Add time:08/07/2019         Source:sciencedirect.com

Ortho/para-aminobenzoic acids and corresponding methyl esters were selected as the diazo components to couple with two mono functional group transformation (FGT) precursors with different pyridine N-substituted groups [1-ethyl-6-((3-methoxypropyl)amino)-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile and 1-(3-isopropoxypropyl)-6-((3-methoxypropyl)amino)-4-methyl-2-oxo- 1,2-dihydropyridine-3-carbonitrile] and one double FGT precursor [2,6-bis((3-methoxypropyl)amino)-4-methylnicotinonitrile], so as to produce a series of isomeric pairs of heterocyclic azo dyes. In comparison with conventional 2,6-pyridone based hydrazone dyes bearing the same ortho/para-aminobenzoic acids and corresponding methyl esters azo components, the pH stability of synthesized mono and double FGT azo dyes has been improved to different extents because of the removal of the active hydrazone proton via 3-methoxypropylamine substitution. Among them, double FGT azo dyes with methyl ester units show extremely high pH stability, but the ones with benzoic acid groups exhibit low pH stability due to the presence of carboxylic acid proton. In contrast, methyl ester involved mono FGT products display less pH stability (pH ˜ 10) owing to intra-ring azo-keto⇌azo-enol tautomerism under strong alkaline condition. It is noted that two distinguishable deprotonated processes could be observed for mono FGT products with benzoic acid groups, and two-step deprotonation mechanism is proposed by forming monovalent and bivalent anions under weak and strong alkaline conditions involving intra-ring azo-keto⇌azo-enol tautomerism.

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