Spectral and Structural Comparisons on Four Pairs of Isomeric 4- and 2-Aminobenzoic Acid Based Aromatic Heterocyclic Dyes

Abstract

Four pairs of isomeric aromatic heterocyclic dyes 1 – 8, synthesized from the diazotization and subsequent coupling reactions between 4 or 2-aminobenzoic acid as the diazo component and different N-substituted pyridone precursors as the coupling component, have been characterized and compared including UV–Vis and 1H NMR spectra, X-ray single-crystal structures and related DFT computations. It is noted that X-ray single-crystal diffraction analyses reveal that all of them adopt the same hydrazone-tautomeric form and planar molecular conformation between the pyridine and phenyl rings. However, different intramolecular and intermolecular hydrogen bonding interactions are observed because of the introduction of an electron-withdrawing carboxylic acid group at the ortho or para position in the phenyl ring. 4-Aminobenzoic acid and 2-aminobenzoic acid have been used as the diazo components to couple with different N-substituted pyridone precursors to produce four pairs of isomeric aromatic heterocyclic dyes, for which 1H NMR spectral, UV–Vis spectral and single-crystal structural comparisons have been made.