Photo-oxidation of phenylazonaphthol dyes and their reactivity analysis in the gas phase and adsorbed on cellulose fibers states using DFT and TD-DFT

Reactivity of gaseous 1-arylazo-2-naphthol dyes and their complexes with cellulose, after adsorption on cellulose, towards singlet molecular oxygen, 1O2, are investigated based on frontier molecular orbital theory. Results reveal that electrophilic reactions may occur predominately for the studied species, with the oxidizing agent 1O2 as electrophile. The −SO3− functionalized 1-arylazo-2-naphtol tautomers (soft nucleophiles) and their complexes with the cellulose are shown to be less reactive towards 1O2 and under thermodynamic control; while the −SO3H−SO3H functionalized 1-arylazo-2-naphtol tautomers (hard nucleuphiles) and their cellulose complexes are fairly reactive and under kinetic control. According to the frontier molecular orbital theory, the sites more vulnerable for 1O2 attack (the atomic positions and double bounds) are similar for both azo dyes and their complexes with cellulose. Thermodynamic study reveals that the photo-oxidation reactions are exothermic and spontaneous, except for cycloaddition of hydrazone tautomers. TD-DFT calculations confirm the decolorization and color fading phenomenon during the photo-decomposition reaction.