To be or not to be an azo pigment: Chemistry for the preservation of historical β-naphthol reds in cultural heritage. (June 2021)

• Record Type: Journal Article
• Authors: Angelin, Eva Mariasole; Oliveira, M. Conceição; Nevin, Austin; Picollo, Marcello; Melo, Maria J.
• Is Part Of: Dyes and pigments. Volume 190(2021)
• Publication Date: 2021-06
• Subjects: Art conservation -- β-Naphthols hydrazone pigments -- Azo pigments -- Photophysics -- Photodegradation; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2021.109244
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: 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. Highlights: The stability of 20th C. red pigments found in works of art is studied. Quantum yields of degradation identify unstable β -naphthol reds. Molecular markers of β -naphthol red degradation identified by mass spectrometry. Science & Art to safeguard the 20th century cultural heritage.