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

The properties of materials containing azobenzene depend strongly on the dynamics of and 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 H-N HMBC NMR provided a direct measure of the hydrazone percentage of the dyes in solution which was complimented by H ultrafast magic-angle spinning (MAS) and C 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.