Acid–Base Forms of Dimethylaminoazobenzene Derivatives, According to UV–Vis Spectroscopic Data

Abstract

UV–Vis spectra of neutral and acidic solutions of C-methyl derivatives of dimethylaminoazobenzene are compared to the analogous spectra of dimethylaminoazobenzene (DAB), the main state of which is Rydberg dimers (rydimers) DAB₂. The identical nature of the spectra of DAB and its 2-, 2′-, 3′-, 4′-methyl derivatives in neutral ethanol is established, testifying to the rydimer structure of these azo dyes and the identical nature of their chromogens, which have intense Vis bands at 400–430 nm. Uniform spectral changes are observed for solutions of 2-, 3′-, 4-methyl-substituted DAB and DAB₂ during protonation at low acidities, demonstrating the stability of the respective rydimers and the identical protonation mechanism of their dimethylamino groups. UV bands at 320 nm and Vis bands at 500–530 nm are observed in the spectra of the resulting rydimers with ammonia groups. It is concluded that as acidity rises, these rydimers first attach a proton to the azo group, change into a triprotonated form, and then decompose with the formation of diprotonated monomers. At low acidity, rydimers with methyl groups at positions 2′ and 3 on phenylene rings are also protonated on amino groups but dissociate with the formation of the corresponding monomeric ammonium cations, the spectra of which copy the UV–Vis spectrum of azobenzene. Based on an analogy between the observed spectral characteristics and those known from the literature on a large number of azo dyes, it is concluded that the rydimer structure is a common property of azo dyes.