A leuco dye is a dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 whose molecules can acquire two forms, one of which is colorless.

For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by an sp³-hybridized "spiro" carbon. After protonating a part of the molecule, irradiation

Irradiation

Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

 with UV light (see Photochromism

Photochromism

Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light...

), or introducing other kind of such change, the bond between the spiro carbon and the oxazine interrupts, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals

Pi bond

In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...

 with the rest of the molecule, and a conjugated system

Conjugated system

In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...

 forms, with ability to absorb photons of visible light, and therefore appear colorful.

Another example of a leuco dye is the crystal violet lactone

Crystal violet lactone

Crystal violet lactone is a leuco dye, a lactone derivate of crystal violet 10B. In pure state it is a slightly yellowish crystalline powder, soluble in nonpolar or slightly polar organic solvents....

, which in its lactone form is colorless or slightly yellowish, but in low pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

, when it is protonated, it becomes intensely violet. Other examples are phenolphthalein

Phenolphthalein

Phenolphthalein is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions...

 and thymolphthalein

Thymolphthalein

Thymolphthalein is an acid-base indicator. Its transition range is at approximately pH 9.3-10.5. Below this pH, it is colorless; above this pH, it is blue. The molar extinction coefficient for the blue thymolphthalein dianion is 38000 M-1cm-1 at 595 nm....

, colorless in acidic to neutral pH, but becoming pink and blue in alkaline environment. Other example are many redox indicator

Redox indicator

A redox indicator is an indicator that undergoes a definite color change at a specific electrode potential....

s, which undergo reversible color change between colored and colorless form at a specific electrode potential

Electrode potential

Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...

.

Leuco dyes are a key component of some thermochromic dyes and thermal printer

Thermal printer

A thermal printer produces a printed image by selectively heating coated thermochromic paper, or thermal paper as it is commonly known, when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, producing an image...

 papers, photochromic materials, and of the Flexplay

Flexplay

Flexplay is a trademark for a DVD-compatible optical video disc format with a time-limited playback time. They are often described as "self-destructing" although the disc merely turns black and does not physically disintegrate. The same technology was used by Disney's Buena Vista Home...

 DVD

DVD

A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

 discs with limited play time, where leuco form of methylene blue

Methylene blue

Methylene blue is a heterocyclic aromatic chemical compound with the molecular formula C16H18N3SCl. It has many uses in a range of different fields, such as biology and chemistry. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in...

 is used.

See also

• Flexplay
  Flexplay
  Flexplay is a trademark for a DVD-compatible optical video disc format with a time-limited playback time. They are often described as "self-destructing" although the disc merely turns black and does not physically disintegrate. The same technology was used by Disney's Buena Vista Home...
  
  , trademark for a DVD-compatible optical video disk format that uses leuco dye to intentionally "wear away" after a limited number of playings.
• Hypercolor
  Hypercolor
  Hypercolor was a line of clothing, mainly T-shirts and shorts, that changed color with heat.They were manufactured by Generra Sportswear Company of Seattle and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and outside the US as Global Hypercolor...
  
  , clothing that changes colour with heat