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…show more content… 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.
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.  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