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Type of molecule
A photoswitch is a type of molecule that can change its structural geometry and chemical properties upon irradiation with electromagnetic radiation. Although often used interchangeably with the term molecular machine, a switch does not perform work upon a change in its shape whereas a machine does. However, photochromic compounds are the necessary building blocks for light driven molecular motors and machines. Upon irradiation with light, photoisomerization about double bonds in the molecule can lead to changes in the cis or trans configuration. These photochromic molecules are being considered for a range of applications.

A photochromic compound can change its configuration or structure upon irradiation with light. Several examples of photochromic compounds include: azobenzene, spiropyran, merocyanine, diarylethene, spirooxazine, fulgide, hydrazone, nobormadiene, thioindigo, acrylamide azobenzene quaternary ammonia, donor acceptor Stenhouse adducts, stilbene, etc.

Upon isomerization from the absorption of light, a π to π* or n to π* electronic transition can occur with the subsequent release of light (fluorescence or phosphorescence) or heat when electrons transit from an excited state to a ground state. A photostationary state can be achieved when the irradiation of light no longer converts one form of an isomer into another; however, a mixture of cis and trans isomers will always exist with a higher percentage of one versus the other depending on the photoconditions.

Although the mechanism for photoisomerization is still debated amongst most scientists, increasing evidence supports cis /trans isomerization of polyenes favoring the hula twist rather than the one bond flip. The one bond flip isomerizes at the reactive double bond while the hula twist undergoes a conformational isomerization at the adjacent single bond. However, the interconversion of stereoisomers of stilbene proceeds via one bond flip.

[[File:Understanding Photoisomerization from the Photoisomerization from A to B. png|thumb|Photoisomerization from A to B: The three rates completely describe the isomerization from A to B where ϕA is the quantum yield, I is the photon flux, β is the fraction of photons absorbed by A, NA is the Avogadro constant, V is the volume of the sample. Photoswitches can be in solution or in the solid state; however, switching in the solid state is more difficult to observe due to the lack of molecular freedom of motion, solid packing, and the fast thermal reversion to the ground state. Through chemical modification, red shifting the wavelengths of absorption needed to cause isomerizaiton leads to low light induced switching which has applications in photopharmacology.

When a photochromic compound is incorporated into a suitable catalytic molecule, photoswitchable catalysis can result from the reversible changes in geometric conformation upon irradiation with light. As one of the most widely studied photoswitches, azobenzene has been shown to be an effective switch for regulating catalytic activity due to its isomerization from the E to Z conformation with light, and its ability to thermally relax back to the E isomer in dark conditions.

thumb|Retinal Photoswitch: The absorption of a photon converting cis retinal to trans retinal. Once converted, the trans retinal can disassociate from Opsin. Once converted back to the cis isomer, it can reform Rhodopsin.

One of the more prevalent biological examples in the human body that undergoes structural changes upon light irradiation includes the class of membrane bound photoreceptors, Rhodopsins. These include the regulation of melanocytes, vision, the release of melatonin and the control of the circadian rhythm, etc. Rhodopsins are highly efficient photochromic compounds that can undergo fast photoisomerization and are associated with various retinal proteins along with light gated channels and pumps in microbes.

Diarylethene photoswitches have been promising for use in rewritable optical storage. Through irradiation of light, writing, erasing, and reading can parallel CD/DVD storage with better performance. Novel azo carrying photoswitches are introduced as molecular hinges, which can be used in the design of molecular machines and optical devices.

In the field of photopharmacology, photoswitches are being investigated as a means to control activity. By including a photoswitch in a drug, the drug assumes several biological active states. Light can be used to switch between these states, resulting in remote control of a drug's activity. Photoswitches have also been shown modulate surface energy properties which can control how the photoswitchable shell interacts with nanoparticles. Pharmaceutical encapsulation and distribution at targeted locations with light has been demonstrated due to the unique change in properties and size of microencapsulated nanostructures with photochromic components.

Photoswitches have been investigated for self healable polymer materials. The first incorporates the phototunability of various functional groups so reactivity can be modulated in one of the isomeric forms, while the second strategy incorporates light driven valence bond tautomerization.