Azomethine dyes revisited. Photobleaching of azomethine dyes under photoreducing conditions

This paper presents the results of a study on the bleaching process of azom ethine dyes (AMDs) during their irradiation in the presence of an electron donor N-phenylglycine (NPG). The bleaching process and singlet oxygen forma tion for the dyes under study occurred with very low quantum yields. Experi mental results showed that the bleaching of azomethine dyes may be due to b oth singlet and triplet states. The prominence of the triplet state was sug gested by an analysis of double reciprocal plots for bleaching quantum yiel ds and [NPG]. Additional support for this mechanism was given by results fr om laser flash experiments with a cyclized form of the dye. In these experi ments, a transient optical absorption was attributed to a triplet state, an d this state was quenched by NPG with a rate constant of 1.2 x 10(7) M-1 s( -1). A similar experiment performed for a branched dye shows a broad, weak transient absorption which may also indicate a small amount of triplet-stat e formation. Changes in the dye structure affected the rate of photobleachi ng. The introduction of heavy atoms into a dye molecule only slightly incre ased the color-loss efficiency. The decrease or restriction of the freedom of the phenyl-group rotation did not increase the rate of the bleaching pro cess. Significant influence of the azomethine dye structure on photobleachi ng rates was observed only when there was a strong electron withdrawing gro up in the R-2 position. The most significant increase of the bleaching rate was observed when the branching of the dye was limited, especially when th e rotation of substituents around the C=N bond was prevented by structural constraints.