Insights into the electronic properties of 6-substituted quinolone derivatives: A solvatochromic study

Abstract

Solvent dependent photophysical properties of 6 -substituted N-methyl quinolone molecules namely 6-amino-1-methylquinolin-2(1H)-one (AMQ), 1-methyl-6-(methylamino)quinolin-2(1H)-one (MAMQ), 1-methyl-6-nitroquinolin-2(1H)-one (NMQ) have been studied using steady state UV–Visible absorption, fluorescence emission spectroscopy, fluorescence lifetime measurements and quantum mechanical calculations. It was found that amine and methylamine substitution at 6-position of quinolone moiety enhanced the fluorescent property whereas nitro substitution rendered it non-fluorescent in all the experimental solvent systems. Maximum redshifted spectra were found in dimethyl sulfoxide (DMSO) compared to both alcoholic solvents and pure water. Significant charge transfer in HOMO to LUMO transition was revealed by quantum mechanical calculations but the direction was different depending on the nature of substitution. The pH response of the molecules were also investigated in pure water and the pKa values were found to be 3.70 and 3.90 for AMQ and MAMQ respectively. Analysis of radiative and non-radiative rate constants in all the solvents showed that the deactivation process may be different depending on hydrogen bond donation ability of the solvent. The spectral shift was rationalised by grouping the solvents based on their hydrogen bonding ability.