The photo-oxidation of reactive azobenzene dyes and an analysis of their reactivity for the azo and hydrazone tautomers using the semiempirical molecular orbital PM5 method

The photo-oxidative reaction of eight reactive azobenzene dyes with singlet oxygen (1O2) was examined by exposing the dyed cellulose films on immersion in an aerobic aqueous Rose Bengal solution, and the second-order rate constants, k0, of the reaction were determined. The standard enthalpies of formation for these dyes in the gas phase and water, as calculated by the PM5 method, indicated that six of the dyes existed as azo tautomers in both the phases without exhibiting the azo–hydrazone tautomerism, and that the remaining two phenylazo dyes existed as hydrazone tautomers. The reaction sites and their reactivity for the corresponding tautomer on cellulose with 1O2 were analyzed in accordance with frontier orbital theory. The positions were primarily the carbon atom of the coupling component at which the coupling reaction occurred for both of the tautomers. The reaction modes occurring between the azo dyes and 1O2 were the ene and/or [2 + 2] cycloaddition reactions, and they occurred at the double bonds whose constituent atoms had larger Fukui electrophilic frontier densities, fr(E). The reactivities of the azo dyes against 1O2 were expressed by the sum of fr(E) at the corresponding double bonds. The plots of log k0 against the sum of fr(E) for all the dyes examined gave a close mutual correlation. The sum of fr(E) thus became a molecular descriptor of the reactivity of aromatics against 1O2.