Rosario Pardo

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.

Ellipsometric analysis of the spectral properties and dynamic transitions of photochromic thin films

Solgel phenyl functionalized silica thin films were used as a host matrix to embed photochromic molecules (1,3,3,5,6-pentamethyl-spiro[indoline-2-3′-[quinolino]oxazine], 3-(2,4-dimethoxyphenyl)-3-(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran and 3,3-diphenyl-3H-naphtho[2,1-b]pyran). The coatings were doped with individual dyes and mixtures of them. The optical properties of the colored and colorless forms of these photochromic materials were studied using variable angle spectroscopic ellipsometry. Light polarization analyses allowed an exhaustive ellipsometric characterization of these materials. Important changes were obtained in the refractive indices of the colored and colorless samples, with differences of up to 0.012 and 0.018 in the real and imaginary components, respectively. Good correlation was observed on the optical parameters of samples doped with individual dyes and mixtures of them. Standard transmission measurements were used to study the dynamic photochromic transitions and obtain the kinetic constants. The high capabilities of ellipsometry for the high-accuracy characterization of dynamic photochromic transitions were demonstrated. This technique allowed measuring variations in the populations of colored and colorless molecules, resulting from variations of temperature lower than ±0.1°C.

Stability against photodegradation of a photochromic spirooxazine dye embedded in amino-functionalized sol–gel hybrid coatings

A photochromic spirooxazine derivative, 1-propyl-3,3,5,6-tetramethyl-spiro[indoline-2-3′-[quinolino]oxazine], was successfully embedded in sol–gel thin silica films functionalized with different amino groups. The resulting films show high transparency and exhibit a strong blue coloration upon irradiation with UV light. The composition of the embedding matrix has an important effect on the photostability of the photochromic molecules upon exposure to sunlight, and can therefore be used to design coatings in which the dye molecules have improved durability. In this sense, the incorporation of different amino groups (–PrNH2, –PrNMe2 and –PhNH2) in the ormosil network, results in an enhanced stabilization of the photochromic dye, as compared with unfunctionalized matrices. In matrices modified with aminophenyl groups (–PhNH2), the photostability of the dye has been increased, reaching a factor of 8, due to the formation of hydrogen bonds between the amino groups and the OH groups of the pore surface, limiting the availability of these groups to undergo side reactions with the dye during irradiation that lead to its degradation. Increasing the photostability of the photochromic dye is an important issue for the long term usage of photochromic materials in outdoors applications, limited, nowadays, by their low durability when exposed to sunlight.