Leuco dyes are a very popular class of dyes. Their uniqueness lies in the fact that in Leuco Dyes, the molecules show the capacity to acquire two distinct forms, and one of it is colourless. A prominent example can be given is that of the spiro form of an oxazine, that is typically a colorless Leuco Dyes. Another example of leuco dye is the crystal violet lactone. This dye while in its lactone form is typically colourless or perhaps slight yellowish, but in conditions of low pH, as it is protonated, it shows the colour that is intense violet. Other examples can include phenolphthalein and thymolphthalein.

• As earlier said, one of the unique features of Leuco Dyes is that the colour changes upon heating. That is from coloured (when cool) to colourless. In fact it is the uniqueness of leuco dyes that they are also reversible. They change colour back and forth with fluctuating temperatures usually for Leuco Dyes to change colour it takes about 5ºF (3ºC) temperature change.
• Leuco Dyes are available in 15 distinct colours and a broad range of temperatures.
• Leuco Dye products can be found in wide variety of forms. This includes slurry, powder, water-based ink, epoxy, master batch, etc.

Leuco Dyes are different from other dyes in its functioning. They require typically a combination of chemicals in a system. For protection of this special system of materials a process called Microencapsulation is put to good use. The process of microencapsulation works like this, in the begining it extracts a small droplet out of the Leuco Dye system and puts protective wall coating around it. This process is effectively shown in the photomicrograph.

As the image shows, actually it is these Leuco microcapsules that has in it the complete color changing thermochromic system. For example Leuco Dyes microcapsules when added in inks makes the ink display the necessary thermochromic properties.

Leuco Dyes are used for a wide array of applications, materials and products. Examples of application of Leuco Dyes include security printing, novelty bath toys, swimming pool toys, battery testers etc.

It is generally seen that the life expectancy of Leuco Dyes are shortened by:

• Very high temperature applications.
• Sunlight (Ultra Violet exposure).
• Contact with some solvents.