chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, photoisomerization is a form of

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

ization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a reversible process.

Applications

Photoisomerization of the compound

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

in the eye allows for vision. Photoisomerizable substrates have been put to practical use, for instance, in

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

s for rewritable CDs,

DVDs The DVD (common abbreviation for digital video disc or digital versatile disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kin ...

, and 3D optical data storage solutions. In addition, interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches,

molecular motor Molecular motors are natural (biological) or artificial molecular machines that are the essential agents of movement in living organisms. In general terms, a motor is a device that consumes energy in one form and converts it into motion or mech ...

s, and

molecular electronics Molecular electronics is the study and application of molecular building blocks for the fabrication of electronic components. It is an interdisciplinary area that spans physics, chemistry, and materials science. It provides a potential means to ...

. Another class of device that uses the photoisomerization process is as an additive in

liquid crystals Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...

to change their linear and nonlinear properties. Due to the photoisomerization is possible to induce a molecular reorientation in the liquid crystal bulk, which is used in

holography Holography is a technique that allows a wavefront to be recorded and later reconstructed. It is best known as a method of generating three-dimensional images, and has a wide range of other uses, including data storage, microscopy, and interfe ...

, as

spatial filter A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of light or other electromagnetic radiation, typically coherent laser light. Spatial filtering is commonly used to "clean up" the ...

or optical switching. Methyl red

Examples

Azobenzenes,

stilbene Stilbene may refer to one of the two stereoisomers of 1,2-diphenylethene: * (''E'')-Stilbene (''trans'' isomer) * (''Z'')-Stilbene (''cis'' isomer) See also * Stilbenoid Stilbenoids are hydroxylated derivatives of stilbene. They have a C6–C ...

spiropyran A spiropyran is a type of photochromic organic chemical compound, characterized by their ability to reversibly switch between two structural forms—spiropyran and merocyanine—upon exposure to light or other external stimuli. This reversible trans ...

s, are prominent classes of compounds subject to photoisomerism. : Synthesis of quadricyclane from norbornadiene

In the presence of a catalyst, norbornadiene converts to quadricyclane via ~300nm

UV radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

. When converted back to norbornadiene, quadryicyclane’s ring strain energy is liberated in the form of heat (''ΔH'' = −89 kJ/mol). This reaction has been proposed to store

solar energy Solar energy is the radiant energy from the Sun's sunlight, light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy (including solar water heating) and solar architecture. It is a ...

( photoswitchs). Photoisomerization behavior can be roughly categorized into several classes. Two major classes are ''trans''–''cis'' (or ''E''–''Z'') conversion, and open-closed ring transition. Examples of the former include

and

azobenzene Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a azo compound, N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide c ...

. This type 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. This type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the

di-π-methane rearrangement In organic chemistry, the di-π-methane rearrangement is the Organic photochemistry, photochemical Rearrangement reaction, rearrangement of a molecule that contains two pi bond, π-systems separated by a saturated carbon atom. In the Aliphatic c ...

Coordination chemistry

Many complexes are often photosensitive and many of these complexes undergo photoisomerization. One case is the conversion of the colorless cis- bis(triphenylphosphine)platinum chloride to the yellow trans isomer. Cis-trans-PtCl2P2

Some

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

es undergo change in their spin state upon illumination, i.e. these are photosensitive

spin crossover Spin crossover (SCO) is a phenomenon that occurs in some metal Coordination complex, complexes wherein the Spin states (d electrons), spin state of the complex changes due to an external stimulus. The stimuli can include temperature, pressure or ...

complexes.

References

{{reflist Chemical reactions