Photo-oxidation and -reduction of vat dyes on water-swollen cellulose and their lightfastness on dry cellulose

Abstract

Photostability of nine (seven anthraquinone and two thioindigoid) vat dyes on cellulosic films was examined. The ease (ko) with which the dyes were photo-oxidized was estimated by the relative fading of vat dyes on cellophane immersed in aqueous Rose Bengal solution on exposure, a condition resulting in only the photo-oxidative fading. The photosensitivity (f) was estimated by the relative fading of an aminopyrazolinyl azo vinylsulfonyl dye on cellophane dyed in admixture with vat dyes examined under the same conditions as above. The values of ko were small, compared with those of reactive dyes, while those of f comparable. The lightfastness of nine vat dyes on dry cotton fabric had close correlation with log (f ko). This fact shows that the fading of vat dyes on dry cellulose occurs mainly by photo-oxidation on exposure in air. On exposure of vat dyes on cellulosic substrate in an aqueous anaerobic DL-mandelate solution, a condition resulting in only the photo-reduction, vat dyes displayed a colour change in the direction of the acid leuco-compounds, many of which returned to the original dyes by re-oxidation. Vat dyes with high photo-reductivity showed the same colour change on dry cellulose by long exposure, implying that photo-oxidation occurs in the superficial part of fibre substrate and photo-reduction in the inner part. The photofading properties of anthraqui-none vat dyes were attributed to the excited triplet T1 (nπ*) state of the carbonyl groups. The quantum yields to T1 (ππ*) was very small by an effective internal conversion of the exited singlet states. Since the ππ* absorption of the annelated anthraquinones with the large extinction coefficients shielded the carbonyl groups with the small coefficients from the nπ* absorption, the photofading of the vat dyes with the highest lightfastness became very small. As a result, those vat dyes perform the highest lightfastness. © 2002 Elsevier Science Ltd. All rights reserved.