Marcos Zayat

Preventing UV-light damage of light sensitive materials using a highly protective UV-absorbing coating.

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.

Highly efficient UV-absorbing thin-film coatings for protection of organic materials against photodegradation

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 µm. 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.

Photochromic naphthopyrans in sol–gel ormosil coatings

The sol–gel method has been used to entrap photochromic naphthopyran molecules in ormosil coatings. The resulting films exhibited a rapid and reversible color change upon irradiation with UV light, leading to deeply colored coatings. The bleaching of the samples can be attained by both thermal and photo-induced processes. The photochromic properties of different naphthopyran derivatives in the sol–gel matrix were measured, such as the absorption spectra of bleached and colored states as well as the thermal and photoinduced decay kinetics.

Ultrafast random laser emission in a dye-doped silica gel powder

We report efficient random lasing in a ground powder of a novel solid-state material based on silica gel containing SiO(2) nanoparticles embedding rhodamine 6G (Rh6G) dye. Basic properties of random lasing such as emission kinetics, emission spectrum, and threshold of stimulated emission are investigated by using real-time spectroscopy. The laser-like emission dynamics can be accurately described by a light diffusive propagation model. The device behavior is close to a conventional ultrafast Q-switched laser, which is an interesting fact aimed to further applications.

The role of organic groups in ormosil matrices in the photochromism of naphthopyrans in sol–gel thin films

Photochromic 3,3-diphenyl-3H-naphtho[2,1-b]pyran molecules were embedded in sol–gel prepared thin organically modified silica films. The introduction of organic functional groups into a silica matrix allows the surfaces of its pores to be tailored and thus the polarity of the pores to be modified. The photochromic properties of the naphthopyran molecules, such as the spectral properties of the colored forms and the kinetics of thermal bleaching, were found to depend strongly on the polarity of the pores in which the molecules are located and hence, on the nature and loading of the organic groups (Me, Ph or iBu) in the ormosil matrix. The large isobutyl substituents showed the largest reduction in the polarity of the pore, due to the effective screening of the polar OH groups at the surfaces of the pores. Hence, the absorption band of the open colored form of the photochromic dye could be adjusted by as much as 35 nm in the yellow-to-orange range by changing the composition of the embedding matrix. Reducing the polarity of the pore walls in the ormosil matrix resulted in a substantial acceleration of the bleaching kinetics of the photochromic effect.

The influence of sol–gel processing parameters on the photochromic spectral and dynamic behaviour of a naphthopyran dye in an ormosil coating

A photochromic naphthopyran derivative was embedded in a Ph-modified ormosil coating. Important changes in the photochromic properties of the films were induced by modifications in the preparation parameters of the sol–gel processing, such as the hydrolysis and condensation of the sols, the photochromic-dye loading in the film and the curing of the samples after deposition. In this way, it is possible to control the thickness of the resulting coatings, their absorption spectra after irradiation and the kinetics of thermal bleaching. The coatings were prepared having a thickness in the 0.8–2.1 µm range, allowing the incorporation of large amounts of dye in the matrix (up to a dye/Si molar ratio of 0.02) and showing a coloration change upon irradiation with UV-light in the ΔOD 0.04–0.22 range.

Reaching bistability in a photochromic spirooxazine embedded sol–gel hybrid coatings

Novel photochromic coatings were designed having a remarkable long-term stability of both the coloured state and colourless states in the dark (bistability). The coatings consist of a dispersion of a commercial spirooxazine in a functionalized sol–gel hybrid matrix, acquiring a deep purple coloration upon irradiation with UV-light. In order to stabilize the coloured form of the dye, a zinc salt, capable of forming a chelate with the photochromic molecule when irradiated, was incorporated into the matrix, resulting in a drastic reduction of the thermal bleaching kinetics. The coloured coatings can only be reverted to their original colourless state by irradiation with visible light. This recording–erasing process is reversible and can, therefore, be repeated by alternating irradiation with UV and visible light. Adding the bistable behaviour to the specific properties of reversible photochromic systems makes a new range of applications in the field of optics and light-controlling media available.

Temperature dependence of the photochromism of naphthopyrans in functionalized sol–gel thin films

A photochromic naphthopyran derivative, 3-(2,4-dimethoxyphenyl)-3-(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran, was embedded in sol–gel prepared phenyl (Ph) and pentafluorophenyl (pFPh) functionalized silica thin films. The functionalization of the matrix surface and the temperature both play important roles in the photochromic properties of the dye molecules as they affect their interaction with the embedding ormosil matrix and, therefore, the equilibrium between coloured and colourless forms of the molecule. The bleaching kinetics of the dye embedded in these ormosil matrices were strongly accelerated by temperature in all samples, particularly in pFPh functionalized matrices, due to the reduced mobility of the molecules at low temperatures. The usage of different amounts of organic functional substituents in the matrix has an important effect on the size of the pores where the photochromic molecules are located affecting the steric hindrance of the mechanism of formation of the coloured forms. The ability to control the dye–matrix interactions allows the design of materials with defined photochromic properties.

Surface Area Study of High Area Cobalt Aluminate Particles Prepared by the Sol-Gel Method

Cobalt aluminate particles were prepared by the sol-gel method, starting from aluminum sec-butoxide and cobalt salts with a Co:Al ratio of 1:3. Samples with the same composition were also prepared by the citrate-gel method from cobalt and aluminum nitrates and citric acid. The particles were calcined to temperatures between 400 and 1000°C, for the formation of the mixed oxide having spinel structure. The surface properties of the different samples (BET surface area and pore size distribution) were measured. The highest BET surface area obtained (about 339 m2/g) corresponds to a sample prepared by cobalt acetate and aluminum sec-butoxide, calcined at 400°C. The surface area of the sample is reduced progressively as the sample is calcined to higher temperatures (to about 65 m2/g at 1000°C). Narrow pore size distributions were observed with average pore radius ranging from 17–20 Å, for samples heated to 400°C, to about 55–65 Å, for samples heated to 1000°C. The different surface areas and porosities obtained for particles prepared by different methods, different precursors or calcination temperatures, are discussed.

An Electro-optical Device from a Biofilm Structure Created by Bacterial Activity

This work was supported by grants of the Ministerio de Ciencia e Innovacion (Grant No. MAT2008-00010/NAN) and by contracts of the Autonomous Community of Madrid and the European Union. EC and MC are grateful to Universidad de Costa Rica and the CSIC for doctoral scholarships.