Synthesis and spectral properties of H acid-containing dyes and their Mannich-type dyeing performances on silk fibroin

The colouration of silk fibroin through a Mannich-type reaction with aromatic primary amine-containing dyes ( APAD s), named Mannich-type dyeing, is a novel dyeing method with the advantages of moderate conditions, high selectivity, and high wet fastness. In this study, three novel APAD s, including an H acid structure, D1 - D3 , were designed and synthesized. A similar dye, C1 , without an aromatic primary amine group, was selected as the control. Their spectral and colour fastness properties in aqueous solutions and on silk fabrics were measured and evaluated. In addition, their Mannich-type reactivity to silk fabric was evaluated via experiments and simulated calculations. It was revealed that for the H acid-containing dye with the same chromosome structure, the electron accepting/donating properties of the diazo and coupling components affect the colour of the dye after the Mannich-type dyeing. The substituent effect of dyes C1 – D3 demonstrated that with the changing properties of electron donation and acceptance in the case of the diazonium component, the absorption spectrum ( λ max ) of the neutral aqueous dye solution changed from reddish-orange (505 nm) to bluish-violet (541 nm). The λ max of Mannich-type dyed fabrics was generally red-shifted in comparison with that of dyed fabrics by acidic dyeing, which implied the formation of a reactive bond between the dye and the silk fibroin. The presence of aromatic primary amine groups in dyes is the prerequisite for Mannich-type reactivity. The primary amine groups on both benzene and naphthalene rings have Mannich-type reactivity, which is related to the electron cloud density of amino N atoms. With the increase in electron cloud density of the amino N atom, the colour fixation of the dye and the fibre was improved. After the Mannich-type dyeing, the colour fastness of the dyed fabrics and DMF stripping resistance significantly improved, also indicating the formation of a covalent bond between the dye and the fibre. The simulated calculation predicted the colour of APAD in an aqueous solution and on dyed silk fabric after Mannich-type dyeing, which guides the design and enrichment of APAD s’ chromatography. • The relationship between the electron cloud density of primary amino group on naphthalene ring and Mannich reaction efficiency was revealed. • The presence of tautomerism in azo dyes can affect the color and Mannich reactivity of aromatic amine dyes. • The color of silk fabric after Mannich-type dyeing with azo dye containing H acid structure can be predicted by simulation.