Conjugated Schiff's bases. Part 8. Structures and spectral properties of p-NN-dimethylaminoanils of some vicinal diketo-compounds. Positive solvatochromic effect

Abstract

Solvent effects on the long wavelength u.v. absorption of some conjugated Schiff's bases containing strong electron-releasing substituents in the anil fragment have been investigated. As found from theoretical calculations the absorption in question is due to the n→π* transitions of the unshared electrons of the azomethine nitrogen atom. This absorption exhibits a bathochromic shift when the polarity of the solvent increases. Such abnormal behaviour can be explained by assuming a superdonating character for the azomethine nitrogen atom. The solute forms a molecular arrangement with the solvent which is stabilized in high degree by charge transfer. This seems to be the cause of the bathochromic shift of the n→π* absorption. The charge transfer model considered for the interactions between the solute and aromatic solvents corresponds well to the experimental data. On the other hand the experimental data found for non-aromatic solvents correlate with the Dimroth parameter E_T. Considering the complementary influences of Lewis acidity and basicity of the solvent the acidic interactions were estimated by the parameter Z⁺ defined similarly to the Kossower parameter Z. The estimates of the percentage contribution from solvent acidity and bascity were 96.5 and 3.5%, respectively.