Azomethine dyes revisited. Photobleaching of azomethine dyes under photoreducing conditions

Abstract

This paper presents the results of a study on the bleaching process of azomethine dyes (AMDs) during their irradiation in the presence of an electron donor N-phenylglycine (NPG). The bleaching process and singlet oxygen formation for the dyes under study occurred with very low quantum yields. Experimental results showed that the bleaching of azomethine dyes may be due to both singlet and triplet states. The prominence of the triplet state was suggested by an analysis of double reciprocal plots for bleaching quantum yields and [NPG]. Additional support for this mechanism was given by results from laser flash experiments with a cyclized form of the dye. In these experiments, a transient optical absorption was attributed to a triplet state, and this state was quenched by NPG with a rate constant of 1.2 × 10⁷ 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-state formation. Changes in the dye structure affected the rate of photobleaching. The introduction of heavy atoms into a dye molecule only slightly increased the color-loss efficiency. The decrease or restriction of the freedom of the phenyl-group rotation did not increase the rate of the bleaching process. Significant influence of the azomethine dye structure on photobleaching rates was observed only when there was a strong electron withdrawing group in the R² position. The most significant increase of the bleaching rate was observed when the branching of the dye was limited, especially when the rotation of substituents around the CN bond was prevented by structural constraints.