Light‐induced colour changes in dyes and materials

The present knowledge of the properties and reactions of photoexcited states of dyes and materials is presented and discussed. It is shown that the most probable result of the absorption of daylight by the majority of coloured products in practical use is the decomposition of the colourless substrate or additives, or their photooxidation. In both reactions, radicals are produced which can convert dyes or coloured pigments into compounds that are colourless or possess a different colour. The conclusions drawn may be useful in the interpretation of anomalous light stabilities of coloured products, and help develop products with improved lightfastness.