Thermochromism

Thermochromism is the property of a material to change color as its temperature changes. The effect can be reversible or irreversible and is observed in dyes, polymers, and inorganic compounds. In reversible systems, color change generally reflects a temperature-driven change in electronic structure, molecular conformation, or phase state.

Main mechanisms fall into three classes. Thermochromic liquid crystals (TLCs) respond to temperature by changing their molecular order, producing visible color shifts used in mood displays and temperature indicators. Leuco dye systems contain a colorless form that shifts to a colored form at certain temperatures, commonly used in inks, coatings, and textiles. Inorganic thermochromic materials, including vanadium oxide (VO2) and related oxides, change optical properties at characteristic temperatures via phase transitions or changes in reflectance.

Applications include temperature indicators, smart windows, energy-saving coatings, sensors, and textiles. The transition temperature, color palette, and reversibility are engineered through material choice, encapsulation, and formulation. Durability and cycling stability can limit long-term performance, particularly for dye- or polymer-based systems.

Examples and notes: Mood rings historically used TLC-based materials to reflect skin temperature. VO2-based coatings can modulate infrared transmission on glass, contributing to thermostat-driven shading. Leuco-dye inks are common in temperature-sensitive packaging and novelty products.