Steady‐State and Time‐Resolved Emission Studies of 6‐Methoxy Quinoline

Abstract

Steady‐state absorption, fluorescence excitation, and emission spectra of 6‐methoxy quinoline (6‐MQ) were measured at room temperature in cyclohexane, dioxane, ethanol, acetonitrile, water, and water–dioxane solvents. Absorption spectra of cyclohexane, n‐hexane, and isopentane solutions show resolved vibronic structure at room temperature. However, the excitation spectrum of cyclohexane solution is structureless and is found to be emission wavelength dependent, indicating the formation of at least two distinct species in the ground state. Similar behavior was observed in dioxane and water–dioxane solutions. For all other solutions, the fluorescence excitation spectrum of 6‐MQ was found to be the same for different emissions. Emission of 6‐MQ in all solvents consisted of two bands with their maxima around 355 nm (I) and 430 nm (II), the actual positions and the relative intensities being dependent on the solvent used. The bands I and II were respectively attributed to normal and protonated/H‐bonded species of either 1La or 1Lb states or mixed (1La/1Lb) state of ππ* character. Fluorescence decay of this dye in all solvents monitored over each emission maximum showed biexponential behavior, and the analysis yielded two different lifetime components for each emission band. The short and long fluorescence decay components were respectively in the range of 0.30–3.00 ns and 18–20 ns. The observed emission characteristics coupled with the nature of the fluorescence polarization spectra and two different decay components for each emission suggest the existence of two different conformers having two different excited electronic states.