Reduction of dyes by free radicals in solution. Correlation between reaction rate constants and redox potentials

Abstract

The reactions of some free radicals (RH·) with a number of organic dyes (D) have been examined in aqueous solutions using the fast-reaction technique of pulse radiolysis. The radicals studied include ·CH2OH, (CH3)2ĊOH, NH2ĊHCOO-, ·C-OH (C = cytosine), ·TH (TH2 = dihydrothymine), and ·CH2- and were produced by reaction of the substrates with OH radicals. The dyes used were Crystal Violet, Safranine T, Phenosafranine, Indigo Di- and Tetrasulfonate, Methylene Blue, Thionine, Indophenol and 2,6-Dichloroindophenol. The electron transfer reaction, RH· + D → R + ·D- + H+ (k1), were determined by following the bleaching kinetics of the dyes at the wavelengths where they absorb. The rates were found to increase monotonically with increase in the redox potential Em of the dyes examined. Similarly the efficiencies (expressed in percentage) of these reactions were obtained and found to be markedly dependent upon the Em values of the dyes in the range from -0.357 to +0.217 V. These results are discussed and the suggestion made that this correlation represents the dependence upon ΔEm, the difference in the redox potentials of the donor radical and the acceptor dyes.