When light betrays colour: the photodegradation of alizarin, a historical dye, in basic media

Abstract

International audienceArchaeological and historical evidence indicates that natural dyes have been employed by human societies for millennia to achieve colouration in various materials. And light reveals this world of colours by painting everything it touches. However, there is much evidence that natural dyes photodegrade when exposed to light, fading cultural heritage work. Upon photon absorption, a molecule reaches an excited state, which promotes photochemical reactions leading to the formation of new compounds and thus the photodegradation. Understanding the underlying mechanisms of these processes helps elaborate effective strategies to mitigate them. These problematics have long been relevant across various fields. There have been many studies about the photodegradation of dyes, and this continues until today. Our study focuses on alizarin, a historical natural dye that can be extracted from madder roots. It has been used in many fields, textiles, paintings, since antiquity. A distinctive aspect of our study is the investigation of alizarin's less-studied basic (di-deprotonated) form, using a solution model under controlled conditions. By using techniques such as EPR and HPLC-UV-MS, we identified radical oxygen intermediates and degradation products while evaluating the influence of pH, solvent and light wavelength. These findings have important implications for preserving alizarin-based artworks, improving industrial dyeing practices, and informing environmental efforts like wastewater treatment and shedding light on computational modelling as a crucial research field in these topics. Overall, the study deepens our understanding of anthraquinone dye degradation and promotes the sustainable use of natural dyes