Thermochromism

Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an example of this property used in a consumer product, although thermochromism also has more practical uses, such as for baby bottles that change to a different color when cool enough to drink, or kettles that change color when water is at or near boiling point. Thermochromism is one of several types of chromism.

Organic materials

Leuco dyes

From the applications perspective, leuco dyes are " the most important class of thermochromic compounds". The dyes are typically applied in the form of microcapsules with the mixture sealed inside. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric. When the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. Dyes most commonly used are spirolactones, fluorans, spiropyrans, and fulgides.

Thermochromatic liquid crystals

Thermochromic liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures. Their color range is limited by their principle of operation.