Photo-isomerization is the name given to the structural re-arrangement of asymmetric chemical compound molecules from one 'isomer' (e.g. 'left-handed' or 'right handed') to another, when illuminated.

The process is normally reversible, is dependent on the wavelength ('colour') of the light, and is extremely rapid, in the order of picoseconds.

Examples include relatively simple organic chemicals such as stilbene and azobenzene - and also more complicated compounds such as retinal, which is responsible for the detection of light in animal eyes.

The isomer-flipping is widely used in various chemical production processes, and in 'writable' digital optical media etc.

Other than the observation that the energy of the photons somehow flips the molecules, there is currently no exact description of the physical process behind the light-induced changes - even for the simplest chemicals ( See : J. Phys. Chem. A 2004, 108, 6, 950–95

Editor's note : This article has been given a 3 star importance rating. Without the as-yet-unexplained light-induced isomer flips of Retinal, you would not be reading this.

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