Thermochromism is the property of temperature dependence of the electronic absorption spectrum of a material, resulting in a color that depends on temperature. Formerly, the term thermochromism, also known as thermochromatism, was reserved for isolated compounds and their solutions. However, the advancement of the field has led to a broadening of this definition; the term thermochromism now can also be used to describe multicomponent mixtures that are able to change color in response to changes in temperature. For both isolated compounds and mixtures, reversibility of the color change generally is regarded as a necessary condition for thermochromic behavior.
Interestingly, thermal copy and receipt paper, which have been the most commercially important thermally responsive color-changing products for the past several decades, undergo an irreversible coloring reaction and therefore do not fall under the narrow technical definition of thermochromism. High-technology products such as thermal copy paper, which make use of multicomponent thermochromic mixtures, become thermochromic due to the thermal initiation of a secondary color-changing reaction. Halochromism, the color change of a compound in response to changing pH, is the temperature-dependent auxiliary process occurring in thermal copy paper which causes the color change.
Thermochromic compounds change color in response to temperature changes . Most thermochromic compounds are organic in nature and undergo thermally activated chemical modifications which give rise to the color change. In many cases the chemical modification is a result of tautomerism. Tautomerism refers to reversible structural isomerism that consists of multiple steps usually involving bond cleavage, molecular reconfiguration, and subsequent bond reformation . Thermochromic behavior can be observed in a wide variety of isolated compounds. Common organic thermochromic compounds include crowded ethenes, Schiff bases, spiro heterocycles (e.g., spiropyrans, spironaphthalenes, etc.), and macromolecular systems including liquid crystals and polymeric materials . There are comparatively fewer examples of inorganic thermochromism. However, vanadium (IV) oxide (VO2) has recently garnered much attention from researchers as a potential smart coating material due to its thermally tunable infrared and near-infrared absorption spectrum .
Today, significant research efforts in the academia and industry focus on the development of new technologies and devices based on multicomponent thermochromic mixtures including smart coatings, erasable printing media, and temperature sensors. This article provides a brief review of important examples of thermochromic compounds, followed by a description of the most recent advancements in the field with an emphasis on applications for new high-technology materials. Advanced thermochromic materials take advantage of the growing field of functional dye chemistry, and a few examples are presented.
Thermochromic behavior in organic compounds is often caused by thermally activated chemical rearrangements, i.e., thermal tautomerism. Some of the more common examples of tautomerism include acid-base reactions, keto-enol rearrangement, and lactim-lactam equilibria. Tautomerism is usually influenced by changes in temperature and solvent properties such as composition, polarity, and pH, and thermally activated tautomerism can lead to thermochromism.
Bianthrone and Crowded Ethenes
The increased planarity at higher temperature permits π-conjugation to extend more effectively across each anthrone moiety, decreasing the HOMO-LUMO gap and concomitant electronic absorption energy and thereby giving rise to a change in color of the compound with a change in temperature. Bianthrone is yellow in the solid (absorbing violet light at low temperature) and green (absorbing red light) in the melt. Substituents play a strong role in determining if these crowded ethenes will be thermochromic. Dixanthylene is colorless in the solid and green in the melt. Bulky groups at the 1’ and 8’ positions would cause excessive steric repulsions and prevent the formation of the bent anthrone state, which is required for the color-changing process .
Schiff Bases, also known as Salicylidene-Anilines
Substituent effects play a very important role in this system and define which tautomeric form of the enol-imine-keto-enamine equilibrium dominates. To switch between forms, thermal energy sufficient to exceed the activation energy of the tautomeric reaction must be added to the system. A recent review of the Schiff bases by Minkin et al. demonstrates the vast variability in this family of compounds, where simple modifications and ring substitutions can push the enol-imine-keto-enamine equilibrium in either direction .
Note that photons (light) also can be used to provide sufficient energy to initiate tautomerism. In that case, the process is considered to be photochromic. Thermochromic and photochromic properties in this class of compounds were long thought to be mutually exclusive. However, recent studies have indicated that salicylidene-anilines are almost always thermochromic in the solid state and are occasionally also photochromic .
Spiro Compounds, Including Spiropyrans
Spiro compounds are arguably the most important class of compound used in thermochromic applications; halochromic triarylmethane and fluoran dyes are widely used as colorants in multicomponent thermochromic mixtures (e.g., in thermal receipt paper and erasable printing media). This class includes spiropyrans, spironaphthalenes, spirooxazines, and fluoran and triarylmethane dyes. Spiro compounds, so named for the “spiro” central sp3 tetrahedral carbon center that all members share, are subject to numerous tautomeric equilibria including lactim-lactam, acid-base, and the aforementioned enol-keto equilibria. These equilibria result in chemical modifications that have significant impact on the electronic structure and, subsequently, on the color of these compounds . Many functional dyes belong to this category of compounds: some of the more important examples are the triarylmethane dyes (e.g., crystal violet lactone, CVL) and the fluoran dyes.
Ring-opened spiropyrans usually adopt the quinoid structure as shown in the bottom right of Fig. 4. However, if the R-group on the pyran moiety (see Fig. 3) is a strong electron-withdrawing group, the negative charge on the oxygen in the zwitterionic form will be stabilized, allowing the molecule to have zwitterionic character. Substitutive modifications to either of the aromatic functionalities in spiropyrans can significantly modify the thermochromic properties by stabilizing the charges formed in the ring-opened configuration.
Structure and thermochromic temperature ranges of some cholesterol esters 
Light reflected by the layer in the chiral nematic phase at location A (see Fig. 5) can constructively interfere with light reflected from the layer at position B (see Fig. 5) if the extra distance traveled (layer A compared with layer B) is an integer number of wavelengths of light. This phenomenon is analogous to Bragg reflection in layered crystalline solids. In such a way, chiral nematic phase liquid crystals act as a diffraction grating, or, more precisely, a monochromator. Temperature variations in the sample can cause the pitch length to change via thermal expansion, giving rise to variations in the wavelength of light that is constructively reflected (aka selective reflection). An important practical consideration arises from this selective reflection; light that is not reflected by the liquid crystal must be transmitted or absorbed. If the backing material is lightly colored, any transmitted light can be reflected back through the liquid crystal, interfering with the single selected wavelength of reflected light, changing the color. Therefore, thermochromic liquid crystal devices are almost always printed on black backings to absorb the light of wavelengths other than the one selected for reflection .
In general, the two important categories of thermochromic liquid crystals behave in much the same way. The major difference can be found in the applicable temperature range for each of the materials. Cholesteric liquid crystals generally have much higher transition temperatures and tend to find applications in thermometers on pasteurization equipment, ovens, and warning indicators on hot surfaces. The (S)-4-(2-methylbutyl)phenol derivatives have transitions at much lower temperatures, including physiological temperatures and find use in thermometers, in mood rings, and in refrigerator and food spoilage warning labels. Thermochromic liquid crystal devices can be engineered to behave in both reversible and irreversible manners, with the latter being particularly important if the thermal history of a product (e.g., perishable food products) is of particular importance.
Thermochromism in conjugated polymers arises when sufficient thermal energy causes an order-disorder transition involving the bulky side chains of the polymer. The side chains in polymers generally keep the polymer backbone organized in some fashion; the backbone bonds tend to be in either all-trans or helical conformations. Above a certain temperature, the side-chain groups become dynamically disordered and can no longer keep the backbone chain in its original conformation. The most common transformation is from all-trans to gauche conformation. Thermochromism results from the change in the HOMO-LUMO gap.
Polythiophenes are widely employed in organic electronics as a conducting layer. They are highly conjugated when the thiophene rings are in a trans-planar configuration. Regioregular poly-3-alkylthiophenes (Fig. 7) undergo a reversible color change from red-violet to yellow when heated under vacuum. This change arises from weakening side-chain interactions that are no longer able to maintain the coplanarity of the thiophene rings, resulting in twisting along the chain, a decrease in conjugation, and a change in the wavelength of light absorbed .
Thermochromism in inorganic materials can have many different origins: changes in ligand geometry, changes in metal coordination, changes in solvation, changes in bandgap energy, changes in reflectance properties, changes in distribution of defects in the material, and phase transitions .
An example of thermochromism arising from a phase transition is the compound Ag2HgI4. At room temperature, the compound adopts a tetragonal crystal structure and is yellow. Upon heating to 50 °C, Ag2HgI4 undergoes a first-order phase transition from tetragonal to a cubic phase concomitant with a color change to orange. Upon further heating, it undergoes a gradual (second-order) order-disorder transition to a phase in which the silver ions become mobile in the lattice and the color of the compound changes to black. Therefore, across a temperature range from 25 °C to 75 °C, the material changes from yellow to orange to black .
Perhaps the most interesting inorganic thermochromic compound is vanadium (IV) dioxide, VO2, which undergoes a semiconductor to metal transition at 68 °C. The transition modifies the absorption spectrum in the infrared and near-infrared regions. Vanadium dioxide is infrared transmissive below ~ 68 °C and infrared reflecting at higher temperatures . Vanadium dioxide is being considered for use in smart coatings which would allow visible sunlight to pass through a thin film coating but block infrared radiation, thus reducing building cooling requirements. Inorganic thermochromic compounds are of great interest for building coatings owing to their stability to light, which is substantially better than organic thermochromic compounds which are notoriously susceptible to decomposition under extended light exposure .
The term thermochromic materials refers to multicomponent mixtures of chemicals which, although not necessarily thermochromic individually, create a thermochromic system when mixed in the appropriate proportions. Two popular examples of commercial products incorporating this type of thermochromic material are the Pilot FriXion erasable pen and the Coors Light beer bottle label. The color of ink from the FriXion pen can be erased thermally by the friction created by rubbing the eraser head on the page [follow this “http://myweb.dal.ca/mawhite/Video/Frixion%20Pen%20Erasing.MOV” for a video]. The label on the Coors Light beer bottle contains a color-changing dye system that reversibly changes from colorless to blue upon cooling the container to below ~ 6 °C [follow this “http://myweb.dal.ca/mawhite/Video/Coors%20Light%20Bottle.MOV” for a video showing warming].
In such mixtures, a color-forming agent, the chromophore, reacts with a color-developing agent, the developer, to initiate the color-changing reaction. The color-change reaction also is controlled by another component of the mixture, usually referred to as the cosolvent, which forms the bulk of the mixture. The cosolvent melts and its melting point determines the color-change temperature. The cosolvent’s interactions with the other components also determine if the colored form of the mixture occurs at high or low temperature.
Most of the receipt paper used today in commercial enterprises employs this type of technology, although the chemicals employed are changing. Fluoran dyes are generally used to produce the black color of modern receipt paper, and bisphenol A is being replaced by other, less harmful, phenolic compounds. Today, the chromophores and developers are separated via microencapsulation of the chromophore. Heating the receipt paper causes the microcapsules to rupture, releasing the contents and initiating the coloring reaction. Although this process is technically not thermochromic due to the lack of reversibility, the widespread use of thermal receipt paper warrants its inclusion in this section .
An interesting commercial development from Japan is Toshiba’s e-Blue erasable laser jet toner. The toner is composed of a blue-colored spirolactone dye and phenolic developer embedded in a polymer matrix. When printed, the toner is blue. Heating a printed page will cause a decolorization reaction in which the developer is segregated from the dye, returning the initial uncolored state of the dye and erasing the printed image . The potential benefits of reducing the amount of paper that enters the recycling waste stream are substantial, although poor resistance to color fade, and low image quality thus far have precluded wide usage of such rewritable printing media. These examples of thermochromic materials also fall under the umbrella of the broad functional dye field and are discussed further in another chapter in this text.
Thermochromic leuco dyes and liquid crystal systems are used in the textile industry for both functional and artistic purposes. The breadth of variation in leuco dye structure permits the formulation of thermochromic products demonstrating an amazing array of colors. The choice of cosolvent allows for precise control of activation temperatures (i.e., the color-changing temperature). Companies supplying thermochromic products can design products to suit the needs of the textile manufacturer. Virtually any color imaginable can be produced by precise mixing of primary colors (e.g., red, green, blue, etc.), while clever selection of activation temperatures can result in interesting and aesthetically pleasing color-play effects .
Thermochromic colorants need to be isolated from their surroundings prior to use in textile applications in order to preserve the intended coloring behavior of the colorant system. To this end, microencapsulation is used to isolate the thermochromic system, with the coacervation method being the most common. The microcapsules are dried to form a powder, after which they are usually referred to as thermochromic pigments. The pigments can then be made into slurries, emulsions, or pellets, dissolved into inks and paints, or applied directly to a fabric.
One of the major problems concerning the use of thermochromic pigments is dilution of the dye throughout the processing steps; final dye concentrations can range from 3 %–5 % for pellets to 15–30 % for inks and paints . Other problems include poor stability against UV radiation (e.g., photobleaching), poor resistance to the effects of water and detergents, the cost of the thermochromic material, and, for some, toxicity of the components. In the case of liquid crystals, the fiber onto which the thermochromic pigment is printed must be black to prevent unwanted reflection effects . Additionally, the microencapsulation process can disrupt carefully engineered interactions in multicomponent thermochromic mixtures (i.e., dye, developer, cosolvent mixtures) such that the final microencapsulated product does not behave in the same way as the isolated system.
The use of thermochromic colorants in the textile industry has been mainly limited to novelty applications (e.g., hypercolor T-shirts). More recent textile applications have been focused on using the thermochromic effect for artistic purposes . Many of the thermochromic artistic works reviewed by Christie et al.  employed fabric-bound, microencapsulated thermochromic dyes coupled with heat-producing microelectronic devices to initiate the color-changing behavior. Smart materials including electronics-coupled textiles , multisensory interactive wallpapers , surface coatings containing heat-storing phase change materials (PCMs) , and soft-woven thermochromic fabrics  have been reported. These artistic applications of thermochromic colorants demonstrate the important link between scientific and technological developments and the creativity of the artistic world.
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