Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Colorant, Thermochromic

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_165

Synonyms

Definition

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.

Introduction

Thermochromic compounds change color in response to temperature changes [1]. 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 [2]. 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 [3]. 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 [4].

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.

Organic Thermochromism

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

One of the first examples of reversible thermochromism observed in organic compounds was that of bianthrone [1]. The structures of bianthrone and some of its analogues are shown in Fig. 1. Bianthrone’s structure can be considered in terms of the planarity of each anthrone moiety (i.e., the upper and lower halves of the molecule, as shown in Fig. 1). At room temperature, the anthrone moieties are curved such that the aromatic rings within each anthrone moiety are not coplanar. When the temperature is increased, the central carbon-carbon double bond expands. This slight bond expansion allows the two anthrone moieties to rotate with respect to one another such that the dihedral angle across the double bond approaches 90°. When this occurs, the previously bent anthrone moieties each take a more planar structure as the steric repulsion created by the proximity of the other anthrone moiety is reduced by rotation about the central bond [5].
Colorant, Thermochromic, Fig. 1

(a) The structure of bianthrone and (b) related compound dixanthylene. (c) Bulky groups at the 1, 1′, 8, and/or 8′ positions would result in non-thermochromic compounds

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 [3].

Schiff Bases, also known as Salicylidene-Anilines

Thermochromic Schiff bases, also known as imines or azomethines, are formed by the condensation of an aromatic amine with either an aldehyde or ketone. The simplest compound in this class, salicylidene-aniline, is generated by the reaction of salicylaldehyde with aniline. The thermochromic Schiff bases exist in a state of equilibrium between two forms, the enol-imine and keto-enamine forms. For salicylidene-aniline, the enol-imine form is more stable at room temperature (Fig. 2). Increased temperature allows an intramolecular tautomerization to occur: the proton from the hydroxyl oxygen migrates to the imine nitrogen. Due to the rearrangement of the π-electrons, the keto-enamine form has more extended π-bonding than the parent enol-imine, which gives rise to a change in color [6].
Colorant, Thermochromic, Fig. 2

Tautomeric equilibrium between enol-imine and keto-enamine forms observed for thermochromic Schiff bases [7]

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 [7].

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 [8].

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 [9]. 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.

The spiropyrans form one of the longest-known and best characterized classes in this group. Figure 3 shows an equilibrium that is observed commonly in this class of compounds. The spiro center is located within the pyran ring which, upon excitation with thermal energy, can undergo a ring-opening reaction that alters the electronic structure. As a result of the disruption of the spiropyran (SP) form, the molecule undergoes electronic rearrangement giving the merocyanine (MC) form, which is deeply colored (e.g., violet, red, blue) for nearly all members of this family [1].
Colorant, Thermochromic, Fig. 3

Schematic view of the tautomeric equilibrium between a ring-closed spiropyran (SP) and a ring-open merocyanine (MC) form as observed for spiro compounds [3]

An important structural aspect of this class of compounds is that in the uncolored spiropyran form, two or more aromatic moieties are segregated from each other by the central spiro carbon atom. After the tautomeric rearrangement to the more planar merocyanine form, the formerly segregated π-electronic domains are able to form resonance structures which extend across the entire molecule. This delocalization of the π-electronic structure lowers the HOMO-LUMO gap (which would be in the UV typically for the ring-closed structure) and gives rise to the formation of intense color in these compounds. The mechanism for this process is shown in Fig. 4 for di-β-naphthopyran, which was first studied by Dickinson [10]. An important observation is that ring opening leads to a zwitterion, which has significant implications concerning the solubility of the ring-opened merocyanine form.
Colorant, Thermochromic, Fig. 4

Mechanism of the ring-opening reaction in the spiropyran di-β-naphthopyran. Cleavage of the spiro carbon-oxygen bond yields a structure with greater planarity and extended π-conjugation. The compound becomes colored due to this extended conjugation [3]

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.

Liquid Crystals

A well-known example of thermochromic behavior which results from macromolecular interactions is the thermochromic liquid crystals. Thermochromic liquid crystals can be found in products as diverse as mood rings, “stress testers,” warning indicators, and thermometers. Thermochromic effects have been exploited in many different types of liquid crystals, but only two basic series of liquid crystals have found widespread use in thermochromic applications. The esters of cholesterol (Table 1) were first studied by Reinitzer and led to the identification of the liquid crystalline phase [11]. The ester derivatives of (S)-4-(2-methylbutyl)phenol (Table 2) form the basis of the majority of today’s thermochromic liquid crystal devices.
Colorant, Thermochromic, Table 1

Structure and thermochromic temperature ranges of some cholesterol esters [12]

Colorant, Thermochromic, Table 2

Structure and thermochromic temperature ranges of some 2- and 3-ring 2-methylbutyl phenol esters [12]

Thermochromic liquid crystals take advantage of the special optical properties of the chiral nematic phase (abbreviated Ch and/or N*). The nematic phase is formed by calamitic (“rod-shaped”) liquid crystals and demonstrates only orientational order; this phase lacks long-range positional order [13]. The angular distribution of the long molecular axes of the calamitic molecules is central to the special optical properties of chiral nematic phase liquid crystals. The most probable direction of the long molecular axes defines the director which coincides with the principal optical axis of the uniaxial phase. In the chiral nematic phase, the director spirals about a helical axis. The pitch length, p, corresponds to the distance along the helical axis required for the director to make a full rotation about the helical axis. The pitch length can be on the order of a few hundred nanometers, i.e., the wavelength of visible light [12]. This effect is shown schematically in Fig. 5 [14].
Colorant, Thermochromic, Fig. 5

Pitch length corresponds to the distance required for the director to make a complete rotation about the central helical axis, from layer A to layer B (Reproduced with permission from Journal of Chemical Education, 1999, 76(9), 1201-120. Copyright (1999) American Chemical Society)

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 [12].

To obtain thermochromic behavior in chiral nematic liquid crystals, the pitch must vary rapidly with temperature. Phase transitions are usually exploited to control variation in pitch. The most common transition exploited is the S-N* (smectic to chiral nematic) phase transition. As the material cools within the N* phase, the pitch elongates (i.e., the liquid crystal becomes more locally structured), and the reflected color changes from blue (at higher temperatures) to red (nearer to the transition temperature). This effect is shown graphically in Fig. 6. Note that further heating of the liquid crystal brings about a transition to the isotropic liquid phase concomitant with a loss of selective reflection and color.
Colorant, Thermochromic, Fig. 6

An illustrative representation of the change in pitch length and corresponding color of a chiral nematic phase liquid crystal above the S-N* transition [12]

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.

Polymers

Highly conjugated organic polymers can demonstrate interesting electrical properties resulting from very long conjugation lengths and a high degree of delocalized electron density. As with conjugated small molecules, conjugated oligomers and polymers are susceptible to thermally induced structural modifications and can exhibit thermochromism. Some common thermochromic polymers include polythiophenes (Fig. 7a), polydiacetylenes (Fig. 7b), and α-conjugated polysilanes [15].
Colorant, Thermochromic, Fig. 7

(a) The polymer 3-hexyl polythiophene, an important material for organic electronics, has coplanar thiophene rings at low temperature. (b) Polydiacetylenes also can display thermochromic behavior depending on the choice of side-chain groups

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 [16].

Inorganic Thermochromism

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 [17].

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 [18].

Reversible thermochromism also can be observed for inorganic compounds in solution. In solution, the color changes are often associated with modification of the solvation sphere, changes in coordination number, or ligand exchange with the solvent. A commonly cited example is CoCl2 in water which is blue at room temperature and green at 0 °C, as shown in Fig. 8 [19]. As the system is heated, the Co(II) coordination changes from octahedral to tetrahedral and is coupled with ligand exchange. This structural change alters the electronic field experienced by the central Co2+ as a function of temperature, changing the wavelength of light absorbed and giving rise to thermochromism.
Colorant, Thermochromic, Fig. 8

The equilibrium form for Co2+ in aqueous solution and in the presence of Cl changes from green to blue upon heating from 0 °C to room temperature, resulting in thermochromism

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 [4]. 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 [4].

Thermochromic Materials

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.

Significant effort within industry has been aimed toward the development of thermally erasable printing inks and toners for large-scale printing in an office setting. National Cash Register Co. developed an irreversible “thermochromic” receipt paper in the 1950s using the spirolactone dye crystal violet lactone (CVL) as the chromophore [20]. The closed ring SP form of CVL is colorless; upon interaction with an acidic compound (or an electron acceptor), the lactone ring opens forming the intensely blue, ring-opened MC form (Fig. 9). Attapulgus clay was originally used to develop the color although more recently phenolic compounds such as bisphenol A (BPA) have been the developers of choice in the United States.
Colorant, Thermochromic, Fig. 9

Ring-opening reaction of crystal violet lactone (CVL) in the presence of an acidic developer. The ring-opened charged form has multiple resonance forms, only two of which are shown here

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 [20].

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 [21]. 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 Colorants

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 [22].

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 [22]. 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 [23]. 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 [23]. Many of the thermochromic artistic works reviewed by Christie et al. [23] employed fabric-bound, microencapsulated thermochromic dyes coupled with heat-producing microelectronic devices to initiate the color-changing behavior. Smart materials including electronics-coupled textiles [24], multisensory interactive wallpapers [25], surface coatings containing heat-storing phase change materials (PCMs) [26], and soft-woven thermochromic fabrics [27] 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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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Department of ChemistryDalhousie UniversityHalifaxCanada