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Other types of intelligent textiles are those which change their colour reversibly according to external environmental conditions, for this reason they are also called chameleon fibres. Chromic materials are the general term referring to materials which radiate the colour, erase the colour or just change it because its induction caused by the external stimuli, as "chromic" is a suffix that means colour. So we can classify chromic materials depending on the stimuli affecting them: • • • • • •
Photochromic: external stimuli energy is light. Thermochromic: external stimuli energy is heat. Electrochromic: external stimuli energy is electricity. Piezorochromic: external stimuli energy is pressure. Solvatechromic: external stimuli energy is liquid. Carsolchromic: external stimuli energy is electron beam.
The phenomenon produced in photochromic materials is called photochromism, where the change in colour is due to incident light. However, to date, photochromism is most important for optical switching data and imaging systems, rather than in textile applications. It is possible to classify photochromic fibres into different groups: those which emit the colour when activated by the visible light and those fibres which emit the colour when activated by ultraviolet radiation. Although in the first group we can find both organic and inorganic materials the most studied are the former because they are colourful, have high density and a wide range of application. Organic photochromic materials generally do not show this phenomenon in crystal form, they show their photochromism after melting in some solvent. The problem is that material behaviour (such as colour emitted, reaction speed, resistance, density, etc) is largely effected in a positive or negative way by the solvent nature. For this reason, in order to apply these materials to fibres, it is important to consider which solvent needs to be used. There are fibres which emit fluorescent colour, for example red, green or blue under ultraviolet radiation in a dark place, though they maintain their original colour when exposed to natural light. The inorganic fluorescent paints used for this purpose are mixed at an approximate rate of 10% in the liquid during the spinning operation. It is important to note that the colour can be freely controlled by mixing various inorganic paints together or by adding the paints to the natural colour of the threads. Thermochromic materials are those whose colour changes as a result of reaction to heat, especially through the application of thermochromic dyes whose colours change at particular temperatures. Two types of thermochromic systems that have been used successfully in textiles are: the liquid crystal type and the molecular rearrangement type. In both cases, the dyes are entrapped in microcapsules, applied to garment fabric like a pigment in a resin binder. The most important types of liquid crystal for thermochromic systems are the so-called cholesteric types, where adjacent molecules are arranged so that they form helices. Thermochromism results from the selective reflection of light by the liquid crystal. The wavelength of the light reflected is governed by the refractive index of the liquid crystal and by the
pitch of the helical arrangement of its molecules. Since the length of the pitch varies with temperature, the wavelength of the reflected light is also altered, and colour changes result. An alternative means of inducing thermochromism is by means of a rearrangement of the molecular structure of a dye, as a result of a change in temperature. The most common types of dye which exhibit thermochromism through molecular rearrangement are the spirolactones, although other types have also been identified. A colourless dye precursor and a colour developer are both dissolved in an organic solvent. The solution is then microencapsulated, and is solid at lower temperatures. Upon heating, the system becomes coloured or loses colour at the melting point of the mixture. The reverse change occurs at this temperature if the mixture is then cooled. However, although thermochromism through molecular rearrangement in dyes has aroused a degree of commercial interest, the overall mechanism underlying the changes in colour is far from clear-cut and is still very much open to speculation. In addition to the changing of colour due to reaction to light or heat there are other chromic fibres presenting other characteristics. These fibres have raised the interest of people because of their surprising and interesting nature. Therefore, there is the problem that this "boom" will soon come to an end because these fibres are only considered to be a temporary fashion material. In order to establish these fibres in everyday life it is specially necessary to improve their endurance to light and to their accuracy. Some of these fibres are those that present the phenomenon called solvate chromism, whose colour changes when in with a liquid, for example water. These materials are normally used for "design" swimsuits. Other materials have applied paints which can store light and these are used in working clothes for road works/repairs in bad light situations or for marking arrows on carpets to guide people during a power failure. Apart from this, the most important application for chromic materials is fashion, to create fantasy designs changing its colour depending on the volume of incident light.
Chromism From Wikipedia, the free encyclopedia
Jump to: navigation, search In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances. It is known that there are many natural compounds that have chromism, and many artificial compounds with specific chromism have been synthesized to date. Chromism is classified by what kind of stimuli are used. The major kinds of chromism are as follows. •
thermochromism is chromism that is induced by heat, that is, a change of temperature. This is the most common chromism of all.
•
•
• •
photochromism is induced by light irradiation. This phenomenon is based on the isomerization between two different molecular structures, light-induced formation of color centers in crystals, precipitation of metal particles in a glass, or other mechanisms. electrochromism is induced by the gain and loss of electrons. This phenomenon occurs in compounds with redox active sites, such as metal ions or organic radicals. solvatochromism depends on the polarity of the solvent. Most solvatochromic compounds are metal complexes. cathodochromism is induced by electron beam irradiation.
Chromic phenomena Chromic phenomena are those phenomena in which color is produced when light interacts with materials in a variety of ways. These can be categorized under the following five headings: • • • • •
Stimulated (reversible) color change The absorption and reflection of light The absorption of energy followed by the emission of light The absorption of light and energy transfer (or conversion) The manipulation of light.
Color changing phenomena Phenomena which involve the change in color of a chemical compound take their name from the type of external influence, either chemical or physical, which is involved. Many of these phenomena are reversible. The following list includes all the classic chromisms plus others of increasing interest in newer outlets. • • • • • • • • • • • • • • • • • •
Photochromism - color change caused by light. Thermochromism - color change caused by heat. Electrochromism - color change caused by an electrical current. Gasochromism- color change caused by a gas - hydrogen/oxygen redox. Solvatochromism - color change caused by solvent polarity. Vapochromism - color change caused by vapour of an organic compound due to chemical polarity/polarisation. Ionochromism - color change caused by ions. Halochromism - color change caused by a change in pH. Mechanochromism - color change caused by mechanical actions. Tribochromism - color change caused by mechanical friction. Piezochromism - color change caused by mechanical pressure. Cathodochromism - color change caused by electron beam irradiation. Radiochromism - color change caused by ionising radiation. Magnetochromism - color change caused by magnetic field. Biochromism - color change caused by interfacing with biological entity. Chronochromism - color change indirectly as a result of the age of time. Aggregachromism - color change on dimerisation/aggregation of chromophores. Crystallochromism - color change due to changes in crystal structure of a chromophore.
Commercial applications of color change materials are very common and include photochromics in ophthalmics, fashion/cosmetics and optical memory and optical switches, thermochromics in paints, plastics and textiles and architecture, electrochromics in car mirrors and smart windows, and solvatochromics in biological probes.
Dyes and Pigments Classical dyes and pigments produce color by the absorption and reflection of light; these are the materials that make a major impact on the color of our daily lives. In 2000, world production of organic dyes was 800,000 tonnes and of organic pigments, 250,000 tonnes. There is also a very large production of inorganic pigments. Organic dyes are used mainly to color textile fibers, paper, hair, leather, while pigments are used largely in inks, paints and plastics. Dyes are also made using the properties of chromic substances: Photochromic dyes and Thermochromic dyes
Luminescence The absorption of energy followed by the emission of light is often described by the term luminescence. The exact term used is based on the energy source responsible for the luminescence as in color-change phenomena. • • • • • •
Electrical - electroluminescence Galvanoluminescence Sonoluminescence. Photons (light) - Photoluminescence Fluorescence Phosphorescence Biofluorescence. Chemical - Chemiluminescence Bioluminescence Electrochemiluminescence. Thermal - Thermoluminescence Pyroluminescence Candololuminescence. Electron Beam Cathodoluminescence Anodoluminescence Radioluminescence. Mechanical - Triboluminescence Fractoluminescence Mechanoluminescence Crystalloluminescence Lyoluminescence Elasticoluminescence.
Many of these phenomena are widely used in consumer products and other important outlets. Cathodoluminescence is used in cathode ray tubes, photoluminescence in fluorescent lighting and plasma display s, phosphorescence in safety signs and low energy lighting, fluorescence in pigments, inks, optical brighteners, safety clothing, and biological and medicinal analysis and diagnostics, chemoluminescence and bioluminescence in analysis, diagnostics and sensors, and electroluminescence in the burgeoning areas of light-emitting diodes (LEDs/OLEDs), displays and lighting. Important new developments are taking place in the areas of quantum dots and metallic nanoparticles.
Light and energy transfer Absorption of light and energy transfer (or conversion) involves colored molecules that can transfer electromagnetic energy, usually from a laser light source, to other molecules in another form of energy, such as thermal or electrical. These laser addressable colorants are used in optical data storage, organic photoconductors, in photomedicine (such as photodynamic therapy of cancer, photodiagnosis and photoinsecticides). The absorption of natural sunlight solar energy by chromophores is exploited in solar cells for the production of electrical energy by inorganic
photovoltaics and dye sensitized solar cell (DSSC) and also in the production of useful chemicals via artificial photosynthesis.
Thermochromism From Wikipedia, the free encyclopedia
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A mood ring shown face front. Note the band of color change. Thermochromism is the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses such as baby bottles (changes to a different color when cool enough to drink) and kettles. Thermochromism is one of several types of chromism. The two basic approaches are based on liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.
Principles Thermochromatic liquid crystals Some liquid crystals are capable of displaying different colors at different temperatures. This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low-temperature crystallic phase, through anisotropic chiral or twisted nematic phase, to the hightemperature isotropic liquid phase. Only the nematic mesophase has thermochromic properties; this restricts the effective temperature range of the material. The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light ing through the crystal undergoes Bragg diffraction on these layers, and the wavelength with the greatest constructive interference is reflected back, which is perceived as a spectral color. A change in the crystal temperature can result in a change of spacing between the layers and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from non-reflective (black) through the spectral colors to black again, depending on the temperature. Typically, the high temperature state will reflect blue-violet, while the low-temperature state will reflect
red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state. Some such materials are cholesteryl nonanoate or cyanobiphenyls. Liquid crystals used in dyes and inks often come microencapsulated, in the form of suspension. Liquid crystals are used in applications where the color change has to be accurately defined. They find applications in thermometers for room, refrigerator, aquarium, and medical use, and in indicators of level of propane in tanks. Liquid crystals are difficult to work with and require specialized printing equipment. The material itself is also typically more expensive than alternative technologies. High temperatures, ultraviolet radiation, some chemicals and/or solvents have a negative impact on their lifespan.
Leuco dyes Main article: Leuco dye
Example of a Hypercolor t-shirt. A hairdryer was used to change the blue to turquoise.
Another example of a Hypercolor t-shirt. Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a color change (usually between the colorless leuco form and the colored form) in dependence on temperature. The dyes are rarely applied on materials directly; they are usually in the form of microcapsules with the mixture sealed inside. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric; when the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact halochromism.
The dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazole derivates, and 4hydroxycoumarin and act as proton donors, changing the dye molecule between its leuco form and its protonated colored form; stronger acids would make the change irreversible. Leuco dyes have less accurate temperature response than liquid crystals. They are suitable for general indicators of approximate temperature ("too cool", "too hot", "about OK"), or for various novelty items. They are usually used in combination with some other pigment, producing a color change between the color of the base pigment and the color of the pigment combined with the color of the non-leuco form of the leuco dye. Organic leuco dyes are available for temperature ranges between about −5 °C and 60 °C, in wide range of colors. The color change usually happens in a 3 °C interval. Leuco dyes are used in applications where temperature response accuracy is not critical: eg. novelties, bath toys, flying discs, and approximate temperature indicators for microwave-heated foods. Microencapsulation allows their use in wide range of materials and products. The size of the microcapsules typically ranges between 3– 5 µm (over 10 times larger than regular pigment particles), which requires some adjustments to printing and manufacturing processes. An application of leuco dyes is in the Duracell battery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply. The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temperature for the leuco dye then becomes colored. Exposure to ultraviolet radiation, solvents and high temperatures reduce the lifespan of leuco dyes. Temperatures above about 200–230 °C typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C is allowed during manufacturing.
Materials Inks Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change color with exposure to heat. They come in two forms, liquid crystals and leuco dyes. Liquid crystals are used in mood rings. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometers, battery testers, clothing, and the indicator on bottles of maple syrup that change color when the syrup is warm. The most well-known line of clothing utilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperature via the forehead. Coors light uses thermochromic ink on its cans now, changing from white to blue to indicate the can is cold.
Paints Thermochromic paint is a relatively recent development in the area of color-changing pigments. It involves the use of liquid crystals or leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the
pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or transparent, therefore changing the appearance of the mug.
Papers Thermochromic papers are used for thermal printers. One example is the paper impregnated with the solid mixture of a fluoran dye with octadecylphosphonic acid. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes chemical reaction in the liquid phase, and assumes the protonated colored form. This state is then conserved when the matrix solidifies again, if the cooling process is fast enough. As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years; this may lead to interesting effects in combination with ing records, receipts from a thermal printer, and a tax audit.
Others Another good example of this is the color indicators on batteries. The indicator turns green if the battery still possesses a charge. This works by ing the charge of the battery through a small resistor on the battery, and causes the pigment to absorb heat. Once the paint has absorbed enough heat from the current of the battery, it changes from black to green (usually), thus indicating that the battery still has a fair amount of charge left in it. Another approach is using a resistor in the shape of a thin triangular layer, under a thermochromic pigment. The variable width of the resistor causes it to be heated unevenly, with the position of transition threshold temperature varying depending on the current the battery is providing. A simple-to-make thermochromic compound is zinc oxide, which is white at room temperature but when heated changes to yellow due to various types of crystal lattice defects. On cooling the zinc oxide reverts to white. Also lead(II) oxide has a similar color change on heating. These solids are technically semiconductors, and the color change is linked to their electronic properties. Cuprous mercury iodide (Cu2HgI4) undergoes a phase transition at 55 °C, reversibly changing from a bright red solid material at low temperature to a dark brown solid at high temperature. Other such material is mercury(II) iodide, a crystalline material which at 126 °C undergoes reversible phase transition from red alpha phase to pale yellow beta phase. Yet another example is nickel sulfate, green at room temperature but becoming yellow at 155 °C. Ag2HgI4 is yellow at low temperatures and orange above 47–51 °C.[1] Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infrared transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductor at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. [2][3] The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungsten lowers the transition temperature to 29 °C. Other thermochromic solid semiconductor materials investigated for commercial use are CdxZn1–xSySe1–y (x=0.5...1, y=0.5...1), ZnxCdyHg1–x–yOaSbSecTe1–a–b–c (x=0...0.5,
y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), HgxCdyZn1–x–ySbSe1–b (x=0...1, y=0...1, b=0.5...1).[4] Some minerals are thermochromic as well; for example some chromium-rich pyropes, normally reddish-purplish, become green when heated to about 80 °C.[5]
References 1. ^ "Thoughts of Amherst". .amherst.edu. 2. 3. 4. 5.
http://www3.amherst.edu/~thoughts/contents/amberger-thermochromism.html. Retrieved 2010-07-12. ^ "Sol-Gel Vanadium oxide". Solgel.com. http://www.solgel.com/articles/August00/thermo/Guzman.htm. Retrieved 2010-0712.[dead link] ^ "Intelligent Window Coatings that Allow Light In but Keep Heat Out – News Item". Azom.com. http://www.azom.com/details.asp?ArticleID=2587. Retrieved 2010-07-12. ^ Optical temperature indicator using thermochromic semiconductors U.S. Patent 5,499,597 ^ "Thermochromic Garnets". Minerals.gps.caltech.edu. http://minerals.gps.caltech.edu/mineralogy/undergrad/garnet_2001/garnet.html. Retrieved 2010-07-12.
Photochromism From Wikipedia, the free encyclopedia
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A photochromic eyeglass lens, after exposure to sunlight with part of the lens covered by paper. Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra.[1][2] Trivially, this can be described as a reversible change of colour upon exposure to light. The phenomenon was discovered in the late 1880s, including work by Markwald, who studied the reversible change of color of 2,3,4,4tetrachloronaphthalen-1(4H)-one in the solid state. He labeled this phenomenon "phototropy", and this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel proposed the term "photochromism".[3] Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example retinal in the vision process).
Overview Photochromism does not have a rigorous definition, but is usually used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes dramatically in strength or wavelength. In many cases, an absorbance band is present in only one form. The degree of change required for a photochemical reaction to be dubbed "photochromic" is that which appears dramatic by eye, but in essence there is no dividing line between photochromic reactions and other photochemistry. Therefore, while the trans-cis isomerization of azobenzene is considered a photochromic reaction, the analogous reaction of stilbene is not. Since photochromism is just a special case of a photochemical reaction, almost any photochemical reaction type may be used to produce photochromism with appropriate molecular design. Some of the most common processes involved in photochromism are pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction). Another requirement of photochromism is two states of the molecule should be thermally stable under ambient conditions for a reasonable time. All the same, nitrospiropyran (which back-isomerizes in the dark over ~10 minutes at room temperature) is considered photochromic. All photochromic molecules back-
isomerize to their more stable form at some rate, and this back-isomerization is accelerated by heating. There is therefore a close relationship between photochromic and thermochromic compounds. The timescale of thermal back-isomerization is important for applications, and may be molecularly engineered. Photochromic compounds considered to be "thermally stable" include some diarylethenes, which do not back isomerize even after heating at 80 C for 3 months. Since photochromic chromophores are dyes, and operate according to well-known reactions, their molecular engineering to fine-tune their properties can be achieved relatively easily using known design models, quantum mechanics calculations, and experimentation. In particular, the tuning of absorbance bands to particular parts of the spectrum and the engineering of thermal stability have received much attention. Sometimes, and particularly in the dye industry, the term "irreversible photochromic" is used to describe materials that undergo a permanent color change upon exposure to ultraviolet or visible light radiation. Because by definition photochromics are reversible, there is technically no such thing as an "irreversible photochromic"—this is loose usage, and these compounds are better referred to as "photochangable" or "photoreactive" dyes. Apart from the qualities already mentioned, several other properties of photochromics are important for their use. These include •
•
•
•
Quantum yield of the photochemical reaction. This determined the efficiency of the photochromic change with respect to the amount of light absorbed. The quantum yield of isomerization can be strongly dependent on conditions (see below). Fatigue resistance. In photochromic materials, fatigue refers to the loss of reversibility by processes such as photodegradation, photobleaching, photooxidation, and other side reactions. All photochromics suffer fatigue to some extent, and its rate is strongly dependent on the activating light and the conditions of the sample. Photostationary state. Photochromic materials have two states, and their interconversion can be controlled using different wavelengths of light. Excitation with any given wavelength of light will result in a mixture of the two states at a particular ratio, called the "photostationary state". In a perfect system, there would exist wavelengths that can be used to provide 1:0 and 0:1 ratios of the isomers, but in real systems this is not possible, since the active absorbance bands always overlap to some extent. Polarity and solubility. In order to incorporate photochromics in working systems, they suffer the same issues as other dyes. They are often charged in one or more state, leading to very high polarity and possible large changes in polarity. They also often contain large conjugated systems that limit their solubility.
Photochromic complexes A photochromic complex is a kind of chemical compound that has photoresponsive parts on its ligand. These complexes have a specific structure: photoswitchable organic compounds are attached to metal complexes. For the photocontrollable parts, thermally and photochemically stable chromophores (azobenzene, diarylethene, spiropyran, etc.) are usually used. And for the metal complexes, a wide variety of compounds that have various functions (redox response, luminescence, magnetism, etc.) are applied.
The photochromic parts and metal parts are so close that they can affect each other's molecular orbitals. The physical properties of these compounds shown by parts of them (i.e., chromophores or metals) thus can be controlled by switching their other sites by external stimuli. For example, photoisomerization behaviors of some complexes can be switched by oxidation and reduction of their metal parts. Some other compounds can be changed in their luminescence behavior, magnetic interaction of metal sites, or stability of metal-to-ligand coordination by photoisomerization of their photochromic parts.
Classes of photochromic materials Photochromic molecules can belong to various classes: triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spiro-oxazines, quinones and others.
[edit] Spiropyrans and spirooxazines
Spiro-mero photochromism. One of the oldest, and perhaps the most studied, families of photochromes are the spiropyrans. Very closely related to these are the spirooxazines. For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp³-hybridized "spiro" carbon. After irradiation with UV light, the bond between the spiro-carbon and the oxazine breaks, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms with ability to absorb photons of visible light, and therefore appear colorful. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond reforms, the spirocarbon becomes sp³ hybridized again, and the molecule returns to its colorless state. This class of photochromes in particular are thermodynamically unstable in one form and revert to the stable form in the dark unless cooled to low temperatures. Their lifetime can also be affected by exposure to UV light. Like most organic dyes they are susceptible to degradation by oxygen and free radicals. Incorporation of the dyes into a polymer matrix, adding a stabilizer, or providing a barrier to oxygen and chemicals by other means prolongs their lifetime.
Diarylethenes
Dithienylethene photochemistry. The "diarylethenes" were first introduced by Irie and have since gained widespread interest, largely on of their high thermodynamic stability. They operate by means of a 6-pi electrocyclic reaction, the thermal analog of which is impossible due to steric hindrance. Pure photochromic dyes usually have the appearance of a crystalline powder, and in order to achieve the color change, they usually have to be dissolved in a solvent or dispersed in a suitable matrix. However, some diarylethenes have so little shape change upon isomerization that they can be converted while remaining in crystalline form.
Azobenzenes
Azobenzene photoisomerization. The photochromic trans-cis isomerization of azobenzenes has been used extensively in molecular switches, often taking advantage of its shape change upon isomerization to produce a supramolecular result. In particular, azobenzenes incorporated into crown ethers give switchable receptors and azobenzenes in monolayers can provide light-controlled changes in surface properties.
Photochromic quinones Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the phenyl group to migrate from one oxygen atome to another. Quinones with good thermal stability have been prepared, and they also have the additional feature of redox activity, leading to the construction of many-state molecular switches that operate by a mixture of photonic and electronic stimuli.
Inorganic photochromics Many inorganic substances also exhibit photochromic properties, often with much better resistance to fatigue than organic photochromics. In particular, silver chloride is extensively used in the manufacture of photochromic lenses. Other silver and zinc halides are also photochromic.
Applications Sunglasses One of the most famous reversible photochromic applications is color changing lenses for sunglasses, as found in eye-glasses. The largest limitation in using PC technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time. The switching speed of photochromic dyes is highly sensitive to the rigidity of the environment around the dye. As result, they switch most rapidly in solution and slowest in the rigid environment like a polymer lens. Recently it has been reported that attaching flexible polymers with low glass transition temperature (for example siloxanes or poly(butyl acrylate)) to the dyes allows them to switch much more rapidly in a rigid lens.[4][5] Some spirooxazines with siloxane polymers attached switch at near solution-like speeds even though they are in a rigid lens matrix.
Supramolecular chemistry Photochromic units have been employed extensively in supramolecular chemistry. Their ability to give a light-controlled reversible shape change means that they can be used to make or break molecular recognition motifs, or to cause a consequent shape change in their surroundings. Thus, photochromic units have been demonstrated as components of molecular switches. The coupling of photochromic units to enzymes or enzyme cofactors even provides the ability to reversibly turn enzymes "on" and "off", by altering their shape or orientation in such a way that their functions are either "working" or "broken".
Data storage The possibility of using photochromic compounds for data storage was first suggested in 1956 by Yehuda Hirshberg.[6] Since that time, there have been many investigations by various academic and commercial groups, particularly in the area of 3D optical data storage which promises discs that can hold a terabyte of data. Initially, issues with thermal back-reactions and destructive reading dogged these studies, but more recently more-stable systems have been developed.[citation needed]
Novelty items Reversible photochromics are also found in applications such as toys, cosmetics, clothing and industrial applications. If necessary, they can be made to change between desired colors by combination with a permanent pigment.
Photochromic: external stimuli energy is light. Thermochromic: external stimuli energy is heat. Electrochromic: external stimuli energy is electricity. Piezorochromic: external stimuli energy is pressure. Solvatechromic: external stimuli energy is liquid. Carsolchromic: external stimuli energy is electron beam.
The phenomenon produced in photochromic materials is called photochromism, where the change in colour is due to incident light. However, to date, photochromism is most important for optical switching data and imaging systems, rather than in textile applications. It is possible to classify photochromic fibres into different groups: those which emit the colour when activated by the visible light and those fibres which emit the colour when activated by ultraviolet radiation. Although in the first group we can find both organic and inorganic materials the most studied are the former because they are colourful, have high density and a wide range of application. Organic photochromic materials generally do not show this phenomenon in crystal form, they show their photochromism after melting in some solvent. The problem is that material behaviour (such as colour emitted, reaction speed, resistance, density, etc) is largely effected in a positive or negative way by the solvent nature. For this reason, in order to apply these materials to fibres, it is important to consider which solvent needs to be used. There are fibres which emit fluorescent colour, for example red, green or blue under ultraviolet radiation in a dark place, though they maintain their original colour when exposed to natural light. The inorganic fluorescent paints used for this purpose are mixed at an approximate rate of 10% in the liquid during the spinning operation. It is important to note that the colour can be freely controlled by mixing various inorganic paints together or by adding the paints to the natural colour of the threads. Thermochromic materials are those whose colour changes as a result of reaction to heat, especially through the application of thermochromic dyes whose colours change at particular temperatures. Two types of thermochromic systems that have been used successfully in textiles are: the liquid crystal type and the molecular rearrangement type. In both cases, the dyes are entrapped in microcapsules, applied to garment fabric like a pigment in a resin binder. The most important types of liquid crystal for thermochromic systems are the so-called cholesteric types, where adjacent molecules are arranged so that they form helices. Thermochromism results from the selective reflection of light by the liquid crystal. The wavelength of the light reflected is governed by the refractive index of the liquid crystal and by the
pitch of the helical arrangement of its molecules. Since the length of the pitch varies with temperature, the wavelength of the reflected light is also altered, and colour changes result. An alternative means of inducing thermochromism is by means of a rearrangement of the molecular structure of a dye, as a result of a change in temperature. The most common types of dye which exhibit thermochromism through molecular rearrangement are the spirolactones, although other types have also been identified. A colourless dye precursor and a colour developer are both dissolved in an organic solvent. The solution is then microencapsulated, and is solid at lower temperatures. Upon heating, the system becomes coloured or loses colour at the melting point of the mixture. The reverse change occurs at this temperature if the mixture is then cooled. However, although thermochromism through molecular rearrangement in dyes has aroused a degree of commercial interest, the overall mechanism underlying the changes in colour is far from clear-cut and is still very much open to speculation. In addition to the changing of colour due to reaction to light or heat there are other chromic fibres presenting other characteristics. These fibres have raised the interest of people because of their surprising and interesting nature. Therefore, there is the problem that this "boom" will soon come to an end because these fibres are only considered to be a temporary fashion material. In order to establish these fibres in everyday life it is specially necessary to improve their endurance to light and to their accuracy. Some of these fibres are those that present the phenomenon called solvate chromism, whose colour changes when in with a liquid, for example water. These materials are normally used for "design" swimsuits. Other materials have applied paints which can store light and these are used in working clothes for road works/repairs in bad light situations or for marking arrows on carpets to guide people during a power failure. Apart from this, the most important application for chromic materials is fashion, to create fantasy designs changing its colour depending on the volume of incident light.
Chromism From Wikipedia, the free encyclopedia
Jump to: navigation, search In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances. It is known that there are many natural compounds that have chromism, and many artificial compounds with specific chromism have been synthesized to date. Chromism is classified by what kind of stimuli are used. The major kinds of chromism are as follows. •
thermochromism is chromism that is induced by heat, that is, a change of temperature. This is the most common chromism of all.
•
•
• •
photochromism is induced by light irradiation. This phenomenon is based on the isomerization between two different molecular structures, light-induced formation of color centers in crystals, precipitation of metal particles in a glass, or other mechanisms. electrochromism is induced by the gain and loss of electrons. This phenomenon occurs in compounds with redox active sites, such as metal ions or organic radicals. solvatochromism depends on the polarity of the solvent. Most solvatochromic compounds are metal complexes. cathodochromism is induced by electron beam irradiation.
Chromic phenomena Chromic phenomena are those phenomena in which color is produced when light interacts with materials in a variety of ways. These can be categorized under the following five headings: • • • • •
Stimulated (reversible) color change The absorption and reflection of light The absorption of energy followed by the emission of light The absorption of light and energy transfer (or conversion) The manipulation of light.
Color changing phenomena Phenomena which involve the change in color of a chemical compound take their name from the type of external influence, either chemical or physical, which is involved. Many of these phenomena are reversible. The following list includes all the classic chromisms plus others of increasing interest in newer outlets. • • • • • • • • • • • • • • • • • •
Photochromism - color change caused by light. Thermochromism - color change caused by heat. Electrochromism - color change caused by an electrical current. Gasochromism- color change caused by a gas - hydrogen/oxygen redox. Solvatochromism - color change caused by solvent polarity. Vapochromism - color change caused by vapour of an organic compound due to chemical polarity/polarisation. Ionochromism - color change caused by ions. Halochromism - color change caused by a change in pH. Mechanochromism - color change caused by mechanical actions. Tribochromism - color change caused by mechanical friction. Piezochromism - color change caused by mechanical pressure. Cathodochromism - color change caused by electron beam irradiation. Radiochromism - color change caused by ionising radiation. Magnetochromism - color change caused by magnetic field. Biochromism - color change caused by interfacing with biological entity. Chronochromism - color change indirectly as a result of the age of time. Aggregachromism - color change on dimerisation/aggregation of chromophores. Crystallochromism - color change due to changes in crystal structure of a chromophore.
Commercial applications of color change materials are very common and include photochromics in ophthalmics, fashion/cosmetics and optical memory and optical switches, thermochromics in paints, plastics and textiles and architecture, electrochromics in car mirrors and smart windows, and solvatochromics in biological probes.
Dyes and Pigments Classical dyes and pigments produce color by the absorption and reflection of light; these are the materials that make a major impact on the color of our daily lives. In 2000, world production of organic dyes was 800,000 tonnes and of organic pigments, 250,000 tonnes. There is also a very large production of inorganic pigments. Organic dyes are used mainly to color textile fibers, paper, hair, leather, while pigments are used largely in inks, paints and plastics. Dyes are also made using the properties of chromic substances: Photochromic dyes and Thermochromic dyes
Luminescence The absorption of energy followed by the emission of light is often described by the term luminescence. The exact term used is based on the energy source responsible for the luminescence as in color-change phenomena. • • • • • •
Electrical - electroluminescence Galvanoluminescence Sonoluminescence. Photons (light) - Photoluminescence Fluorescence Phosphorescence Biofluorescence. Chemical - Chemiluminescence Bioluminescence Electrochemiluminescence. Thermal - Thermoluminescence Pyroluminescence Candololuminescence. Electron Beam Cathodoluminescence Anodoluminescence Radioluminescence. Mechanical - Triboluminescence Fractoluminescence Mechanoluminescence Crystalloluminescence Lyoluminescence Elasticoluminescence.
Many of these phenomena are widely used in consumer products and other important outlets. Cathodoluminescence is used in cathode ray tubes, photoluminescence in fluorescent lighting and plasma display s, phosphorescence in safety signs and low energy lighting, fluorescence in pigments, inks, optical brighteners, safety clothing, and biological and medicinal analysis and diagnostics, chemoluminescence and bioluminescence in analysis, diagnostics and sensors, and electroluminescence in the burgeoning areas of light-emitting diodes (LEDs/OLEDs), displays and lighting. Important new developments are taking place in the areas of quantum dots and metallic nanoparticles.
Light and energy transfer Absorption of light and energy transfer (or conversion) involves colored molecules that can transfer electromagnetic energy, usually from a laser light source, to other molecules in another form of energy, such as thermal or electrical. These laser addressable colorants are used in optical data storage, organic photoconductors, in photomedicine (such as photodynamic therapy of cancer, photodiagnosis and photoinsecticides). The absorption of natural sunlight solar energy by chromophores is exploited in solar cells for the production of electrical energy by inorganic
photovoltaics and dye sensitized solar cell (DSSC) and also in the production of useful chemicals via artificial photosynthesis.
Thermochromism From Wikipedia, the free encyclopedia
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A mood ring shown face front. Note the band of color change. Thermochromism is the ability of substance to change color due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses such as baby bottles (changes to a different color when cool enough to drink) and kettles. Thermochromism is one of several types of chromism. The two basic approaches are based on liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.
Principles Thermochromatic liquid crystals Some liquid crystals are capable of displaying different colors at different temperatures. This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low-temperature crystallic phase, through anisotropic chiral or twisted nematic phase, to the hightemperature isotropic liquid phase. Only the nematic mesophase has thermochromic properties; this restricts the effective temperature range of the material. The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light ing through the crystal undergoes Bragg diffraction on these layers, and the wavelength with the greatest constructive interference is reflected back, which is perceived as a spectral color. A change in the crystal temperature can result in a change of spacing between the layers and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from non-reflective (black) through the spectral colors to black again, depending on the temperature. Typically, the high temperature state will reflect blue-violet, while the low-temperature state will reflect
red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state. Some such materials are cholesteryl nonanoate or cyanobiphenyls. Liquid crystals used in dyes and inks often come microencapsulated, in the form of suspension. Liquid crystals are used in applications where the color change has to be accurately defined. They find applications in thermometers for room, refrigerator, aquarium, and medical use, and in indicators of level of propane in tanks. Liquid crystals are difficult to work with and require specialized printing equipment. The material itself is also typically more expensive than alternative technologies. High temperatures, ultraviolet radiation, some chemicals and/or solvents have a negative impact on their lifespan.
Leuco dyes Main article: Leuco dye
Example of a Hypercolor t-shirt. A hairdryer was used to change the blue to turquoise.
Another example of a Hypercolor t-shirt. Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a color change (usually between the colorless leuco form and the colored form) in dependence on temperature. The dyes are rarely applied on materials directly; they are usually in the form of microcapsules with the mixture sealed inside. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric; when the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact halochromism.
The dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazole derivates, and 4hydroxycoumarin and act as proton donors, changing the dye molecule between its leuco form and its protonated colored form; stronger acids would make the change irreversible. Leuco dyes have less accurate temperature response than liquid crystals. They are suitable for general indicators of approximate temperature ("too cool", "too hot", "about OK"), or for various novelty items. They are usually used in combination with some other pigment, producing a color change between the color of the base pigment and the color of the pigment combined with the color of the non-leuco form of the leuco dye. Organic leuco dyes are available for temperature ranges between about −5 °C and 60 °C, in wide range of colors. The color change usually happens in a 3 °C interval. Leuco dyes are used in applications where temperature response accuracy is not critical: eg. novelties, bath toys, flying discs, and approximate temperature indicators for microwave-heated foods. Microencapsulation allows their use in wide range of materials and products. The size of the microcapsules typically ranges between 3– 5 µm (over 10 times larger than regular pigment particles), which requires some adjustments to printing and manufacturing processes. An application of leuco dyes is in the Duracell battery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply. The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temperature for the leuco dye then becomes colored. Exposure to ultraviolet radiation, solvents and high temperatures reduce the lifespan of leuco dyes. Temperatures above about 200–230 °C typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C is allowed during manufacturing.
Materials Inks Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change color with exposure to heat. They come in two forms, liquid crystals and leuco dyes. Liquid crystals are used in mood rings. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometers, battery testers, clothing, and the indicator on bottles of maple syrup that change color when the syrup is warm. The most well-known line of clothing utilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperature via the forehead. Coors light uses thermochromic ink on its cans now, changing from white to blue to indicate the can is cold.
Paints Thermochromic paint is a relatively recent development in the area of color-changing pigments. It involves the use of liquid crystals or leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the
pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or transparent, therefore changing the appearance of the mug.
Papers Thermochromic papers are used for thermal printers. One example is the paper impregnated with the solid mixture of a fluoran dye with octadecylphosphonic acid. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes chemical reaction in the liquid phase, and assumes the protonated colored form. This state is then conserved when the matrix solidifies again, if the cooling process is fast enough. As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years; this may lead to interesting effects in combination with ing records, receipts from a thermal printer, and a tax audit.
Others Another good example of this is the color indicators on batteries. The indicator turns green if the battery still possesses a charge. This works by ing the charge of the battery through a small resistor on the battery, and causes the pigment to absorb heat. Once the paint has absorbed enough heat from the current of the battery, it changes from black to green (usually), thus indicating that the battery still has a fair amount of charge left in it. Another approach is using a resistor in the shape of a thin triangular layer, under a thermochromic pigment. The variable width of the resistor causes it to be heated unevenly, with the position of transition threshold temperature varying depending on the current the battery is providing. A simple-to-make thermochromic compound is zinc oxide, which is white at room temperature but when heated changes to yellow due to various types of crystal lattice defects. On cooling the zinc oxide reverts to white. Also lead(II) oxide has a similar color change on heating. These solids are technically semiconductors, and the color change is linked to their electronic properties. Cuprous mercury iodide (Cu2HgI4) undergoes a phase transition at 55 °C, reversibly changing from a bright red solid material at low temperature to a dark brown solid at high temperature. Other such material is mercury(II) iodide, a crystalline material which at 126 °C undergoes reversible phase transition from red alpha phase to pale yellow beta phase. Yet another example is nickel sulfate, green at room temperature but becoming yellow at 155 °C. Ag2HgI4 is yellow at low temperatures and orange above 47–51 °C.[1] Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infrared transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductor at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. [2][3] The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungsten lowers the transition temperature to 29 °C. Other thermochromic solid semiconductor materials investigated for commercial use are CdxZn1–xSySe1–y (x=0.5...1, y=0.5...1), ZnxCdyHg1–x–yOaSbSecTe1–a–b–c (x=0...0.5,
y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), HgxCdyZn1–x–ySbSe1–b (x=0...1, y=0...1, b=0.5...1).[4] Some minerals are thermochromic as well; for example some chromium-rich pyropes, normally reddish-purplish, become green when heated to about 80 °C.[5]
References 1. ^ "Thoughts of Amherst". .amherst.edu. 2. 3. 4. 5.
http://www3.amherst.edu/~thoughts/contents/amberger-thermochromism.html. Retrieved 2010-07-12. ^ "Sol-Gel Vanadium oxide". Solgel.com. http://www.solgel.com/articles/August00/thermo/Guzman.htm. Retrieved 2010-0712.[dead link] ^ "Intelligent Window Coatings that Allow Light In but Keep Heat Out – News Item". Azom.com. http://www.azom.com/details.asp?ArticleID=2587. Retrieved 2010-07-12. ^ Optical temperature indicator using thermochromic semiconductors U.S. Patent 5,499,597 ^ "Thermochromic Garnets". Minerals.gps.caltech.edu. http://minerals.gps.caltech.edu/mineralogy/undergrad/garnet_2001/garnet.html. Retrieved 2010-07-12.
Photochromism From Wikipedia, the free encyclopedia
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A photochromic eyeglass lens, after exposure to sunlight with part of the lens covered by paper. Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra.[1][2] Trivially, this can be described as a reversible change of colour upon exposure to light. The phenomenon was discovered in the late 1880s, including work by Markwald, who studied the reversible change of color of 2,3,4,4tetrachloronaphthalen-1(4H)-one in the solid state. He labeled this phenomenon "phototropy", and this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel proposed the term "photochromism".[3] Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example retinal in the vision process).
Overview Photochromism does not have a rigorous definition, but is usually used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes dramatically in strength or wavelength. In many cases, an absorbance band is present in only one form. The degree of change required for a photochemical reaction to be dubbed "photochromic" is that which appears dramatic by eye, but in essence there is no dividing line between photochromic reactions and other photochemistry. Therefore, while the trans-cis isomerization of azobenzene is considered a photochromic reaction, the analogous reaction of stilbene is not. Since photochromism is just a special case of a photochemical reaction, almost any photochemical reaction type may be used to produce photochromism with appropriate molecular design. Some of the most common processes involved in photochromism are pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction). Another requirement of photochromism is two states of the molecule should be thermally stable under ambient conditions for a reasonable time. All the same, nitrospiropyran (which back-isomerizes in the dark over ~10 minutes at room temperature) is considered photochromic. All photochromic molecules back-
isomerize to their more stable form at some rate, and this back-isomerization is accelerated by heating. There is therefore a close relationship between photochromic and thermochromic compounds. The timescale of thermal back-isomerization is important for applications, and may be molecularly engineered. Photochromic compounds considered to be "thermally stable" include some diarylethenes, which do not back isomerize even after heating at 80 C for 3 months. Since photochromic chromophores are dyes, and operate according to well-known reactions, their molecular engineering to fine-tune their properties can be achieved relatively easily using known design models, quantum mechanics calculations, and experimentation. In particular, the tuning of absorbance bands to particular parts of the spectrum and the engineering of thermal stability have received much attention. Sometimes, and particularly in the dye industry, the term "irreversible photochromic" is used to describe materials that undergo a permanent color change upon exposure to ultraviolet or visible light radiation. Because by definition photochromics are reversible, there is technically no such thing as an "irreversible photochromic"—this is loose usage, and these compounds are better referred to as "photochangable" or "photoreactive" dyes. Apart from the qualities already mentioned, several other properties of photochromics are important for their use. These include •
•
•
•
Quantum yield of the photochemical reaction. This determined the efficiency of the photochromic change with respect to the amount of light absorbed. The quantum yield of isomerization can be strongly dependent on conditions (see below). Fatigue resistance. In photochromic materials, fatigue refers to the loss of reversibility by processes such as photodegradation, photobleaching, photooxidation, and other side reactions. All photochromics suffer fatigue to some extent, and its rate is strongly dependent on the activating light and the conditions of the sample. Photostationary state. Photochromic materials have two states, and their interconversion can be controlled using different wavelengths of light. Excitation with any given wavelength of light will result in a mixture of the two states at a particular ratio, called the "photostationary state". In a perfect system, there would exist wavelengths that can be used to provide 1:0 and 0:1 ratios of the isomers, but in real systems this is not possible, since the active absorbance bands always overlap to some extent. Polarity and solubility. In order to incorporate photochromics in working systems, they suffer the same issues as other dyes. They are often charged in one or more state, leading to very high polarity and possible large changes in polarity. They also often contain large conjugated systems that limit their solubility.
Photochromic complexes A photochromic complex is a kind of chemical compound that has photoresponsive parts on its ligand. These complexes have a specific structure: photoswitchable organic compounds are attached to metal complexes. For the photocontrollable parts, thermally and photochemically stable chromophores (azobenzene, diarylethene, spiropyran, etc.) are usually used. And for the metal complexes, a wide variety of compounds that have various functions (redox response, luminescence, magnetism, etc.) are applied.
The photochromic parts and metal parts are so close that they can affect each other's molecular orbitals. The physical properties of these compounds shown by parts of them (i.e., chromophores or metals) thus can be controlled by switching their other sites by external stimuli. For example, photoisomerization behaviors of some complexes can be switched by oxidation and reduction of their metal parts. Some other compounds can be changed in their luminescence behavior, magnetic interaction of metal sites, or stability of metal-to-ligand coordination by photoisomerization of their photochromic parts.
Classes of photochromic materials Photochromic molecules can belong to various classes: triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spiro-oxazines, quinones and others.
[edit] Spiropyrans and spirooxazines
Spiro-mero photochromism. One of the oldest, and perhaps the most studied, families of photochromes are the spiropyrans. Very closely related to these are the spirooxazines. For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp³-hybridized "spiro" carbon. After irradiation with UV light, the bond between the spiro-carbon and the oxazine breaks, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms with ability to absorb photons of visible light, and therefore appear colorful. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond reforms, the spirocarbon becomes sp³ hybridized again, and the molecule returns to its colorless state. This class of photochromes in particular are thermodynamically unstable in one form and revert to the stable form in the dark unless cooled to low temperatures. Their lifetime can also be affected by exposure to UV light. Like most organic dyes they are susceptible to degradation by oxygen and free radicals. Incorporation of the dyes into a polymer matrix, adding a stabilizer, or providing a barrier to oxygen and chemicals by other means prolongs their lifetime.
Diarylethenes
Dithienylethene photochemistry. The "diarylethenes" were first introduced by Irie and have since gained widespread interest, largely on of their high thermodynamic stability. They operate by means of a 6-pi electrocyclic reaction, the thermal analog of which is impossible due to steric hindrance. Pure photochromic dyes usually have the appearance of a crystalline powder, and in order to achieve the color change, they usually have to be dissolved in a solvent or dispersed in a suitable matrix. However, some diarylethenes have so little shape change upon isomerization that they can be converted while remaining in crystalline form.
Azobenzenes
Azobenzene photoisomerization. The photochromic trans-cis isomerization of azobenzenes has been used extensively in molecular switches, often taking advantage of its shape change upon isomerization to produce a supramolecular result. In particular, azobenzenes incorporated into crown ethers give switchable receptors and azobenzenes in monolayers can provide light-controlled changes in surface properties.
Photochromic quinones Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the phenyl group to migrate from one oxygen atome to another. Quinones with good thermal stability have been prepared, and they also have the additional feature of redox activity, leading to the construction of many-state molecular switches that operate by a mixture of photonic and electronic stimuli.
Inorganic photochromics Many inorganic substances also exhibit photochromic properties, often with much better resistance to fatigue than organic photochromics. In particular, silver chloride is extensively used in the manufacture of photochromic lenses. Other silver and zinc halides are also photochromic.
Applications Sunglasses One of the most famous reversible photochromic applications is color changing lenses for sunglasses, as found in eye-glasses. The largest limitation in using PC technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time. The switching speed of photochromic dyes is highly sensitive to the rigidity of the environment around the dye. As result, they switch most rapidly in solution and slowest in the rigid environment like a polymer lens. Recently it has been reported that attaching flexible polymers with low glass transition temperature (for example siloxanes or poly(butyl acrylate)) to the dyes allows them to switch much more rapidly in a rigid lens.[4][5] Some spirooxazines with siloxane polymers attached switch at near solution-like speeds even though they are in a rigid lens matrix.
Supramolecular chemistry Photochromic units have been employed extensively in supramolecular chemistry. Their ability to give a light-controlled reversible shape change means that they can be used to make or break molecular recognition motifs, or to cause a consequent shape change in their surroundings. Thus, photochromic units have been demonstrated as components of molecular switches. The coupling of photochromic units to enzymes or enzyme cofactors even provides the ability to reversibly turn enzymes "on" and "off", by altering their shape or orientation in such a way that their functions are either "working" or "broken".
Data storage The possibility of using photochromic compounds for data storage was first suggested in 1956 by Yehuda Hirshberg.[6] Since that time, there have been many investigations by various academic and commercial groups, particularly in the area of 3D optical data storage which promises discs that can hold a terabyte of data. Initially, issues with thermal back-reactions and destructive reading dogged these studies, but more recently more-stable systems have been developed.[citation needed]
Novelty items Reversible photochromics are also found in applications such as toys, cosmetics, clothing and industrial applications. If necessary, they can be made to change between desired colors by combination with a permanent pigment.
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