Insights into dissociative electron transfer in esterified shikonin semiquinones by in situ ESR/UV-Vis spectroelectrochemistry

Abstract

In this work, electrogenerated anion and dianion species from shikonin and its ester derivative isovalerylshikonin were characterized by means of ESR/UV-Vis spectroelectrochemistry. Analysis of the spectra supported the proposal that stepwise dissociative electron transfer (DET) takes place during the second reduction process of the esterified compound. Quantum chemical calculations were performed for validating the occurrence of this mechanistic pathway and for obtaining thermodynamic information on the electron transfer process; ΔG⁰_cleavage was estimated to be −0.45 eV, considering that the two possible products of the overall reaction scheme are both a quinone and carboxylate anions.