Azobenzene (and derivatives) undergo photoisomerization of trans and cis isomers. cis-Azobenzene relaxes back, in dark, to the trans isomer. Such thermal relaxation is slow at room temperature. The two isomers can be switched with particular wavelengths of light: ultraviolet light, which corresponds to the energy gap of the π-π* ("S2" state) transition, for trans-to-cis conversion, and blue light, which is equivalent to that of the n-π* ("S1" state) transition, for cis-to-trans isomerization. For a variety of reasons, the "cis" isomer is less stable than the trans (for instance, it has a distorted configuration and is less delocalized than the trans configuration). Photoisomerization allows for reversible energy storage (as photoswitches).