¹O₂-driven photobleaching pathways of xanthene dyes: A comparative study via LC-QTOF-MS/MS analysis and FT-IR...

Singlet oxygen (¹O₂)-driven photodegradation is a key factor that controls the stability of xanthene dyes and a mechanism of their photobleaching is still lacking. Here, we report a comparative photodegradation study of five xanthene dyes (i.e., erythrosin B, eosin Y, fluorescein, phloxine B and rose bengal) under illumination conditions maximizing ¹O₂ generation. Time-resolved degradation was analyzed mainly by LC-QTOF-MS and FTIR spectroscopy, enabling identification of major intermediates and allowing the description of a general degradation mechanism. Despite the different molecular structures, only two different pathways were identified: i) all dyes were consecutively oxidized, yielding a major product with partial cleavage of the tricyclic xanthene core; ii) dyes lacking chlorine substitution on the peripheral benzoate ring presented a faster route, leading to complete chromophore breakdown and formation of low molecular weight oxidation products. These results show a relationship between structure and degradation pathway and provide important insights relevant to design, lifetime and conservation of xanthene-based dyes.