or non-linear optics materials. The growing interest in this field has largely expanded the use of photochromic materials for the purpose of improving existing materials and exploring new photochromic hybrid systems. This tutorial review summarizes the design and preparation of photochromic hybrid materials, and particularly those based on the incorporation of organic molecules in organic-inorganic matrices by the sol-gel method. This is the most commonly used method for the preparation of these materials as it allows vitreous hybrid materials to be obtained at low temperatures, and controls the interaction between the organic molecule and its embedding matrix, and hence allows tailoring of the performance of the resulting devices.

Bright blue CoAl 2 O 4 particles were prepared by the sol-gel and citrate-gel methods using aluminum sec-butoxide, cobalt salts, and citric acid as oxides precursors. Both methods start from sols of the precursor alkoxides and salts, and involve formation of homogeneous solid intermediates, reducing atomic diffusion processes during thermal treatment. This important feature results in a substantial lowering of the time and temperature needed for the formation of the desired compounds. The stages of the formation of CoAl 2 O 4 , as well as the characterization of the resulting compounds were done using XRD, FTIR, UV-VIS, SEM, and TGA/DTA techniques. The structure, coloration, particle size, and temperature of formation of the resulting CoAl 2 O 4 phases were found to depend on the precursors and methods used for preparation and the calcination temperature. The lowest temperature for preparation of the blue cobalt aluminate of about 700 °C was obtained using the citrategel method. This temperature is much lower than that needed for preparation of the compound through traditional solid-state reactions (above 1000 °C).

One of the most important drawbacks of classical and new advanced functional materials for applications outdoors, or in environments with high UV irradiation, is the light induced damage that reduces drastically their effective operation lifetime or durability. This makes protecting light sensitive materials against UV irradiation a nowadays important technological demand in almost every industrial field. This tutorial review incorporates the main aspects of UV damage to materials and describes the recently developed highly effective thin UV-protective coatings, based on UV-absorber molecules entrapped in a Sol-Gel derived ormosil matrix.

γ-Fe2O3/SiO2 nanocomposites have been prepared using a sol-gel procedure, starting from iron nitrate and triethyl orthosilicate. The addition of acids to the sols resulted in a way to increase particle size, keeping low iron concentrations, and to narrow the particle size distribution of γ-Fe2O3 in the glass composite. Thus, the addition of 0.56 mmol HCl (≈0.1 M) to a solution having an Fe/Si molar ratio of 18% leads to an increase of γ-Fe2O3 particle size from 6 to 13 nm together with a remarkable decrease in the polydispersity degree of the particle size from 66% to 15%. The iron oxide crystalline phase, the particle size and shape, and the homogeneity of the resulting nanocomposites were studied by X-ray diffraction, Mo ¨ssbauer spectroscopy, and transmission electron microscopy. 29 Si NMR and Fourier transform infrared techniques were used to study the changes in matrix structure due to the presence of acids. The influence of acid addition on the size of the resulting magnetic particles was found to depend on the matrix microstructure, the charge environment, and the presence of the counteranions of the acids. Magnetization studies at room temperature showed superparamagnetic behavior for all the nanocomposites, except for the sample with the largest particle size (13 nm), which exhibits ferromagnetism. Optical measurements showed that as the size of the magnetic particle decreases, a blue shift in the UV-vis absorption spectra occurs, which is related to quantum confinement effects.

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The preparation and characterization of highly transparent (see Figure) and magnetic nanocomposites exhibiting Faraday rotation is reported. Superparamagnetic behavior results from the confinement of the γ‐Fe2O3 nanoparticles within the pores of a Vycor glass. The remarkable Faraday rotation exhibited by the resulting composites opens their application to a wide range of magneto‐optical devices.

UV-protective coatings have been prepared by the sol-gel method, to reduce the destructive effects of UV radiation on easily photodegradable devices, i.e. those containing organic compounds, such as dyes and pigments or plastic materials to be used in outdoor applications. A benzophenone derivative (2,2-dihydroxy, 4-methoxybenzophenone), showing high photostability and strong absorption in the UV range, was embedded in an ormosil matrix. The usage of an organically modified silica matrix enhances the solubility of the UV absorber in the matrix allowing the preparation of highly loaded coatings. The protective coatings, prepared at room temperature, require no thermal treatment after deposition, allowing therefore their application on temperature sensitive materials. The resulting films have a strong absorption in the UV range with a thickness of only 1.0 mm. In addition, the UV-absorbing coatings are transparent, colourless, and exhibit high optical quality. The UV-protective coatings offer an easy method to prevent the photodegradation of organic materials without altering their optical properties in the visible region.Fluorescent rhodamine dye-doped thin ormosil films were coated with a UV-protective layer in order to study their effectiveness in the reduction of the photodegradation of the dye upon irradiation with UV light. The degradation of 20% of the molecules in coated samples was 14 times slower than that of the uncoated samples. The effective temperature range of the UV-protective coatings was established by measuring the photodegradation of the samples at different temperatures.

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