The role of azo-hydrazone tautomerism in the photofading of 1-phenylazo-4-naphthol in polymer substrates

Abstract

The photofading of 1-phenylazo-4-naphthol has been studied in films of cellulose acetate and cellulose triacetate. The relative rates of fading of the azo and hydrazone tautomers were monitored by resolving the visible absorbance spectrum of the dye into two overlapping bands corresponding to each tautomer. Permanent photofading of 1-phenylazo-4-naphthol was found to be due to oxidative attack on the ground state hydrazone tautomer of the dye by singlet excited oxygen. The lowest excited triplet state of the hydrazone tautomer was responsible for sensitizing singlet oxygen. The azo tautomer was resistant to permanent photofading, but its concentration was depleted during irradiation while it temporarily existed in the hydrazone form. As well as thermal tautomeric interchange, tautomeric conversion following trans → cis isomerization of the azo tautomer could have been responsible for this temporary existence. Relative fading rates were influenced by the nature of the polymer concerned, and the method used to incorporate the dye. Temporary fading occurred as a result of isomeric and conformational changes in each tautomer.