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

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.