Photo-oxidation of pyrazolinylazo dyes and analysis of reactivity as azo and hydrazone tautomers using semiempirical molecular orbital PM5 method

Photo-oxidative fading of seven pyrazolinylazo dyes was examined by exposing the dyed cellulose films immersed in an aerobic aqueous Rose Bengal solution to a carbon arc. The ease with which the dye was photo-oxidised was determined as k0, the second-order rate constant of the reaction of the dye with singlet oxygen, from the initial rate of the fading. Calculating standard enthalpies of formation for the dyes in the gas phase and water by the PM5 method determined the azo–hydrazone tautomerism (AHT) of the dyes. Four dyes existed as a hydrazone tautomer (HT) and one dye as an azo tautomer (AT) in both phases without exhibiting AHT, and two dyes as HT in the gas phase and as an azo/keto tautomer (A/KT) in water. According to frontier orbital theory, the reaction sites for the corresponding tautomer(s) of azo dyes on cellulose to molecular singlet oxygen and their reactivity were analysed. The reaction sites were primarily the double bonds (CN) of the hydrazone structure and other double bonds in the pyrazoline ring and secondarily several double bonds around carbon atoms connected with substituents. The probable reaction modes are ene and [2 + 2] cycloaddition toward the double bonds. These reaction modes were confirmed to be consistent with the absorption spectra of decomposed products combined with cellulose. The k0-values obtained had a close correlation with the sum of Fukui's electrophilic frontier densities, fr(E), at both atoms of the corresponding double bonds (i.e. a linear correlation between log k0 and the sum of fr(E)). The sum was demonstrated to be a molecular descriptor of the reactivity toward singlet oxygen of a molecule.