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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.

Rhodamine 6G (R6G)-doped silica gels were prepared through the sol−gel process using dye concentrations ranging from 4.00 × 10-6 to 6.40 × 10-4 M. The adsorption of the R6G on the porous surface of the silica gels determined the formation of R6G fluorescent J-dimers as concentration increased, in concordance with the behavior reported for other xanthene molecules as rhodamine B (RB) and rhodamine 110 (R110) also doped in silica gels. The photophysical study of the R6G fluorescent dimers doped in silica gels was done through the recording of the excitation and emission fluorescence spectra as well as the measurement of the lifetime values. The exciton theory was used for the elucidation of the adsorption angle (θ), the angle between the monomer units (α), and the separation distance between the molecules (R) of the R6G fluorescent dimers. The angle between the monomer units (α) and the separation distance between the molecules (R) were also determined for RB and R110 fluorescent dimers doped in silica gels.

Rhodamine B (RB) dimers were observed in doped gel glasses made by sol-gel processing in a wide range of dye concentrations. The photophysical properties such as excitation/fluorescence spectra and lifetimes of the studied doped gel glasses within a high dye concentration range are characteristic of adsorbed fluorescent J-dimers. To our knowledge, this is the first time that these species have been reported at the xerogel state of doped sol-gel glasses. The formation of fluorescent J-dimers rather than nonfluorescent H-dimers has been studied and was attributed to the geometry that the dimers adopt upon adsorption on the silica gel surface. Geometry is also related to the specific concentration of RB molecules inside the pores. Through the study of the photophysical properties of the doped gel glasses, conversion from fluorescent J-dimers to nonfluorescent H-dimers was observed over a defined RB concentration range. The appearance of these fluorescent J-dimers, even at high dye concentrations, demonstrates the improved capability of these matrices to be used as solid dye lasers.

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.

Isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3‚9H2O and tetraethylorthosilicate (TEOS). However, when FeCl3‚6H2O was used as precursor, well-developed hematite particles were obtained in the final composite. This different behavior was already manifest in the initial gels. Thus, the gel obtained from iron nitrate salt shows a compact appearance as a result of its higher degree of network connectivity (polymeric gel) whereas the one from the iron chloride appears more loose and highly hygroscopic (colloidal gel). In addition, small superparamagnetic nuclei are formed during the hydrolysis and condensation of the gel obtained from the iron nitrate salt. The γ-Fe 2O3 nanoparticle formation takes place through a reduction-oxidation reaction which occurs during the burning of the organic species trapped inside the gel pore. The growth mechanism of the γ-Fe2O3 nanoparticles in the silica network has been studied as well as the optimum conditions for their preparation. Thus, γ-Fe2O3 nanocomposites with different particle sizes and distributions can be prepared by adequate modification of the initial gel microstructure through different gelation times, salt concentrations, and mechanical treatment. Superparamagnetic behavior has been found in all nanocomposites at room temperature, meanwhile at 70 K, a transition from superparamagnetic to ferrimagnetic behavior is observed as the particle size increases. In all cases, the variation in particle size observed by X-ray diffraction corresponds well with changes in the saturation magnetization for the γ-Fe 2O3 nanocomposites. Similar size effects are also found via the coercivity values at 70 and 5 K.

Low-water content reverse micelles have been formed in cyclohexane employing Triton X-100 or AOT (bis(2-ethylhexyl) sulfosuccinate sodium (salt) as the surfactant. Steady-state and time-resolved luminescence quenching studies, using Ru(bpy)3 2+ as the luminophore and Fe(CN)6 3-or MV 2+ as the quencher, have shown that at low-water content, reverse micelles can not properly solubilize polar species but the surfactant molecules tend to reorganize themselves around the polar molecules forming structures that depend on the nature of the surfactant and the charge. When titanium isopropoxide is solubilized in such reverse micelles, it is slowly hydrolyzed and it polymerizes to give an -O-Ti-O-Ti-network. Gelling is faster in the case of AOT than in the case of Triton X-100-based solutions. Gelling, i.e., polymerization, is a complex process that mainly depends on the surfactant, which tends to organize itself around the polar TiO 2 particles. Gels can be deposited by dip coating on glass slides as thin films of thickness of the order of 100 nm. When dipping at an early stage of gelation, films are transparent and optically uniform. Study with steady-state and time-resolved pyrene fluorescence reveals that AOT-based films consist of domains of lower dimensionality than Triton-based films. Indeed, when films are heated up to 450°C giving pure inorganic (oxide) material, after burning the organic part, Triton-based TiO 2 shows up as monodisperse spherical particles of a size of a few tens of nanometers. On the contrary, AOT-based TiO2 films consist of long particles with a high degree of orientation. It is obvious that with AOT reverse micelles one can obtain both hybrid organic/inorganic and pure inorganic mesoporous films of highly structured and oriented domains. It is believed that the difference between the materials created by the two surfactants mainly originates from the higher hydrolysis rates obtained in the AOT-based system.