Fotoisomerisering

Fotoisomerisering is a photochemical reaction where a molecule undergoes a structural rearrangement upon absorption of light, resulting in an isomer of the original molecule. This process typically involves a change in the geometry around a double bond or a ring system. A common example is the cis-trans isomerization of alkenes, where light energy can cause the rotation around the carbon-carbon double bond, converting a cis isomer to a trans isomer or vice versa. Another significant example is the photoisomerization of retinal in rhodopsin, a protein in the retina responsible for vision. This light-induced change initiates a cascade of events leading to signal transduction in the eye.

The efficiency of fotoisomerisering is often measured by its quantum yield, which is the ratio of molecules isomerized to photons absorbed. The wavelength of light absorbed is crucial, as it must match the energy required to promote the molecule to an excited state from which isomerization can occur. After light absorption, the molecule typically enters an excited singlet state and may then undergo intersystem crossing to a triplet state before proceeding to the isomeric product. The reverse reaction, or the thermal relaxation back to the original isomer, can also occur. This phenomenon has applications in areas such as molecular switches, optical data storage, and the design of photosensitive materials.