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. [1] However, photochromic compounds are the necessary building blocks for light driven molecular motors and machines. [2] Upon irradiation with light, photoisomerization about double bonds in the molecule can lead to changes in the cis- or trans- configuration. [3] 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, [6] spiropyran, [7] merocyanine, [8] diarylethene, [9] spirooxazine, [10] fulgide, [11] hydrazone, [12] nobormadiene, [13] thioindigo, [14] acrylamide-azobenzene-quaternary ammonia, [15] donor-acceptor Stenhouse adducts, [16] [17] stilbene, [18] 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. [19]
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. [20] 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. [21]
One of the most important properties of a photoswitch is its quantum yield which measures the effectiveness of absorbed light to induce photoisomerization. Quantum yield is modeled and calculated using Arrhenius kinetics. [22] 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. [23] Through chemical modification, red shifting the wavelengths of absorption needed to cause isomerizaiton leads to low light induced switching which has applications in photopharmacology. [24]
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. [25] 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. [26]
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. [28] These include the regulation of melanocytes , vision, the release of melatonin and the control of the circadian rhythm, etc. [29] Rhodopsins are highly efficient photochromic compounds that can undergo fast photoisomerization and are associated with various retinal proteins [30] along with light-gated channels and pumps in microbes. [31]
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. [32] Companies involved in this area include Novartis, Vedere, Allergan, and Nanoscope Therapeutics. [33]
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. [34] For example, optical control of ligand binding in human serum albumin has been demonstrated to influence its allosteric binding properties. [35] Also, red-shifted azobenzenes have been used to control ionotropic glutamate receptors. [36]
Photoswitches are studied in biology, materials chemistry, and physics and have a wide variety of potential applications, especially in the framework of nanotechnology. [37]
Depending on the isomeric state, photoswitches have the potential to replace transistors used in electronics. [38] 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. [39]
Diarylethenes form stable molecular conduction junctions when placed between graphene electrodes at low and room temperature and act as a photo-electrical switch. [40] 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. [39] A photo-electric cell is connected to a circuit that measures how much electricity the cell generates. The circuit decides and gives the output, according to the setting of minimum and maximum lux level. [41]
Photoswitches have been used in the generation of three-dimensional animations and images. [42] 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.
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. [43]
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. [44]
Incorporation of photoswitchable molecules into porous metal organic frameworks that can 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. [44]
Incorporation of photoswitching molecules such as donor-acceptor Stenhouse adducts into polymersomes has been used to form nanoparticles which can selectively expose enzymes in response to light, allowing them to mimic some functions of cells. [45]
Chiral shape driven transformations in liquid crystal structures can be achieved through photoisomerization of bistable hydrazones to generate long term stable polymer shapes. [46] 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. [47] Incorporation of photoswitches into nematic liquid crystals can change self-assembly, crystal packing, and the light reflecting properties of the supramolecular interactions. [48]
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. [49] Novel azo-carrying photoswitches are introduced as molecular hinges, [50] [51] which can be used in the design of molecular machines and optical devices. [52]
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. [53] 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. [54]
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. [44]
In molecular biology and biotechnology, a fluorescent tag, also known as a fluorescent label or fluorescent probe, is a molecule that is attached chemically to aid in the detection of a biomolecule such as a protein, antibody, or amino acid. Generally, fluorescent tagging, or labeling, uses a reactive derivative of a fluorescent molecule known as a fluorophore. The fluorophore selectively binds to a specific region or functional group on the target molecule and can be attached chemically or biologically. Various labeling techniques such as enzymatic labeling, protein labeling, and genetic labeling are widely utilized. Ethidium bromide, fluorescein and green fluorescent protein are common tags. The most commonly labelled molecules are antibodies, proteins, amino acids and peptides which are then used as specific probes for detection of a particular target.
Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a 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 class of similar compounds. These azo compounds are considered as derivatives of diazene (diimide), and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes. Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.
Diarylethene is the general name of a class of chemical compounds that have aromatic functional groups bonded to each end of a carbon–carbon double bond. The simplest example is stilbene, which has two geometric isomers, E and Z.
In chemistry, photoisomerization is a form of isomerization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a reversible process.
Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimicking macromolecular devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as DNA replication and ATP synthesis. Kinesins and ribosomes are examples of molecular machines, and they often take the form of multi-protein complexes. For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. The first example of an artificial molecular machine (AMM) was reported in 1994, featuring a rotaxane with a ring and two different possible binding sites. In 2016 the Nobel Prize in Chemistry was awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa for the design and synthesis of molecular machines.
Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra.
Synthetic molecular motors are molecular machines capable of continuous directional rotation under an energy input. Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion, some groups also use the term when referring to non-biological, non-peptide synthetic motors. Many chemists are pursuing the synthesis of such molecular motors.
Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction. The idea is conceptually similar to photosynthesis in plants, which converts solar energy into the chemical bonds of glucose molecules, but without using living organisms, which is why it is also called artificial photosynthesis.
Light-gated ion channels are a family of ion channels regulated by electromagnetic radiation. Other gating mechanisms for ion channels include voltage-gated ion channels, ligand-gated ion channels, mechanosensitive ion channels, and temperature-gated ion channels. Most light-gated ion channels have been synthesized in the laboratory for study, although two naturally occurring examples, channelrhodopsin and anion-conducting channelrhodopsin, are currently known. Photoreceptor proteins, which act in a similar manner to light-gated ion channels, are generally classified instead as G protein-coupled receptors.
A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electric current, microenvironment, or in the presence of ions and other ligands. In some cases, a combination of stimuli is required. The oldest forms of synthetic molecular switches are pH indicators, which display distinct colors as a function of pH. Currently synthetic molecular switches are of interest in the field of nanotechnology for application in molecular computers or responsive drug delivery systems. Molecular switches are also important in biology because many biological functions are based on it, for instance allosteric regulation and vision. They are also one of the simplest examples of molecular machines.
Stefan Hecht is a German chemist.
Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.
A spiropyran is a type of organic chemical compound, known for photochromic properties that provide this molecule with the ability of being used in medical and technological areas. Spiropyrans were discovered in the early twentieth century. However, it was in the middle twenties when Fisher and Hirshbergin observed their photochromic characteristics and reversible reaction. In 1952, Fisher and co-workers announced for the first time photochromism in spiropyrans. Since then, there have been many studies on photochromic compounds that have continued up to the present.
Photoactivated peptides are modified natural or synthetic peptides the functions of which can be activated with light. This can be done either irreversibly or in a reversible way. Caged peptides which contain photocleavable protecting groups belong to irreversibly activated peptides. Reversible activation/deactivation of peptide function are achieved by incorporation photo-controllable fragments in the side chains or in the backbone of peptide templates to get the photo-controlled peptides, which can reversibly change their structure upon irradiation with light of different wavelength. As the consequence, the properties, function and biological activity of the modified peptides can be controlled by light. Since light can be directed to specific areas, such peptides can be activated only at targeted sites. Azobenzenes, and diarylethenes can be used as the photoswitches. For therapeutic use, photoswitches with longer wavelengths or the use of two-photon excitation are required, coupled with improved methods for peptide delivery to live cells.
Quasi-crystals are supramolecular aggregates exhibiting both crystalline (solid) properties as well as amorphous, liquid-like properties.
Bernard Lucas Feringa is a Dutch synthetic organic chemist, specializing in molecular nanotechnology and homogeneous catalysis. He is the Jacobus van 't Hoff Distinguished Professor of Molecular Sciences, at the Stratingh Institute for Chemistry, University of Groningen, Netherlands, and an Academy Professor of the Royal Netherlands Academy of Arts and Sciences. He was awarded the 2016 Nobel Prize in Chemistry, together with Sir J. Fraser Stoddart and Jean-Pierre Sauvage, "for the design and synthesis of molecular machines".
Natalia B. Shustova is a Fred M. Weissman Palmetto Professor of Chemistry at the University of South Carolina. She focuses on developing materials for sustainable energy conversion, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and graphitic supramolecular structures.
Lu Shin Wong is a Senior Lecturer in the Department of Chemistry at The University of Manchester. His research in general is based on industrial biotechnology and materials chemistry, specifically on nanofabrication and biocatalysis.
In organic chemistry, a fulgide is any of a class of photochromic compounds consisting of a bismethylene-succinic anhydride core that has an aromatic group as a substituent. The highly conjugated system is a good chromophore. It can undergo reversible photoisomerization induced by ultraviolet light, converting between the E and Z isomers, both of which are typically colorless compounds. Unlike the more-stable Z isomer, the E isomer can also undergo a photochemically-induced electrocyclic reaction, forming a new ring and becoming a distinctly colored product called the C form. It is thus the two-step Z–C isomerization that is the photochromic change starting from the stable uncyclized form.