In situ FT-IR spectroelectrochemical investigation of the electrochemical reduction of duroquinone

Abstract

Quinone is a bioactive compound present in some traditional Chinese medicines and participates in numerous biological processes. The carbonyl group in quinone serves as the active center in electrochemical reactions, functioning as an excellent carrier of electron transfer. In this study, in situ Fourier transform infrared (FT-IR) spectroelectrochemistry enables the real-time tracking of three-dimensional (3D) information at molecular level of different substances throughout the electrochemical process. The detection of absorption peaks corresponding to dimerization and hydrogen bonding in the Fourier transform infrared 3D spectra proves that the electrochemical reduction of duroquinone (DQ) in anhydrous acetonitrile involves dimer formation and development of intramolecular hydrogen bonds within monovalent anion radicals. Consistent with these findings, cyclic voltammetry reveals two pairs of redox peaks and one irreversible anodic peak. Surprisingly, ionic liquids with complex structures of ions, such as 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), make the reduction simpler. Conversely, the transformation of DQ to DQ^•- and subsequently to DQ^2- without dimer or hydrogen bond occurring, as clearly observed in the infrared cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA). DQ undergoes a straightforward, sequential two-step, one-electron transfer process facilitated by the unique composition of ionic liquids, potentially prompting further evaluation of the roles of cations and anions in ionic liquids.