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Leuco dye


A leuco dye (from the Greek leukos: white ) is a dye which can switch between two chemical forms; one of which is colorless. Reversible transformations can be caused by heat, light or pH; resulting in examples of thermochromism, and halochromism respectively. Irreversible transformation typically involve reduction or oxidation. The colorless form is sometimes referred to as the leuco form.

Leuco dyes form the basis of thermal printer papers and certain pH indicators.

The most common example is in applying sulfur dyes and vat dyes; with indigo being a classic case. This is characteristically purple but is also completely insoluble in water, meaning that it cannot be applied to clothes directly. It is instead reduced to indigo white (sometimes Leucoindigo), which is water-soluble but colorless. When a submerged fabric is removed from a dyebath of white indigo the dye quickly combines with oxygen in the air and reverts to the insoluble, intensely colored indigo. The reduction step is typically achieved with sodium dithionite, hydroxyacetone and hydrogen, or by electrochemical methods.

Indigo white (Leucoindigo)

Indigo

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 with UV light (see ), 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 with the rest of the molecule, and a conjugated system forms, with ability to absorb photons of visible light, and therefore appear colorful.


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