Essay on Thermochromism in Ink Chemistry

2461 Words May 6th, 2008 10 Pages

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
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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

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