Photoreduction of organic dyes in ketone amine systems

Abstract

Nucleophilic radicals, such as α-amino and ketyl radicals, produced by reaction of benzophenone triplets with amines and alcohols respectively, react efficiently by electron transfer with cationic organic dyes, such as phenosafranine, thiopyronine, methylene blue, and crystal violet. Time-resolved laser flash photolysis studies reveal a high reactivity of α-amino radicals toward these cationic dyes; in contrast, ketyl radicals are less reactive. The electron transfer rate constants for the reaction between the α-amino radical of triethanol amine and these dyes range from 2.7 × 10 9 M −1 s −1 (phenosafranine) to 7.5 × 10 9 M −1 s −1 (methylene blue). In contrast, rate constants ranging from 1.1 × 10 8 M −1 s −1 (phenosafranine) to 1.2 × 10 9 M −1 s −1 (methylene blue) were measured for the reaction of the dyes with the diphenyl ketyl radical. In dye mixtures used in conjunction with ketone-amine systems, stepwise electron transfer from an initially formed semireduced dye to a molecule of another dye occurs. For example, the primarily formed crystal violet radical reacts very rapidly ( k = 2 × 10 9 M −1 s −1) with phenosafranine, thus forming the semireduced phenosafranine and regenerating crystal violet.