Photoswitch

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. Isomerization 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. Mechanism 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. Quantum yield the wavelengths of absorption needed to cause isomerization leads to low light induced switching which has applications in photopharmacology. Catalysis 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. == Biological ==

Rhodopsins 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. Research Advances in vision restoration with photochromic compounds has been investigated. Fast isomerization allows retinal cells to turn on when activated by light and advances in acrylamide-azobenzene-quaternary ammonia have shown restoration of visual responses in blind mice. Companies involved in this area include Novartis, Vedere, Allergan, and Nanoscope Therapeutics. Through the incorporation of photoswitches into biological molecules, biological processes can be regulated through controlled irradiation with light. This includes photocontrol of peptide conformation and activity, transcription and translation of DNA and RNA, regulation of enzymatic activity, and photoregulated ion channels. For example, optical control of ligand binding in human serum albumin has been demonstrated to influence its allosteric binding properties. Also, red-shifted azobenzenes have been used to control ionotropic glutamate receptors. ==Potential applications==

Photoswitches are studied in biology, materials chemistry, and physics and have a wide variety of potential applications, especially in the framework of nanotechnology. Electronics Depending on the isomeric state, photoswitches have the potential to replace transistors used in electronics. Through the attachment of photoswitches onto the surfaces of various substrates, the work function can be changed. For example, the incorporation of diarylethenes as a self-assembled monolayer on a gold surface shows promise in optoelectronic devices. Diarylethenes form stable molecular conduction junctions when placed between graphene electrodes at low and room temperature and act as a photo-electrical switch. By combining a photoswitch, containing various highest and lowest unoccupied molecular orbital levels in its open and closed geometrical conformation, into a film composed of either p- or n-doped semiconductors, charge transport can be controlled with light. Photoswitches have been used in the generation of three-dimensional animations and images. The display utilizes a medium composed of a class of photoswitches (known as spirhodamines) and digital light processing technology to generate structured light in three dimensions. UV light and green light patterns are aimed at the dye solution, which initiates photoactivation and thus creates the 'on' voxel. Energy storage Due to one of the photoisomers being more stable than the other, isomerization from the stable to metastable isomer results in a conversion of light energy into free energy as a form of a chemical potential and has applications in storing solar energy. Merocyanine has been shown to shuttle protons across a polymeric membrane upon irradiation with light. When UV and visible light were irradiated upon opposites sides of the membrane, a storage potential and pH gradient were generated. Liquid crystals Chiral shape driven transformations in liquid crystal structures can be achieved through photoisomerization of bistable hydrazones to generate long term stable polymer shapes. Light-gated optical windows that can change the absorbance properties can be made by chirally doping liquid crystals with hydrazone photoswitches or by kinetically trapping various cholesteric states as a function of the photostationary state. Incorporation of photoswitches into nematic liquid crystals can change self-assembly, crystal packing, and the light reflecting properties of the supramolecular interactions. Optical storage 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. Photopharmacology 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. Self-healing materials 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. == References ==