Synthesis of azo-dyes based on thiophendiazonium salts and their spectrophotometric properties

Abstract

New thienylazo reagents the methyl 3-[(2-hydroxynaphthalen-1-yl)diazenyl]-4-R 1 -5-R 2 -thiophene-2-carboxylates and methyl 2-[(2-hydroxynaphthalen-1-yl)diazenyl]-4-R 1 -5-R 2 -thiophene-3-carboxylates have been synthesized for the first time. They are obtained by interaction of thienyldiazonium chlorides and β‑naphthol under azo coupling reaction conditions. Methyl 3-[(2-hydroxynaphthalen-1-yl)diazenyl]-4-R 1 -5-R 2 -thiophene-2-carboxylates and methyl 2‑[(2‑hydroxynaphthalen-1-yl)diazenyl]-4-R 1 -5-R 2 -thiophene-3-carboxylates might be perspective reagents for spectrophotometry analytical research. 1 H, 13 C NMR spectra were used to confirm the obtained compounds structure. The influences of structure of the new synthesized dyes and azo-group position in thiophene ring on the spectrophotometric properties were studied. The medium acidity effects on the absorption spectra of the reagents have been investigated. Two forms of the methyl 2‑[(2-hydroxynaphthalen-1-yl)diazenyl]-4-R 1 -5-R 2 -thiophene-3-carboxylates at different acidities of the medium have been observed. Based on spectrophotometric studies, it can be concluded that a significant effect on the color, dependence of the maximum absorption on pH, and the value of the effective molar light absorption coefficient are determined by the structure of a substituted thiophene cycle. Location of the azo group near the 3rd or 2nd carbons of thiophene ring causes a considerable contribution into mentioned properties. At the same time, a type of the substituents at the 4rd and 5th carbon atoms of the thiophene cycle (aliphatic or aromatic substituents) has minor effect on the spectrophotometric characteristics of the tested substances. Key words: 2-Aminothiophenes, 3-Aminothiophenes, Thiophenediazonium chlorides, Thienylazo dyes, Spectrophotometric detection, Azo coupling.