Photostability of Anthraquinone and Azo Dyes in N‐Ethylacetamide (Nylon Model)

Abstract

Measurements of the quantum yields of photodegradation of anthraquinone and azo dyes in N‐ethylacetamide (nylon model) and triplet sensitisation of dye fading showed that the photochemical reactions are initiated by an upper excited n‐π* triplet state. The primary photochemical reaction with both anthraquinone and azo dyes involves hydrogen abstraction from the amide so/vent. In some cases oxygen retards dye fading by reoxidation of the reduced structures. In other cases oxygen accelerates the photochemical reaction via free‐radical initiated oxidation reactions. The quantum yields of dye fading are strongly wavelength dependent. On exposure to a simulated sunlight spectrum photodegradation is mostly caused by radiation in the region 300–400 nm. While unsubstituted anthraquinone is very photoreactive, amino‐substituted anthraquinones are much more photostable in N‐ethylacetam/de. Photostability of mono‐ and di‐substituted anthraquinone derivatives increases with the electron‐donating power of substituent groups. Azo dyes with increased conjugation such as diazo and naphthazo structures are more photostable than simple azobenzene derivatives. Certain electron‐withdrawing substituents, which do not affect dye colour, improve the ‘oxidative’ photostability of both anthraquinone and azo dyes. A cobalt premetallised azo dye (C.I. Acid Red 182) is very photostable in N‐ethylacetamide, showing the effect of metal chelates on photostability. The low values of quantum yields for the dye solutions are comparable with those of acid and disperse dyes in nylon films, indicating that N‐ethylacetamide is a suitable nylon model for mechanistic studies of dye fading.