Abstract
Ionic liquids (ILs) are new and alternative solvent media that offers a unique solvation milieu due to ionic nature of their constituents. Proton-transfer involving a probe and its prototropic forms strongly depends on the nature of the solubilizing media. In this short communication, photophysical properties of 2-Naphthoic acid (2NA) in IL based media have been investigated with the help of electronic absorbance and steady state fluorescence spectroscopy. Fluorescence emission from 2-Naphthoate anion can be observed in neat 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-Butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]). Fluorescence emission from 2-Naphthoate anion can be quenched in presence of [bmim]+ cation of ionic liquids in presence of neutral/basic water whereas protonation of 2-Naphthoate anion takes place when acidic water is added to ionic solutions of 2-Naphthoic acid. Formation of non fluorescent complex takes place in excited state between 2-Naphthoate anion and ionic liquid cation [bmim]+ in presence of neutral/basic water. More specifically, presence of IL [bmim][BF4] up to 5.0 ± 0.1 wt% do not cause any change in the absorbance spectra of 2-Naphthoic acid dissolved in pH 12.0 ± 0.1 water whereas significant fluorescence quenching is observed due to formation of non fluorescent complex in the excited state between 2-Naphthoate anion and IL cation [bmim]+. Excited state protonation of 2-Naphthoate anion can be achieved in [bmim][BF4] by the addition of pH 2.0 ± 0.1 phosphate buffer, whereas addition of pH 6.0 ± 0.1 phosphate buffer causes decrease in fluorescence intensity. Fluorescence quenching of deprotonated form of 2-Naphthoic acid was also observed with the addition of 1 M aqueous NaOH in [bmim][OTf] having same cation [bmim]+ as in IL [bmim][BF4].
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