UV–Vis spectra and structure of acid-base forms of dimethylamino- and aminoazobenzene

• Published in: Russian Journal of Physical Chemistry A 2017-04, Vol.91 (4), p.715-726
• Main Authors: Mikheev, Yu. A., Guseva, L. N., Ershov, Yu. A.
• Format: Article
• Language: English
• Subjects: Absorption spectra; Band spectra; Banded structure; Bonding strength; Cations; Cellulose triacetate; Chemistry; Chemistry and Materials Science; Dimers; Hydrogen chloride; Molecular structure; Monomers; Physical Chemistry; Protonation; Structure of Matter and Quantum Chemistry; Sulfuric acid
• ISSN: 0036-0244; 1531-863X
• DOI: 10.1134/S0036024417040161

The changes in the UV–Vis absorption spectra of dimethylaminoazobenzene (DAB) in the cellulose triacetate (CTA) matrix during the absorption of gaseous hydrogen chloride and protonation of DAB in weakly and strongly acid solutions of hydrochloric and sulfuric acids were studied. The yellow-orange color of DAB and its analog aminoazobenzene (AAB) was shown to be due to the dimers, whose structure involves phenylaminyl type cations formed due to the promotion of the sp 2 electrons of the azo groups to the Rydberg R 3s orbitals of the azo groups and possessing Vis bands at 400–417 nm. The cations exist immanently as the dimers due to the Rydberg intermonomer bonds; the Vis bands of the dimers shifted toward 500–520 nm under the action of HCl (in CTA) and weakly acid media as a result of the formation of phenyl aminyl type pair cations, whose π* levels are split by Simpson’s mechanism, during the protonation of interrelated monomers. In the concentrated sulfuric acid, the Vis bands at 500–520 nm vanished because of the decomposition of the dimers into diprotonated monomers possessing Vis bands at 400–417 nm. The mechanism of transformations of chromogens responsible for the spectral transformation was given.