Synthesis and spectral properties of azo dyes derived from 2-aminothiophenes and 2-aminothiazoles

A series of thienylazo and thiazolylazo disperse dyes has been prepared from their corresponding coupling components, 2-aminothiophenes and 2-aminothiazole. Depending on the various substituents present in the diazo component, absorption maxima varied from 437 to 534 nm in toluene. The spectroscopic properties of the azo dyes were examined with respect to the effects of 3- and 4-substituents present in the coupling component. Thus, the appropriate Hammett substituent constants could be related to the observed bathochromic shifts in the order CONH₂<<CN<CO₂Et<<H at the 3-position of the thiophene ring. However, the resonance effect of the 4-phenyl group contributed to the red shifts observed, unlike the corresponding 4-methyl group. PPP-MO gave reliable predictions of absorption maxima by using new modified parameters for the terminal nitrogen atom in the coupling component.