Molecular Modeling Studies on the Photochemical Stability of Azo Dyes

The structure and electronic properties of a series of colored azobenzenes and azothiophenes containing electron donors and attractors have been calculated with both semiempirical and ab initio methods. A reasonable correlation has been found between the predicted positions of the first and second absorption bands of the azo dyes calculated with the AM1/MECI method and the experimental values determined in methanol. The known photochemical degradation reactions of 4-dialkylaminoazobenzenes containing nitro groups in either the 2'- or 4'-position of the chromophore, which proceed via irradiation of the second absorption band, have been modeled by calculating the structures and electronic properties of the corresponding singlet and triplet states at the AM1 level. The superior photochemical stability of the 4'-nitroazobenzenes over the corresponding 2'-isomers has been rationalized in terms of the magnitude of the calculated spin densities of the azo nitrogen atoms in the second excited triplet state.