Theoretical Study on the Structures and Absorption Properties of Yellow Azomethine Dyes

To examine the relationship between the structural features and the visible absorption spectra of yellow (alpha)-acylacetanilide azomethine dyes, we carried out measurements of the visible absorption spectra, X-ray crystal structure analyses, and density functional theory calculations for 2-benzoyl-2-[N-(4-diethylamino-2methylphenyl)imino]-2ʹ-chloroacetanilide (BA) and 2-[N-(4-diethylamino-2-methylphenyl)imino]-2-pivaloyl-2ʹ-chloroacetanilide (PA). BA shows a visible absorption spectrum, which has a relatively high molar extinction coefficient and an unwanted absorption curve in the green region, while PA shows a visible absorption spectrum, which has a relatively low molar extinction coefficient and a sharp cut absorption curve in the green region. BA and PA have a common structural feature that the paminophenyl fragment is coplanar with the azomethine-carbamoyl (-N=C-CONH-) fragment, resulting in a chromophore, but is perpendicular to the acyl fragment. The stable structures are dominated by the effect of (pi)-conjugation in the chromophore moiety and a repulsive interaction between the p-aminophenyl fragment and the acyl fragment. The potential energy curve for the rotation of the p-aminophenyl fragment shows two minima, and suggests that in BA two conformations coexist in equilibrium, while PA takes only single conformation in equilibrium. The difference in the absorption spectra of BA and PA is mainly attributed to the number of conformers coexisting in equilibrium.