N‐Sulphonatoalkyl indophenine derivatives: Design, synthesis and dyeing properties on wool, silk and nylon fabrics

Abstract The importance of quinonoidal planarity in improving the wet fastness of acid dyes was studied in a new series of N‐sulphonatoalkyl‐substituted indophenine derivatives. These derivatives were synthesised via the sulphonatoalkylation reaction of isatins with alkanesultone or sodium bromoalkanesulphonate followed by the indophenine reaction. The effects of the position of sulphonic acid groups, length of the alkyl chain, and electron‐donating/withdrawing groups on geometry, electronic structure, solubility, and absorption properties were systematically examined and correlated with the performances of the dyes on wool, silk and nylon fabrics. The N‐sulphonatoalkyl indophenine dyes could exhibit a blue colour in N,N‐dimethylformamide (DMF) and on silk or nylon fabric but a purplish colour in water and on wool fabric. Methoxy groups had a negative impact on the molar extinction coefficient and dye exhaustion, leading to relatively poor dyeing performance. In comparison, fluorine or chlorine atoms were less influential. The introduction of nitro groups could induce a yellowness to the dyed fabrics caused by the strengthened absorption at the region of 360 to 420 nm and reduce the dyeing rate and wet fastness level due to the low dye–fibre affinity. In general, the N‐sulphonatoalkyl indophenine dyes exhibited satisfactory washing and rubbing fastness, especially on dyed nylon fabric. This study provides insight into the structure–property relationship of indophenine dyes and demonstrates that sulphonatoalkylation is an efficient methodology for designing high‐performance textile dyestuffs based on quinonoidal heterocyclic structures.