THE SENSITIZED FADING OF TRIPHENYLMETHANE DYES IN POLYMER-FILMS .1.

The photochemistry of four acid triphenylmethane dyes, viz. Acid Blue 1, Acid Green 9, Acid Blue 15 and Acid Violet 17, was studied in model systems of dyed poly(vinyl alcohol), methylcellulose and gelatin films, in order to elucidate the complex photochemical reactions occurring in a dyed-wool/water/air system upon exposure to ultraviolet radiation. A variety of chemicals that model protein-bound amino acids, and sodium azide and cadmium sulphate octahydrate were added to each dyed-polymer system in order to determine the importance of specific photochemical reactions anticipated for a dyed-wool substrate. A comparison between the dye spectra in various solutions and in the polymer substrates revealed that triaminotriphenylmethane dyes, in particular, show a propensity to aggregate. This was particularly evident in poly(vinyl alcohol) and gelatin substrates; in methylcellulose the monomeric forms of all dyes predominated. Dye fading appears to be governed by (a) the ability of the substrate, or residual solvent within the substrate, to donate electrons or hydrogen atoms to the dye and (b) the degree of dye aggregation, which may be partially determined by (c) the chemical and physical structure of the substrate. Two dissociative pathways were proposed for the fading of dimeric dye molecules in protein substrates. For monomeric dye molecules, it was found that fading was photosensitized by components of the protein structure, in particular the guanidino group of arginine and the carboxylate groups of glutamic and aspartic acids. In contrast, neither lysine, histidine nor excited singlet-state oxygen appears to sensitize the fading of triphenylmethane dyes in protein substrates.