A leuco dye (from the Greek λευκός ''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 Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, such as baby bottles which change to a differe ...

photochromism Photochromism is the reversible transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra. In plain language, th ...

and

halochromism A halochromic material or pH indicator is a material which changes colour when pH changes occur. The term ‘ chromic’ is defined for materials that can change colour reversibly with the presence of an external factor. In this case, the factor ...

respectively. Irreversible transformations typically involve reduction or oxidation.Chemistry and Applications of Leuco Dyes. Ramaiah Muthyala. 302 pag. Springer; 1997 edition. The colorless form is sometimes referred to as the leuco form. Leuco dyes form the basis of

thermal printer Thermal printing (or direct thermal printing) is a digital printing process which produces a printed image by passing paper with a thermochromic coating, commonly known as thermal paper, over a print head consisting of tiny electrically heated ...

papers and certain

pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...

Examples

The most common example is in applying

sulfur dye Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. They are inexpensive, generally have good colour fastness, wash-fastness, and are easy to apply. Sulfur dyes are predominantly black, brown, and dark blue. Red s ...

s and

vat dye Vat dyes are a class of dyes that are classified as such because of the method by which they are applied. Vat dyeing is a process that refers to dyeing that takes place in a bucket or vat. The original vat dye is indigo, once obtained only from p ...

s; with

indigo Indigo is a deep color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine, based on the ancient dye of the same name. The word "indigo" comes from the Latin word ''indicum'', m ...

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 Sodium dithionite (also known as sodium hydrosulfite) is a white crystalline powder with a sulfurous odor. Although it is stable in dry air, it decomposes in hot water and in acid solutions. Structure The structure has been examined by Raman s ...

hydroxyacetone Hydroxyacetone, also known as acetol, is the organic chemical with the formula CH3C(O)CH2OH. It consists of a primary alcohol substituent on acetone. It is an α-hydroxyketone, also called a ketol, and is the simplest hydroxy ketone structure. ...

and hydrogen, or by

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

methods. Leucoindigo structure.png, Indigo white (Leucoindigo) File:Indigo skeletal.svg, Indigo The spiro form of an

oxazine Oxazines are heterocyclic compounds containing one oxygen and one nitrogen atom in a doubly unsaturated six-membered ring. Isomers exist depending on the relative position of the heteroatoms and relative position of the double bonds. By extensi ...

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 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 ...

with UV light (see

Photochromism Photochromism is the reversible transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra. In plain language, th ...

), 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 In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as ...

forms, with ability to absorb photons of

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

, and therefore appear colorful. Another example of a leuco dye is the

crystal violet lactone Crystal violet lactone (CVL) 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. The central carbon in the leuco form is ...

, which in its lactone form is colorless or slightly yellowish, but in low pH, when it is protonated, it becomes intensely violet. Other examples are

phenolphthalein Phenolphthalein ( ) is a chemical compound with the formula C20 H14 O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this applica ...

and

thymolphthalein Thymolphthalein is a phthalein dye used as an acid– base ( pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianio ...

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

redox indicator A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential. The requirement for fast and reversible color change means that the oxidation-reduction e ...

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

electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode (S ...

References

{{DEFAULTSORT:Leuco Dye Dyes

Examples

See also

References