Thermochromism in Ink Chemistry

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