Synthesis and characterization of novel yellow azo dyes from 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and study of their azo–hydrazone tautomerism

• Synthesized azo dyes from 2-Morpholin-4-yl-1,3-thiazol-4(5H)-one by simple route.
• Azo–hydrazone tautomerization has been discussed experimentally and theoretically.
• Experimental photophysical properties were supported by computation.
• Thermal properties were evaluated and compounds are thermally stable.

Novel yellow azo dyes were synthesized by diazotization of aromatic amines followed by coupling with 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and fully characterized. The geometries of the synthesized dyes for azo and hydrazone tautomeric forms were optimized using B3LYP, CAM-B3LYP and M06 functional and 6-31G(d) and 6-311++G (d,p) basis sets, also their electronic excitation properties were evaluated using density functional theory. The optimized geometries reveal that the hydrazone for is more stable than the azo form. Photophysical properties of the synthesized dyes were evaluated by UV–Visible spectroscopy and compared with computed vertical excitation obtained from TDDFT. The results clearly illustrate existence of dye 8a, 8b and 8c in hydrazone tautomeric form, while 8d exist in both azo as well as hydrazone form. Thermal stabilities were estimated by using thermo gravimetric analysis, and results revels that the synthesized dyes have good thermal stability.