5-Hydroxy-1-phenyl-1H-pyrazole-3-carboxylic acid based heterocyclic dyes

Abstract 5-Hydroxy-1-phenyl-1H-pyrazole-3-carboxylic acid was selected as the coupling component to react with three S /N-containing heterocyclic (2-amino-4-chloro-5-formylthiophene-3-carbonitrile, 5-nitrobenzo[c]isothiazol-3- amine, 5-amino-1-ethyl-6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3- carbonitrile) diazo salts and three aniline derivatives with different electron-pulling/electron-pushing substituents, so as to produce six carboxylic pyrazolone based mono-/bi-heterocyclic dyes. Our results show that λmax of carboxylic pyrazolone based heterocyclic dyes depends on the substituent effects on aromatic rings and type of the heterocyclic rings. Compared with methyl pyrazolone/2,3-dimethylaniline based mono-heterocyclic dye, corresponding carboxylic pyrazolone based one with stronger D–π–A system brings a red shift of 37 nm. In contrast, carboxylic pyrazolone/thiophene and carboxylic pyrazolone/benzisothiazole based bi-heterocyclic dyes display blue shifts of 39 and 47 nm compared with relative methyl pyrazolone ones because of the role transformation for donor and acceptor (D−π−A to A−π−D) originating from the electron-withdrawing groups of S-containing heterocycles. Furthermore, different azo ⇌ hydrazone tautomeric transformations have been confirmed by the acid-base titration experiments. Namely, the mono-heterocyclic dyes, which would be transferred into the azo form in the alkaline condition, exhibit dominating hydrazone form in MeOH. However, the azo form of bi-heterocyclic dyes, which would convert to the hydrazone one under the acidic condition, is overwhelming in MeOH.