The Effect of Environment and Substituents on the Photochemical Activity of Anthraquinonoid Vat Dyes and the Rôle of n‐π* Transitions

Abstract

The well‐established correlations between dye fading and the phototendering caused by vat dyes on the one hand and dye affinity, aggregation, radical stability, electromeric effects, etc., on the other, are reviewed. Recent work, including flash spectroscopy, photolysis of model systems, studies of fluorescence and absorption spectroscopy, and studies indicating the existence of both singlet and triplet n–* transitions, is discussed, and this leads to the suggestion that n–* transitions are of fundamental importance. Evidence is collected to substantiate this. Factors which may affect the transition, especially hydrogen bonds involving the n‐electrons and electromeric effects, are considered, as well as processes influencing the singlet‐triplet ratio. It is shown, in terms of the relative amounts of singlet and triplet formed, how the observed difference between the efficient cyclic process of photosensitisers can be explained. Finally, these arguments are applied to a few well‐known regularities of commercial dyes.