Photochromism of yellow azo dyes: a specific phenomenon of cotton dyed with reactive dyes

A method that is more sensitive than the ISO standard procedure for the detection and assessment of photochromism has been developed using a colour measuring device. Using this method the photochromism of various yellow azo reactive dyes applied to cotton has been studied. This has shown that azo dyes that can form a hydrazone tautomer tend not to be photochromic. In particular for pyrazole and hydroxynaphthalene azo dyes, the colour change is too small to be perceived by the human eye. Furthermore photochromism decreases with increasing size of the aromatic moieties attached to the azo group, as well as with the increasing number of substituents located ortho to the azo linkage. Finally the photochromism of dyeings with the golden yellow chromophore of structure IV was reduced when they were finished with certain Knittex brands.