Photostability of Mannich‐type dyed silk fibroin with pyrazolone‐containing aromatic primary amine dyes

Abstract Mannich‐type dyeing of silk fibroin with aromatic primary amine dyes (APADs) is a novel reactive dyeing method that requires mild conditions and exhibits high selectivity and good wet fastness. However, the primary amine group in the APADs significantly decreases the photostability of Mannich‐type dyed silk fabrics. To reveal the structure–activity relationship and photofading mechanism of the APADs by Mannich‐type dyeing, six pyrazolone‐containing APADs with similar structures were designed and synthesised. Variation in amino electron density among the dye analogues was related to differences in colour fixation of the Mannich‐dyed fabrics, as determined from the calculated Mulliken charge densities. Using mass spectrometry to monitor the photodecomposition of the dyed silk, it is demonstrated that the Mannich‐type dyed silk fabrics undergo photooxidative fading, in contrast to the conventional acidic‐type dyed fabrics that undergo photoreductive fading. Furthermore, it is shown that the Mannich‐type dyed silk fabric has a higher light fastness than conventional acidic‐type dyed fabrics using the same pyrazolone‐containing APADs. Evaluation of dye dipole moment and Mannich‐type dyeing shows that the APADs with increased dipole moments generally demonstrated increased light fastness.