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.

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Photochromic organic-inorganic hybrid materials.

This critical review focuses on the development of photochromic compounds as sensors for cations, anions, and biologically important molecules. The review commences with a brief description of photochromism and the strategies to exploit photochromic molecular switches' properties for sensing application. This is followed by a summary of photoswitchable receptors emerged to date and classified according to the photochromic structure they are based on. These include azobenzenes, fulgides, dithienylethenes, dihydroindolizines, chromenes and spiropyrans.

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Molecular switches as photocontrollable “smart” receptors

Organic photochromic molecules are well established as colorants in the manufacture of niche products, providing striking colour change effects when irradiated with light. This paper describes the industrially important classes of photochromic dye in terms of their development and chemistries, the applications in which their photochromism is employed commercially, and the technical features behind their success. Photochromic systems form the subject of much industrially funded research into nascent high-tech applications that have the potential to become the most commercially significant outlets for photochromic dyes. Those types being scrutinised are therefore also discussed along with the directions in which the industrial use of photochromic colorants may take.

Industrial organic photochromic dyes

The review describes photoswitchable molecular receptors based on crown-ether systems. The important feature of the systems is reversible change in the capacity of a photochromic host molecule for association with guests upon irradiation. From the other side, the exploration of the complex formation process in crown-ether systems leads to a novel approach to the modification of photochromic behavior. The results obtained show that the introduction of crown-ether moieties into dye molecules affords compounds that can change their spectral and photochromic properties upon complex formation. The researchers believe that the novel photochromic systems can now be regarded as being promising for traditional applications (photochromic ophthalmic lenses or camera filters, reversible holographic systems and cosmetics) and molecular electronics, biomimetic chemistry, and optical information storage.

Photoswitchable molecular receptors

In recent years, stimuli responsive materials have gained significant attention in membrane separation processes due to their ability to change specific properties in response to small external stimuli, such as light, pH, temperature, ionic strength, pressure, magnetic field, antigen, chemical composition, and so on. In this review, we briefly report recent progresses in light-driven materials and membranes. Photo-switching mechanisms, valved-membrane fabrication and light-driven properties are examined. Advances and perspectives of light responsive polymer membranes in biotechnology, chemistry and biology areas are discussed.

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Light responsive polymer membranes: a review.

The preparation of a commercially acceptable plastic photochromic ophthalmic lens requires the incorporation of a number of properties - a neutral activated color, reasonable activation and fade rates, good lifetime, etc.. In this paper we will concentrate on approaches to finding photochromic systems with relatively neutral colors and acceptable rates of activation and fade. While offering good photochromic properties with regard to kinetics and photostability, they suffer from the fact that the visible absorption bands of the activated, or open, forms have narrow band widths. Also, without difficult to achieve substitution patterns, the absorption bands appear within a relatively narrow range of wavelengths. Partial neutralization of the activated color of the naphthoxazines was achieved through the use of indolino spirobenzoxazines. The open forms within this family exhibited broad absorption bands but the photochromic activity of each of the members was moderate to low. Enhancement of the photochromic activity was possible with triplet sensitization but at the expense of reduced lifetime. True color neutralization was achieved by coupling pyridobenzoxazines with members of a completely different family of photochromic compounds, the naphtliopyrans. The indolino spironaphthoxazines and related compounds have been known for over 25 years.

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B. Van Gemert

Papers

Photochromic compounds: Chemistry and application in ophthalmic lenses