The sensitized fading of triphenylmethane dyes in polymer films. Part 2

Abstract

Investigations were performed on four acid triphenylmethane dyes, viz. Acid Blue 1, Acid Green 9, Acid Blue 15, and Acid Violet 17, in order to elucidate the complex photochemical reactions occurring on a dyed-wool/water/air system, upon exposure to ultraviolet radiation. The studies were performed using model systems of dyed poly( vinyl alcohol), methylcellulose and gelatin films. In Part 1 of this work, compounds which model protein-bound amino acids, present in wool, were examined for their ability to promote or retard dye fading on protein substrates. Part 2 focuses on three amino acids, l-arginine, l-glutamic acid and l-aspartic acid and examines the role of the amino and carboxylate groups present in these compounds in promoting dye fading. The work compares the ability of the amino acids to promote dye fading when they exist alone in the presence of dye, and when they participate in peptide bonds. The guanidino, α-amino and carboxylate groups of these compounds appear to photosensitize dye fading. The dye fading mechanism appears to involve an excited triplet-state dye molecule and, in the presence of a photosensitizer, possibly an excited triplet-state exciplex. It also 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, (b) the degree of dye aggregation, which may be partially determined by (c) the chemical and physical structure of the substrate, and (d) the position of the water solubilizing SO 3 − group on the dye. Eight kinetic mechanisms are proposed and discussed in terms of their likelihood of occurring for dye fading in the presence of guanidino and carboxylate groups.