Leuco dye
chromic dye that can switch between colored and colorless forms
A leuco dye (from the Greek λευκό leuko: 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, photochromism and halochromism, respectively. Irreversible transformations 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.
From the applications perspective, leuco dyes are "the most important class of thermochromic compounds". The dyes are typically applied in the form of microcapsules with the mixture sealed inside. An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric. When the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact halochromism.
“Leuco dye” enters the record as chromic dye that can switch between colored and colorless forms. Crown Archives preserves that source wording while asking what Leuco, chromic and switch can confirm, complicate or overturn.
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