User:Btotheotherb/Photoswitch

= Photoswitch = 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 conformation 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- state. These photochromic molecules have a wide range of applications across biology, chemistry, and physics.

= Chemical Photoswitch = A photochromic compound can change its conformation upon irradiation with light and includes the following as examples: azobenzene, spiropyran , merocyanine , diarylethene , spirooxazine , fulgide , hydrazone , nobormadiene , thioindigo , Acrylamide-Azobenzene-Quaternary Ammonia (AAQ) , Donor-Acceptor Stenhouse Adducts (DASA) , 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 transition 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, the increasing evidence supports cis-/trans- isomerization of polyenes favoring the Hula Twist (HT) rather than the One-Bond-Flip (OBF). The OBF isomerizes at

the reactive double bond while the HT undergoes a conformational isomerization at the adjacent single bond. However, the interconversion of stereoisomers of stilbene proceeds via OBF.

Quantum Yield
One of the most important properties of a photoswitch is the quantum yield which measures how well light is absorbed to induce photoisomerization and is modeled and calculated using Arrhenius kinetics. 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 the field of 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. Among the most widely studied photoswitches, azobenzene has been studied as 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 Photoswitch = One of the more prevalent biological examples in the human body that are responsible to changes from light irradiation include 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.

Advances in vision restoration by studying natural photochromic compounds has shown promise in the last ten years. The fast isomerization is what allows retinal cells to turn on when activated by light and recent advances in AAQ have shown restoration of visual responses in blind mice. Companies like Novartis, Vedere, Allergan, Nanoscope Therapeutics, etc. have been advancing the field of optogenetics and have recently entered clinical trials for vision restoration by using various forms of gene therapy and insertion of opsin based derivative molecules into damaged cells.

Biological Applications
Through the incorporation of photoswitches into biological molecules, biological processes can be regulated and controlled by the 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 and can influence the allosteric binding properties. Also, red-shifted azobenzenes have been used to control ionotropic glutamate receptors. = Applications = Photoswitches are utilized by many disciplines of science that include biology, materials chemistry, and physics and have a wide variety of applications that are crucial to the framework of nanotechnology.

Electronics
Depending on the isomeric state, photoswitches can be turned on and off and have the potential to one day replace transistors used in electronics. Through the attachment of photoswitches onto the surfaces of different 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 have been shown to 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 different HOMO and LUMO 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. A photo-electric cell is connected to a circuit that measures how much electricity the cell produces and according to the setting of minimum and maximum lux level, the circuit decides and gives the output.

Photoswitches have recently also 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 (DLP) 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. The device is capable of displaying a minimum voxel size of 0.68 mm^3, with 200 μm resolution, and good stability over hundreds of on-off cycles.

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.

Mercocyanine 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 generated a storage potential as well as a pH gradient.

Guest Uptake and Release
Incorporation of photoswitchable molecules in porous metal organic frameworks can be used for the uptake of gaseous molecules like carbon dioxide as well as contribute to optoelectronics, nanomedicine, and better energy storage. By changing the chemical properties of the pores, adsorption and desorption of gases can be tuned for advancements in smart membrane materials.

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 through the chiral doping of liquid crystals with hydrazone photoswitches, 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.

Photopharmacology
In the field of photopharmacology, photoswitches are used to obtain control over the activity. By including a photoswitch to the drugs, a drug with several states is created, all having their own biological activity. Light can be used to switch between these states, resulting in drugs with remote control over the activity. Photoswiches have been shown to be able to change the surface energy properties of nanoparticles 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
Two strategies have been incorporated to utilize photoswitches 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.

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