ORGANIC DYES AS COLORIMETRIC SENSORS FOR ANIONS: SPECTRAL ANALYSIS

Abstract

A new class of organic dyes containing hydrazone or imidazole chromophores have been synthesized and characterized as new colorimetric and ratiometric optical sensors for fluoride and acetate anions. These receptors show a unique selectivity and reactivity for fluoride and acetate in CH3CN by deprotonation process leading to a colorimetric response which we can detect by naked eye. The binding between dye and anions have been confirmed by quantitative NMR analysis. In the NMR, signal corresponding to hydroxyl and imidazoles NH hydrogens disappeared after the addition of anions to the dye, moreover other signals shifted upfield, which confirms the binding of -OH and -NH hydrogen by the anion. This type of receptor binds fluoride and acetate via electrostatic interaction (anion-n and charge/electron transfer). The presence of strong electron withdrawing group (NO2) enhanced the acidity of —OH and NH, and the reactivity of fluoride and acetate towards the dyes. These sensors exhibited high selectivity for fluoride and acetate anions over other anions with more pronounced change in absorption characteristics. The binding properties of the dyes with the anions were evaluated by Job's, Benesi-Hildebrand and Scott methods..