Photochromic inorganic compounds

Among the photochromic inorganic compounds [88-90], it is well known that silver halide particles in borosilicate or aluminoborosihcate glass exhibit reversible photochromism upon exposure to sunlight [91-93]. Upon irradiation, the silver ions are reduced to metallic silver particles, which are responsible for the... 

By far the greatest number of photochromatic compounds, however, are organic. As in the inorganic compound mercury dithizon-ate, photochromism in organic compounds occurs when one form of a compound is exposed to light, which converts the compound to an isomer of a different color. In most such instances, the two isomers differ in that one is a closed form of the isomer and the other is an open form. In some cases, however, the two isomers differ in that one occurs in a trans form and the other in a cis form. Trans and cis isomers are forms of a compound in which two constituent atoms or groups are located on opposite or the same side of a molecule, respectively. 

Photochromic organic-inorganic hybrid materials are one of the most studied systems. It is believed that the synergy of organic and inorganic compounds in hybrid materials can preserve or even improve the features of the components. Nowadays, it has been demonstrated that hybrid systems can also confer the material new properties, different from those of the single components [6,10]. 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

Less attention has been paid to the thermochromic properties [72-77] of these hybrid materials, although they can be used as temperature sensors [78-82]. Some photochromic compounds such as naphthopyrans and spirooxazines show thermochromic properties, being the naphthopyrans, better photochromic but worse thermochromic compounds than spirooxazines. Inorganic thermochromic compounds such as those based on copper and tungsten embedded in sol-gel matrices are being widely studied as temperature sensors. 

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