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Thermochromism in Ink Chemistry.

Any student of chemistry who had the privilege of having access to a Gilbert chemistry set would recall with nostalgia the pleasures of experimenting with disappearing inks.

Let me commence this essay by illustrating a few general demonstrative experiments that have the halo of chemical magic [1,2]:

* (1) You place a blank cardboard on a flame and mysteriously there appears a black lettered message.

* (2) You heat a pink colored solution and suddenly the color vanishes; on cooling the solution the color reappears and the procedure can be repeated.

* (3) Again, you place a blank paper on a hot plate and there appear the blue words of a love letter.

Now let me uncover the shroud of mystery to throw light into the chemical facts behind these demonstrations as David Copperfield exposes secrets behind his famous magical items in popular TV shows.

In the first one, the magician had written the message with concentrated sulfuric acid.

On heating the cardboard that is made up of cellulose (a carbohydrate, [([C.sub.6][H.sub.10][O.sub.5]).sub.n]), the acid removes elements of water from it, leaving behind black carbon.

In the second, the pink color is due to the presence of phenolphthalein in ammonium hydroxide. On heating, a shift of equilibrium between ionized ammonium hydroxide and unionized ammonia occurs, causing change in the color of the indicator.

In the third, the letter was written with cobaltous chloride solution that was invisible on drying. This turns into blue on heating.

The chemistry as exemplified in the above instances may profitably be utilized to prepare a class of inks called thermochromic inks.

Thermochromism

Thermochromism refers to the phenomenon of color changes by the agency of heat. Obviously, the color changes are made possible by the temperature-induced chemical or physical changes of materials incorporated into the inks.

Sometimes, the color change occurring at a temperature is permanent, and at other times the original color can be regained on cooling.

Accordingly, we have an irreversible or reversible thermochromic system. The required chemistry can be adopted based on the end use. That means one can select an irreversible thermochromic system when a certain temperature crossing is to be monitored and a reversible system when the actual temperature range is to be monitored. The color change may be achieved with a single chemical material or a mixture of them through physical or chemical changes [3,4,5].

In fact, thermochromism is a special case of the phenomenon called "chromotropism," which refers to the changes in color caused by external influences. To this category belongs the phenomenon of "piezochromism," which is the change in color caused by pressure. If the color change is due to the frictional force, it is referred to as "tribochromism."

The color changes observed when certain materials are ground in a mortar come under the purview of this class, though the possibility of color change emanating from a reduction in the particle size during grinding should be ruled out.

Similarly, the color change shown in different solvents is called as "solvatochromism." Added to this is the input from the branch of photochemistry called photochromism that represents light induced color transitions. Other areas such as "electrochromism" (color changes caused by electricity) are also emerging.

Applications

There are many applications where the temperature at which a certain change occurs is required to be registered. For example, it may be necessary to ensure that a delicate material or foodstuff is maintained at a stipulated temperature range or a process does not exceed certain temperature. It is not always convenient to monitor the temperature variations in a system directly, say by the use of a thermometer or a thermocouple device. Such a situation may arise even in high-tech applications like computers where the microchip or the printed circuit board (PCB) should not be allowed to surpass ambient temperatures during production or use.

Thermochromic ink chemistry comes to the rescue in such instances. An efficiently designed ink coating on the PCB can indicate the temperature or temperature profile by showing remarkable color changes in the coating at the transition temperature.

Thermochromic materials may either be inorganic or organic in nature. Most of the early thermochromic chemicals were of inorganic type and a wealth of literature is available on them. However, in modern times, organic thermochromic systems are gaining popularity owing to the vast strides in organic structure design.

A classical example of inorganic thermochromism is the temperature-induced transition between monomeric nitrogen peroxide ([NO.sub.2]) and dimeric nitrogen tetroxide ([N.sub.2][O.sub.4]). When a sealed tube containing brown [NO.sub.2] gas is cooled in ice, the color fades away owing to the formation of the dimer [N.sub.2][O.sub.4] [6].

2 [NO.sub.2] [left arrow]right arrow] [N.sub.2][O.sub.4]

(Brown)

(Colorless)

Due to structural and electronic reasons, the absorption spectrum of the dimer differs drastically from that of the monomeric species. A common reason for color is the absorption of light frequencies by the molecule in the visible and/or near ultraviolet (UV) region of the spectrum, though there are dozens of other minor and major reasons responsible for it.

When light is absorbed, electrons in the molecule rearrange within different energy levels facilitating the absorption process. Usually, these electrons will be distributed in the molecule in locations called orbitals designated as bonding, antibonding and nonbonding orbitals.

These orbitals differ in their energy levels. Absorption in the visible region takes place when low energy electronic transitions involving visible region frequencies occur.

In the present case, the dimer absorbs light in the mid-UV region rendering it colorless, and the monomer absorbs in the visible region causing it to be colored.

The possibility of easy interconversion between the two species coupled with the spectral shifts makes them thermochromic.

CT Complex Formation

An important thermochromic mechanism operating in solutions of a simple molecule like iodine in various solvents is referred to as "Charge-transfer" (CT) complex formation [7]. It is a common observation that iodine shows a violet color in non-polar solvents such as hexane, carbon tetrachloride, carbon disulfide, etc., and a brown color in polar solvents such as acetone, alcohol, pyridine, etc.

The origin of these color differences is manifested in their absorption spectra.

Figure 1 depicts the absorption spectra of iodine in five different solvents with widely differing dielectric constant values, an index of polarity of molecules [8]. Shifts in the absorption bands in the visible and UV regions are discernible in these spectra. The change from the conspicuous violet to brown is attributed to the CT complex formation between the solvent and iodine.

This effect can be made clear as follows. A solvent (D:) like ether which can donate a lone pair of nonbonding electrons to iodine can form a CT complex by the formation of a coordinate bond between D: and iodine leading to an oscillation (resonance) between the two structures shown below.

D: [cdots] I-I [less than] - [greater than] D: -[greater than] I[cdots]I

The symbols [less than]-[greater than] and -[cdots] in the above scheme respectively represent the resonance between the two structures and coordination through electron pair donation from the solvent. In aromatic solvents such as benzene and pyridine, the [phi] electron cloud causing aromaticity in them is partially transferred to iodine.

The above principle may also be used to explain the thermochromism of iodine in solvents such as ethyl stearate. Iodine in ethyl stearate is brown at room temperature which on heating above 80[degrees]C becomes violet. At room temperature, the polar ends of the long chain molecules form CT complexes with iodine.

On heating, this CT complex dissociates, and the iodine thus made free will be surrounded by the long chain hydrocarbon environment provided by the stearyl group. This situation gives the typical aliphatic hydrocarbon surroundings to iodine where it is shown to be violet.

Reversible Thermochromism

Another major mechanism behind thermochromic effect in inorganic compounds can be illustrated with the example of ruby, the red colored gem.

Ruby shows reversible thermochromism changing from red to green through violet on heating. [9] Chemically speaking, ruby is a solid solution of chromium and aluminum oxides. The color change in it is explained with the help of ligand field theory of transition metal complexes. Chromium is a transition metal characterized by d orbitals. The color of transition metal compounds is due to the electronic transition within d orbitals generally called as d-d transitions.

These d orbitals split in terms of energy under the influence of a ligand. Ruby's normal red color is due to the unusually large splitting of the d orbitals caused by the ligand field. On heating, the crystal structure expands, and the [Cr.sup.3+] ions become more relaxed and regain its original green color.

The ligand field splitting theory can explain the thermochromism in other transition metal compounds also. The hydrated green salts of nickelion ([Ni.sup.2+]), on cooling in liquid nitrogen (-196[degrees]C), acquires a sky blue color. [10] Similarly, the red salts of cobalt ion changes to yellow at liquid nitrogen temperature. The Werner's coordination complexes of nickel and cobalt ions with a coordination number of six represented by [[Ni([[H.sub.2]O).sub.6]].sup.2+] and [[Co([[H.sub.2]O).sub.6].sup.2+] undergo strong contraction at increasing ligand fields.

Sometimes, a mere phase transition can result in color change of certain solids. A case in point is that of mercuric iodide, which is red at normal temperatures. Above 127[degrees]C, it exhibits a reversible thermochromism turning into yellow. Here a red tetragonal phase is transformed into a yellow rhombic phase. The mechanism responsible for color changes is a variation of CT process that operates by donating electrons from the ligand to metal. The thermochromism in salts like silver iodide is also explained in similar lines. The same mechanism explains the thermochromism in certain double salts as illustrated by the following examples. [11] [Ag.sub.2][HgI.sub.4], a double salt of mercuric and silver iodides, shows thermochromism at 50.7[degrees]C changing from yellow to orange. Similarly, [Cu.sub.2][HgI.sub.4] turns from red to black at 67[degrees]C.

Sone and Fukuda [12] have compiled the names of important thermochromic systems, both reversible and irreversible, in their monograph on this topic. Interestingly enough, most of these examples utilize the color bearing chemistry of first transition metals such as chromium, vanadium, cobalt and nickel.

The multimillion numbers of organic compounds and polymers can provide many useful examples of thermochromism [3,13]. Normal organic compounds are colorless since their absorption bands lie in the UV region. But factors like conjugated double bonds shift the absorption to the visible region imparting color. Structural factors within the molecule that contribute to the color forming mechanism can be altered by temperature variations, and such molecules can be efficiently designed due to the versatility of organic chemical methods.

In naphthospiropyran derivatives, thermally induced ring opening is responsible for the color change. [3] ACT type mechanism causes thermochromism in compounds like poly(xylylviologen dibromide).

Specifically, the CT energy levels are altered when the polymer undergoes hydration and dehydration processes as a function of temperature.

A hybrid approach assimilating the potentials of inorganic and organic chemistry is also being tried to evolve novel thermochromic systems. For example, Takeda et al. [14] investigated recently the color of polycrystals of the zirconocene complex with 1,4-diphenyl-1,3-butadiene. They performed nuclear magnetic resonance cross polarization/magic angle spinning studies, quasielastic neutron scattering experiments, molecular orbital calculations, etc. in an attempt to reveal the minute details of the molecular transitions in this complex subsequent to temperature variations.

Liquid Crystals

In the recent past, thermochromism has been achieved by the temperature-induced rearrangement of liquid crystals [15].

Liquid crystals are phases exhibiting properties in between liquids and solids. They belong to the two general classes referred to as thermotropic and lyotropic liquid crystals. Thermotropic liquid crystals are formed by heating solids and lyotropic liquid crystals are formed by dissolving solids in liquids.

Liquid crystals that show thermochromism are made by microencapsulating cholesteryl ester-based compounds. The color changes in them can be reversed facilitating temperature detection. These chemicals are thick fluids whose molecules spontaneously align. They form a helically twisted structure like the strands of a rope. With temperature, the pitch of the helix changes resulting in differential reflection leading to color variation. The microencapsulation protects the pure chemicals from contamination.

Finally, irreversible thermochromic systems may be designed based on ninny chemical reactions using multiple components that lead to colored end products on heating.

One can apply the immense potential derived from inorganic qualitative analysis of cationic radicals where many colored salts are formed in the course of experimentation, such as the colorful sulfides of various metals. What is needed is to arrive at a system where these salts may be formed from the component reagents on heating. In this respect, organic chemistry is more resourceful since it is replete with numerous classes of compounds and reactions from which an appropriate thermal reaction yielding colored end product may be identified.

They are incorporated in inks, paints, crayons, adhesive labels, etc. to mark the temperature variation in a wide range. As implied in the beginning of this essay, thermochromic inks and paints can display the temperature in a real time basis on many moving parts of instruments instantly, and these colors can even be recorded.

Ink manufacturers have been successful in translating the chemical knowledge into technology with projected new applications on the horizon such as security inks [16], and established applications such as sterilization indicators.

This branch of technology will be more colorful with the possibility of fusing concepts such as photochromism with thermochromism in a general ink system [17,18].

Joy T. Kunjappu received his Ph.D. inorganic photochemistry in 1985 and D.Sc. in physical chemistry of surfactants in 1996. Prior to his arrival to the U.S. in 1987, he served as a senior scientific officer with the Department of Atomic Energy of India specializing on many aspects of chemistry. He worked as a post-doctoral research scientist (1987-1989) and associate research scientist (1994-1996) at the Langmuir Center for Colloids and Interfaces and the Chemistry Department of Columbia University, New York.

He has authored about 70 publications which include original research papers, review articles, book chapters book reviews and symposium proceedings. He also served as the reviewer of technical and scientific papers of eight international publications. In 1989, he edited a special issue of Colloids and Surfaces (Aspects of Interfaces) as a guest editor. His biography is featured in "Marquis Who's Who in Science and Engineering" (1997) Currently, Dr. Kunjappu is working as a research chemist at Propper M.C., Inc New York. He may be reached at jkunjappu@ aol.com and (212) 942-4828.

References

(1.) L. A. Ford, "Chemical Magic," 2nd edition, Dover Publications, Inc., New York, 1993.

(2.) J. D. Lippy, Jr. and E. L. Palder, "Modern Chemical Magic."

(3.) J. H. Day, Chemical Reviews, 63, (1963) 65.

(4.) J. H. Day, Chemical Reviews, 68, (1968) 649.

(5.) J. H. Day, "Kirk-Othmer's Encyclopedia of Chemical Technology," Vol.6, Wiley-Interscience, New York, 1979, p.129.

(6.) C. F. Bell, "Synthesis and Physical Studies of Inorganic Compounds," Pergamon Press, Oxford, 1972, p.35.

(7.) R. Foster, "Organic Charge-Transfer Complexes," Academic Press, London, 1969.

(8.) H. A. Benesi and J. H. Hildebrand, Journal of American Chemical Society 71 (1949) 2703.

(9.) R. C. Teitelbaum et al., Journal of American Chemical Society, 100 (1978) 3215

(10.) C. K. Jorgensen, "Absorption Spectra and Chemical Bonding in Complexes," Pergamon, Oxford, 1962, p.145.

(11.) J. G. Hughes, Journal of Chemical Education, 75 (1998) 57.

(12.) K. Sone and Y. Fukuda, "Inorganic Thermochromism," Springer-Verlag, Heidelberg, 1987.

(13.) K. Nassau, "The Physics and Chemistry of Color," John Wiley & Sons, New York, 1983, pp. 77,109.

(14.) S. Takeda et al., Journal of Physical Chemistry, 101 (1997) 278.

(15.) P. J. Collins, "Kirk-Othmer Concise Encyclopedia of Chemical Technology," 4th edn. John-Wiley & Sons, Inc., New York, 1999.

(16.) D. Savastano, Ink World, February 1999, p. 70.

(17.) Joy T. Kunjappu, Ink World, August, 1998, p.32.

(18.) Joy T. Kunjappu, Ink World, December, 1999, p.50.
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Author:T. Kunjappu, Dr. Joy
Publication:Ink World
Date:Mar 1, 2000
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