Investigation of the Source of Dual Fluorescence in Quinoline-2- carboxylic acid through Conformational Analysis

Abstract

In this study steady state absorption, fluorescence excitation and emission spectra of quinoline-i-carboxylic acid have been examined in non-polar and polar solvents and computational calculation is also performed to achieve the goal of this research work in finding the origin of dual fluorescence. The fluorescence spectra of quinoline-Z. carboxylic acid are found to exhibit dual emission in polar solvent (dioxatie and DMSO) at a shorter and at longer wctve length region corresponding to I Lb and I La state respectively. The excitation wavelength dependence of the emission spectra has also been studied, and strong dependence was observed in case of dioxane and Dlv!SO. Closer examination of the absorption (excitation) spectra of the sample in these solvents revealed the presence of two different absorptions (excitations) at shorter and longer wavelength region indicating the sources of the two emissive states to be in the ground state. The 3D emission spectra of the sample in these solvents indicate the correspondence between the observed excitation spectra and the emission spectra. Stepwise addition of solution of Zn (II ) ion i n to the solution o f the sample changed the absorption position f r o m shorter wavelength to the longer wavelength region indicating the presence of t wo absorbing species in the ground state. These results together with the observation of t wo energy minima i n computational calculations of the PE S of quinoline- 2-carboxylic acid, indicated the presence of two different conformers of the same molecule i n the ground state that lead to two close l ying excited state; 1 Lb and I La and thereby results in the dual fluorescence of the compound. On the basis of the experimental and theoretical results, it is suggested that the existence of tw o different conformers of quinoline-i-carboxylic acid i n the ground state, which are at equilibrium are the cause for the observed dual fluorescence.