Unveiling the photostability of the molecule of colour shikonin: From solution to the solid state

Shikonin, Shk, is a molecule of colour characterized by its natural phenolic-based compound composition, belonging to the naphthoquinone family. It is known for its low solubility in water and limited resistance to lightfastness. Shk is present in variable amount in the dried roots of Lithospermum erythrorhizon, Lithospermum officinale, and Alkanna tinctoria. The dyestuff extracted from these plants has historically been employed in Asia and Europe for textile dyeing, producing various shades of purple. Despite the historical significance of this dye, it has received limited attention in the context of cultural heritage studies. In the present work, Shk and model hydroxynaphthoquinone dyes were irradiated with polychromatic and monochromatic light in solution and in the solid state, mimicking paint mock-ups, to evaluate their photochemical stability. Low photoreaction quantum yield values (ϕR < 10−5) were observed in solution, providing quantitative evidence for the high stability of this molecule to light exposure. Chromatographic-mass spectrometric analyses carried out on the irradiated solutions and extracts from artificially aged Shk paint mock-ups, detected 5,8-dihydroxy-2-(1‑hydroxy-3-oxo-4-methyl-4-pentenyl)-l,4-naphthoquinone as degradation product, together with newly identified intermediate species involved in the complex photodegradation process. These indicate that, both in solution and in the solid state, the photoreaction of Shk proceeds with a similar mechanism. In the context of heritage science, this work contributes to highlight the potentialities of the adopted noninvasive approach, corroborated by results collected by chromatographic-mass spectrometric techniques, in identifying Shk in historical and archaeological specimens through its absorption and emission features and/or by detecting its degradation products which are, for the first time, characterized in terms of their UV–Vis absorption and emission spectral properties.