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In chemistry, photoisomerization is molecular behavior in which structural change between isomers is caused by photoexcitation. Both reversible and irreversible photoisomerization 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 addition, recent interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches, molecular motors, and molecular electronics.
Photoisomerization behavior can be roughly categorized into two classes: trans (or E) and cis (or Z) conversion, and open ring and closed ring transition. Instances of the former include stilbene and azobenzene. This class 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. These types of compounds undergo bond cleavage and bond creation upon irradiation with particular wavelengths of light.