Fluorescence quenching of etilefrine by acetate anion

Acid dissociation in the excited state of antihypotensor drug etilefrine [2-(ethylamino1-3-hydroxyphenyl)ethanol] is studied. Fluorescence of etilefrine decreases at pHB7 and is related to phenolic group dissociation. However, intensity of etilefrine fluorescence diminishes as the concentration of the acetate anion increases at pH\7. Analyses of the absorption and fluorescence spectra of aqueous solutions of etilefrine in the presence of acetate anions have been made. Considering the existence of an equilibrium in the excited state the values of 3.47×10 and 0.216×10 M s have been obtained for the rate constants for direct and inverse reactions, respectively. Moreover, the lifetime (t0 %=0.58×10 −9 s) and quantum yield (0.01) of non-protonated etilefrine have been determined. Our results seem to support the existence of a dynamic quenching process based on a proton transfer mechanism induced by acetate anions. This process could represent a serious inconvenience in analytical fluorimetric techniques taking into account that the acetic acid/acetate pair is commonly used as a buffer. Additional fluorescence quenching by H ions could be involved in acid aqueous mediums. At high concentrations of acetic acid, a value of 2.98×10 M s for the bimolecular constant for the quenching by H has been calculated. © 1999 Elsevier Science B.V. All rights reserved.