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 Catal & PRIME;an 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 ioni-zation 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.