NMR spectroscopic investigation of benzothiazolylacetonitrile azo dyes: CR7 substitution effect and semiempirical study

Ebead et al. had reported in the synthesis and the UV–Visible solvatochromism combined with semiempirical calculations to study the tautomerism of 1,3-benzothiazol-2-ylacetonitrile azo dyes [1]. In this study, 1,3-benzothiazol-2-ylacetonitrile azo dyes (1–6) have been resynthesized, one and two dimensional ¹H and ¹³C NMR, as well as absorption and emission spectra, were collected and interpreted. The obtained results were used to clarify the tautomerization phenomenon of these dyes. Predominantly, these dyes exist in Z-hydrazone form reinforced through intramolecular hydrogen bonding in deuterated chloroform. Substituents (R) in carbon7 (CR7) are the key player of the changes in ¹H and ¹³C NMR chemical shifts, also in the absorption and emission wavelengths of benzothiazole azo dyes. Also, the relative photoluminescence quantum yields (PL QY) of these dyes (1–6) were estimated and related to the nature of substituents. Finally, PM6 semiempirical calculations were employed to confirm the preferred geometric structure and experimental NMR, absorption, and emission results.