Structure and Reactivity of 2-Arylazonaphthol Dyes in Solution and in Cellophane

The structure and reactivity of four 2-arylazo-l ,8-dihydroxynaphthalene (monoazo) dyes and their 2,7-bisarylazo- (bisazo) analogues in water, DMSO, and cellophane were studied using various techniques. NMR and UV-visible absorption spectroscopic studies ' suggested that the azo tautomer content (::;12 %), intramolecular hydrogen-bonding strength, and pKa (ca. 9.9-10.7), of each monoazo dye in water, increased with the increasing electron-donating ability of substituents on the phenyl ring. Contributions from the four tautomers of each bisazo dye to the overall composition were also found to depend on the electron-donating ability ofphenyl substituents. The intramolecular hydrogen-bonding strength and pKa (ca. 10.4-11.1) of the bisazo dyes were greater than for their monoazo ~ounterparts. None of the dyes aggregated significantly in water at ::;1 x 10-3 mol dm·3 , consistent with their extensive sulfonation. Dye reduction by a.-hydroxyalkyl radicals waS studied using time-resolved UV-visible absorption spectroscopy. The rate constant for electron transfer from radical to dye was similar for each dye (ca. 2-4 x 109 dm3 morl S·l). The radical anions formed by electron transfer disproportionated with rate constants of ca. 2-7 x 107 dm3 morl S·l for the monoazo dyes and one of the bisazo dyes, and ca. 6 x 108 dm3 morl S·l for the remaining bisazo dyes. The hydrazo compound formed by disproportionation ofthe bisazo dyes was observed to decay by two independent first-order processes. Product analysis studies using lH NMR spectroscopy, HPLC, and LC-MS suggested that dye reduction cleaves the N-N bond. Electrochemical studies ofthe dyes were consistent with the four-electron reduction of each hydrazone or azo group to yield aminated products. Product analysis studies suggested that electrochemical reduction of the azo group on one side of a bisazo dye occurred more readily than at the hydrazone group on the other, consistent with the products generated by reaction with photoinitiator radicals.EThOS - Electronic Theses Online ServiceGBUnited Kingdo