SYNTHESIS AND ELECTRONIC SPECTRAL CHARACTERISTICS OF SOME NEW AZO MEROCYANINE DYES

The synthesis of new solvatochromic azo merocyanine dyes (1-6) is described. The preparations involve coupling of 4-aminopyridine with appropriate phenols and subsequent methylation by methyl iodide. N-Methylbenzothiazolylazo-4'-benzoquinone (3) and N-methylbenzothiazolylazo-(3',5'-diphenyl)-4'-benzoquinone (4) were synthesised by coupling with phenols. In an alternative route, N-methylbenzothiazolinone hydrochloride (MBTH) was converted via the sulphonyl hydrazone to the azo sulphone, followed by coupling with various phenols to yield 3 and 4. Naphthoquinones were coupled to MBTH to give N-methylbenzothiazolyl-4'-naphthoquinone (5) and N-methylbenzothiazolyl-2'-naphthoquinone (6). The UV-Vis spectra of the six dyes (1 to 6) were recorded in 29 solvents and their lambda-max values and molar extinction coefficients (epsilon) are presented. The effect of structural change on the electronic spectra, on replacement of the -CH = CH- bridge by -N = N- in merocyanines, is discussed.