Analytical characterization of 5,5′-dibromoindigo and its first discovery in a museum textile

Abstract

Abstract The exploitation of natural sources and later synthetic molecules to generate blue to purple coloration in textiles has a long history in the dyer’s craft. Natural indigoids such as indigo, woad, and Tyrian or shellfish purple served this purpose for millennia, but in the late 1800s synthetic analogs, in particular indigotin, quickly replaced natural sources. Halogenated versions of the dye were also created, and some like 5,5′-dibromoindigo were brought to market. Interestingly, these have not been significantly discussed in the literature, nor have they been found in forensic or technical art history investigations of textiles until now. This paper reports the first identification in a museum context of this unusual synthetic brominated analog of indigo, discovered on three twentieth century Japanese yukata. Analytical data collected on reference materials using liquid chromatography-mass spectrometry, UV–visible spectroscopy, Raman microspectroscopy, Fourier transform infrared spectroscopy, and X-ray fluorescence spectroscopy are provided to assist with future identifications of this relatively unknown colorant. Density functional theory applied to 5,5′-dibromoindigo was used to confirm the experimental Raman spectrum.

Keywords

• 5,5′-Dibromoindigo
• Vat Blue 35
• Textile dye
• Japanese yukata
• Raman spectroscopy
• Liquid chromatography–mass spectrometry