Structural elucidation of two Congo red derivatives on dyed historical objects indicative of formaldehyde exposure and the potential for chemical fading

Smith et al. recently reported dye analysis of a carpet fragment thought by some to be from a 15th century silk knotted Persian carpet [Smith et al., Forensic Science International: Synergy 2021; 3:100130]. Sample threads were found to contain several synthetic dyes from the late 19th century, including Congo red, which was first synthesized in 1883, indicating that the carpet is a modern object. Two mysterious compounds designated as A and B of unknown identity were also detected with Congo red in similar amounts on the object. This paper reports the structural elucidation of A and B using high resolution mass spectrometry and multidimensional NMR spectroscopy. To our knowledge, these compounds have never been reported before. Additionally, experiments are described demonstrating that A and B were products from the reaction of Congo red with formaldehyde. Previously unknown homologs of A and B were also formed when Congo red was reacted with acetaldehyde, pentanal, and hexanal, indicating a common reaction mechanism. Uv-Vis spectra of the homologs suggest that aldehyde exposure can cause an overall color change in objects dyed with Congo red. A historic dye swatch book containing cotton skeins dyed with the colorants benzopurpurine 4B and deltapurpurine 5B, which are of the same chemical class as Congo red, and exhibiting a faded appearance, were found to contain dye derivatives analogous to A and B. Chemical mechanisms proposed for the formation of compounds A and B are discussed, along with the possible origins of the formaldehyde exposure. The concept of 'chemical fading,' is adduced to heighten the awareness of the exposure of historic dyed textiles to the harmful effects of formaldehyde, a common museum and household pollutant.