The photochemistry of sulphuric acid esters of leuco vat dyes and their use in photographic printing and the cross–linking of poly(vinyl alcohol)

Abstract

The photofission of vat esters proceeds through a free radical mechanism to give a vat dye and two bisulphite radicals. The bisulphite radicals may react with water to produce sulphite and sulphate or with each other to produce dithionate. The use of vat esters in photographic printing is hampered mainly because the free radicals generated by irradiation of a single vat ester in a mixture can initiate development of all vat esters in the mixture. However, when vat esters are used in cross–linking of poly(vinyl alcohol) films, their photofission depends on the degree of plasticity of the film. Water in the film acts as a plasticiser and as a source of hydroxyl radicals, which can oxidise more vat ester to vat dye or initiate cross–linking. Dependence of vat dye formation on water is diminished by the use of non–hygroscopic plasticisers, but the extent of cross–linking is also reduced.