Azobenzene (and derivatives) undergo photoisomerization of trans and cis isomers. It is structurally related to azobenzene. The soft materials respond to light, using photoresponsive surfactants that include an azobenzene group. Yellowish-red crystalline flakes of azobenzene were obtained in 1856. Examples of the former include stilbene and azobenzene. For instance, with maleic acid and with azobenzene, often by photoisomerization. The photo-isomerization of azobenzene is extremely rapid, occurring on picosecond timescales. Azobenzene is the prototypical aromatic azo compound. The photo-isomerization of azobenzene is a form of light-induced molecular motion. Azobenzene was first described by Eilhard Mitscherlich in 1834. Condensation with anilines affords azobenzene derivatives in a reaction known as the Mills reaction. Organic compounds have been deployed as photoswitches and a popular example of it is azobenzene. Thus, azobenzene is an example of an organic diazene. In alkaline solution, azobenzene results, whereas arsenic acid produces the violet-coloring matter violaniline.