title: Spectroscopic and quantum chemical investigations of substituent effects on the azo-hydrazone tautomerism and acid-base properties of arylazo pyridone dyes.
abstract: A series of 5-(4-substituted arylazo)-6-hydroxy-4-methyl-3-cyano-2-pyridone dyes was synthesized and the structure of the dyes was confirmed by UV-Vis, FT-IR, (1)H NMR and (13)C NMR spectroscopic techniques. The azo-hydrazone tautomeric equilibrium was found to depend on the substituents as well as on the acidity and basicity of the media. Ionization constant, pKa, of the dyes was determined by UV-Vis spectroscopy and correlated with the Hammett substituent constants, σp and σI. The interpretation of the effect of different substituent in phenyl ring of arylazo pyridone dyes on their spectroscopic and structural properties was based on quantum chemical calculations performed by the density functional theory (DFT/M06-2X) method. The DFT calculations confirmed the existence of two forms in water solution: hydrazone form in acidic and neutral media and anionic form in basic media. The different contribution of azo and hydrazone canonical forms of anionic form is observed for dyes with electron-donating and dyes with electron-withdrawing groups. The dependence of absorption spectra and determined pKa values to the substituent type seems to be mostly due to azo/hydrazone canonical structure ratio in their anionic form.
authors.affiliation: University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegoševa 12, 11000 Belgrade, Serbia. Electronic address: jasmina@nanosys.ihtm.bg.ac.rs.
authors.forename: J
authors.initials: J
authors.lastname: Dostanić
authors.affiliation: Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia.
authors.forename: D
authors.initials: D
authors.lastname: Mijin
authors.affiliation: Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia.
authors.forename: G
authors.initials: G
authors.lastname: Ušćumlić
authors.affiliation: University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegoševa 12, 11000 Belgrade, Serbia.
authors.forename: D M
authors.initials: DM
authors.lastname: Jovanović
authors.affiliation: University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Chemistry, Njegoševa 12, 11000 Belgrade, Serbia.
authors.forename: M
authors.initials: M
authors.lastname: Zlatar
authors.affiliation: University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegoševa 12, 11000 Belgrade, Serbia.
authors.forename: D
authors.initials: D
authors.lastname: Lončarević
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D004396:Coloring Agents / Q000737:chemistry*; D006835:Hydrazones / Q000737:chemistry*; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D001391:Azo Compounds; D004396:Coloring Agents; D006835:Hydrazones
keywords: Absorption spectra; Azo dyes; Azo-hydrazone tautomerism; DFT; pK(a)