Synthesis, structure and study of azo-hydrazone tautomeric equilibrium of 1,3-dimethyl-5-(arylazo)-6-amino-uracil derivatives

Azo dyes, 1,3-dimethyl-5-(arylazo)-6-aminouracil (aryl = -C₆H₅ (1), -p-CH₃C₆H₄ (2), -p-ClC₆H₄ (3), -p-NO₂C₆H₄ (4)) were prepared and characterized by UV-vis, FT-IR, ¹H NMR, ¹³C NMR spectroscopic techniques and single crystal X-ray crystallographic analysis. In the light of spectroscopic analysis it evidences that of the tautomeric forms, the azo-enamine-keto (A) form is the predominant form in the solid state whereas in different solvents it is the hydrazone-imine-keto (B) form. The study also reveals that the hydrazone-imine-keto (B) form exists in an equilibrium mixture with its anionic form in various organic solvents. The solvatochromic and photophysical properties of the dyes in various solvents with different hydrogen bonding parameter were investigated. The dyes exhibit positive solvatochromic property on moving from polar protic to polar aprotic solvents. They are fluorescent active molecules and exhibit high intense fluorescent peak in some solvents like DMSO and DMF. It has been demonstrated that the anionic form of the hydrazone-imine form is responsible for the high intense fluorescent peak. In addition, the acid-base equilibrium in between neutral and anionic form of hydrazone-imine form in buffer solution of varying pH was investigated and evaluated the pK_a values of the dyes by making the use of UV-vis spectroscopic methods. The determined acid dissociation constant (pK_a) values increase according to the sequence of 2 > 1 > 3 > 4.