Synthesis, spectroscopic, and structural studies of substituent effect on the solvatochromic, tautomeric, and halochromic behavior of 2-methylated arylazoquinoline dyes along with DFT calculations

Abstract

Six methylated heteroarylazo quinoline dyes with different electron-withdrawing/donating substituents, namely 5-(4-substituted arylazo)-8-hydroxy-2-methyl quinoline dyes, were synthesized by coupling reaction and their chemical structures were elucidated by conventional spectroscopic methods. The photophysical properties of the dyes were examined and systematically compared in a range of solvents of various solvatochromic parameters and at various pH values through UV–Vis spectroscopy. 2-methylated arylazoquinoline dye (the unsubstituted dye) was prepared and chosen for comparison purposes. The dyes show different chromic behavior including tautomerism, solvatochromism, ionochromism, and halochromism. The chromic and azo/hydrazone tautomeric properties of the dyes were found to depend on the substituent nature, solvent polarity/polarizability, and the acidity/basicity of the solvent environment. The Kamlet-Abboud-Taft, Katritzky, and Catalán linear solvation energy relationships (LSER) were used to correlate the dye spectral data. The acid-base equilibria between neutral and anionic forms of the dyes were studied in buffer solutions in a range of pH 1.0–13.0 and the ionization constants,pKa, were determined. Finally, the DFT calculations on the studied compounds provide an overall supportive and complementary picture of the spectral measurements of representative structure of the azo model.