Spectroelectrochemical studies on redox reactions of eosin Y and its polymerization with Zn2+ ions

Redox reactions of eosin Y (EY) in aqueous solutions in the presence and absence of Zn2+ were studied by spectroelectrochemical measurements combining cyclic voltammetry and in situ monitoring of absorption spectra, employing an optically transparent thin layer electrode (OTTLE). In the absence of Zn2+, the original dianion, EY2−, undergoes one electron reduction to trianion radical that is stable above pH 11, but is protonated and doubly reduced under the lower pH to become highly stabilized against re-oxidation. When Zn2+ is present, the reduction occurs as a two electron process coupled with complexation of two Zn2+ ions to bridge eosin Y molecules, resulting in polymeric mixed aggregates that are also greatly stabilized against re-oxidation. This reaction has been identified as the key process in the electrochemical self-assembly of ZnO/eosin Y hybrid thin films in nanowire structures. On the other hand, two electron oxidation of EY2− was found to convert the molecule into lactone form which is strongly stabilized against re-reduction. Despite of such highly complex redox reactions, these processes in overall were found to be reversible both electrochemically and spectroscopically.

► Redox reactions of eosin Y studied by in situ UV–Vis spectroelectrochemical analysis. ► Stoichiometry of reactions, number of electron and subsequent protonation revealed. ► Two electron reduction of eosin Y complexing with two Zn2+ ions to polymerize. ► Comprehensive reaction scheme of eosin Y presented. ► Essential process for electrochemical self-assembly of ZnO/eosin Y hybrid thin films.