To be or not to be an azo pigment: Chemistry for the preservation of historical β-naphthol reds in cultural heritage

Naphthol reds are historical pigments widely found in cultural heritage, characterized by 1-arylhydrazone-2-naphthol basic skeleton. Little is known about their degradation mechanisms, and this knowledge is essential for their long-term preservation. In this research, for the first time, the photodegradation quantum yields (Φ_R) were calculated for a series of pigment reds selected based on published literature, in order to evaluate the effects of the electron-withdrawing and donating groups present in C(4′), in particular the effect of sulfonate groups on the stability of these compounds. Φ_R values in homogeneous media, ranging from 3 × 10⁻⁶ to 4 × 10⁻⁵, are difficult to obtain in these systems, but provide the quantification of the stability of naphthol reds. One of the main conclusions is that the sulfonate groups have a stabilizing effect and that the keto forms are generally more stable. Another important piece of information is that the solvent, and as such the medium predictably, has a major impact on stability. For Sudan I and Orange II irradiation was also carried with a xenon source (λ_irr ≥ 300 nm), allowing characterizing the main degradation products by HPLC-DAD-MS and UHPLC-HRMS. For the dyes characterized by higher Φ_R, extensive degradation was observed with the formation of volatile compounds as phthalates.