Solvent effect on the anodic oxidation of tannic acids: EPR/UV–Vis spectroelectrochemical and DFT theoretical study

Abstract

Cyclic voltammetry of tannic acid mixture (TAM) in water, dimethylsulfoxide (DMSO), and acetonitrile (ACN) indicates the highest oxidation potential for TAM in ACN, followed by the slightly lowered potential in DMSO and a strongly shifted oxidation potential in water solutions, confirming its pH-dependent redox behavior. In situ EPR and UV–Vis spectroelectrochemical experiments were performed to follow the oxidation reactions of TAM in protic and aprotic media. The formation of an unstable semiquinone anion radical formed upon anodic oxidation of TAM was proved by in situ EPR spectroelectrochemistry both in DMSO and water solutions. The quantum chemical calculations of the model pyrogallol derivatives and tannic acid molecules with four and ten galloyl moieties estimated the role of the spatial hydrogen bonds on the proton affinities and suggested the possible interpretation of experimentally detected redox and spectroelectrochemical behaviors.