Optical and theoretical investigation of Indian yellow (euxanthic acid and euxanthone)

The optical properties (photophysics and imaging) of Indian yellow were investigated both in solid state and in aqueous solution and correlated with its chemical composition. The analyses were corroborated by a theoretical study carried out on the different xanthone derivatives that comprise the pigment under investigation, both as isolated molecules and in a polar (protic) solvent, to help the assignment of the excited states involved in the photo-induced process. Knowledge of its relatively high photoluminescence quantum yield (PLQY 0.6%), excitation and emission spectra and lifetime decays enhances the potential for reliable identification using non-invasive photo-induced luminescence imaging techniques. New insights into the chemical composition of the pigment, such as the identification of a sulphonate derivative of euxanthone, and its extensive occurrence on a 17th-century Indian wall painting are also reported for the first time in this study.