Structural insight into the fiber dyeing ability: Pyridinium arylazo pyridone dyes

Abstract Herein, we report the synthesis and characterization of nine 6-hydroxy-4-methyl-3-pyridinium-2-pyridone based azo dyes, wherein, eight of them being novel. Used building blocks ensured water solubility of six dyes and made them adequate to be used for dyeing in an alkaline environment contrary to the traditional application of arylazo pyridone dyes. The peculiarity of the compounds arises as they exhibit zwitterion-hydrazone equilibrium and shift from neutral to cationic dyes. It is shown that the equilibrium strongly depends on the electronic effects of the p-substituent in the phenyl moiety, as well as, pH values of the solutions. The screening of the dyeing process (by using multifiber fabric) suggested that synthesized compounds are most eligible for dyeing wool and diacetate fabrics at a pH of 8.5. Furthermore, 100% wool and diacetate fabrics were dyed and used for the dyeing assessment. A comprehensive study between dye architectures and fiber dyeing was performed. The interplay of various interactions, responsible for the dye-fiber binding, has been proposed, bearing in mind both, dyes' and fibers’ structures and pH dependency. Namely, dye-wool interactions are mostly based upon charge assisted hydrogen bonds, wherein these interactions are strengthened by C–H···π contact when dyes are affixed to diacetate. The specific nature of the dye is affected by the electronic effect of the p-substituents of the phenyl moiety, upon which, a detailed study on fibers affinity towards zwitterionic and/or hydrazone forms has been outlined. Most of the dyed fabrics exhibit good to excellent ultraviolet protection, while dyed diacetate shows good washing fastness.