In chemistry, photoisomerization is a molecular behavior in which structural change between isomers is caused by photoexcitation. Both reversible and irreversible isomerization reactions exist. However, the word "photoisomerization" usually indicates a reversible process.
Photoisomerizable molecules are already put to practical use, for instance, in pigments for rewritable CDs, DVDs, and 3D optical data storage solutions. In fact, photoisomerization of the molecule retinal in the eye is the mechanism that allows for vision. In addition, recent interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches, molecular motors, and molecular electronics.
Another class of device that uses the photoisomerization process is as an aditive in liquid crystals to change their linear and nonlinear properties. Due to the photoisomerization is possible to induce a molecular reorintation in the liquid crystal bulk, which is used in holography, as spatial filter  or optical switching.
Photoisomerization behavior can be roughly categorized into several classes. Two major classes are trans-cis (or 'E-'Z) conversion, and open-closed ring transition. Examples of the former include stilbene and azobenzene. This type of compounds has a double bond, and rotation or inversion around the double bond affords isomerization between the two states. Examples of the latter include fulgide and diarylethene. This type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the di-pi-methane rearrangement.
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