Solid state NMR and DFT studies of azo–hydrazone tautomerism in azo dyes and chitosan-dye films

Abstract

The properties of materials containing azobenzene depend strongly on the dynamics of cis and trans isomerization which in turn are influenced by the presence of different possible tautomers. Two commonly used food azo dyes, Allura Red (ALR) and Amaranth (AMA), were studied by NMR spectroscopy to experimentally determine their tautomeric forms and then theoretically rationalized. Variable temperature 2D ¹H–¹⁵N HMBC NMR provided a direct measure of the hydrazone percentage of the dyes in solution which was complimented by ¹H ultrafast magic-angle spinning (MAS) and ¹³C CPMAS solid state NMR to characterize the structures of the solid dyes alone and incorporated into chitosan films designed to disassemble upon exposure to light and water. Density functional theory (DFT) calculations were carried out to predict tautomeric equilibria in comparison to NMR measurements and rationalize the observed equilibrium in terms of electronic structure effects computed in the natural bond orbital analysis framework. In agreement with NMR results, the hydrazone form was strongly favored in a polar solvent, driven primarily by stabilizing resonance delocalization and favourable electrostatic interactions with solvent. Our observations show that the ALR and AMA dyes exist predominantly in the hydrazone form of their tautomeric equilibrium in solution, in the solid-state, and in a chitosan-dye film. These results indicate that since disassembly of the chitosan-dye films, potential substitutes for petrol-based polymer films, is based on the cis–trans isomerization of the azo form, other water soluble food dyes with higher azo contents should be explored.