Photochemical fading and photostabilization of the crystal violet lactone colour former system

Abstract

The photochemistry and photofading of the leuco and dye forms of the crystal violet lactone colour former system have been examined under various conditions using UV absorption, flash photolysis and high performance liquid chromatography techniques as well as hydroperoxide analysis. Colour production in solution is very much dependent on the acid concentration attaining a maximum at 90 vol.% acetic acid in acetonitrile. The leuco-dye equilibrium is an important factor in controlling the light stability of the system; photofading is shown to be essentially a photo-oxidative free-radical process followed by oxygen attack generating peroxides. The latter are effectively destroyed by a well-known hydroperoxide decomposer, nickel dialkyldithiocarbamate, which thus imparts stability to both the leuco and the dye forms of the colour former. Mechanisms are proposed to account for the results and the commercial implications of the light stability data to the practical use of colour formers are discussed.