A detailed UV–Vis spectral investigation of six azo dyes derived from benzoic- and cinnamic acids: experimental and theoretical insight

Abstract

Six donor–${\pi }$–acceptor azo dyes, with altered donor- (p-hydroxyphenyl- and naphthalene-2-ol) and carboxylic based acceptor groups, were subjected to detailed solvatochromic and tautomeric investigation in different environments by experimental and theoretical approaches. Naphthalene-2-ol compounds exhibit azo–hydrazone tautomerism in all solvents, wherein benzoic- and cinnamic acid-based compounds appear in solutions as a complex system of neutral forms and corresponding carboxylates. p-Hydroxyphenyl based compounds adopt the azo form in all investigated solvents. A special focus was placed on the definition of the anionic species and the extent of the deprotonation in different alkaline solutions. Excellent agreement between theoretical and experimental data is attained.

Keywords

• Azo–hydrazone equilibrium
• Acid–base equilibrium
• Solvatochromism
• DFT analysis
• Linear solvation energy relationships analysis