Theoretical Investigation of the Substituent Effect on First and Second Reduction Potential of Benzoquinone Derivatives: From a DFT Perspective for Redox Flow Battery Application

Abstract

ABSTRACT This study investigated the electrochemical behavior of benzoquinone (BQ) and its derivatives using density functional theory (DFT) calculations at the B3LYP/6‐311+G(d) level of theory. The first and second reduction potentials of BQ and its derivatives were predicted in both gas and solvent phases, water, dimethyl sulfoxide (DMSO), and cyclohexane. The solvation free energies were determined using the conductor‐like polarizable continuum model (CPCM), highlighting the significant influence of solvent effects on the redox potentials. The DFT results reveal that electron‐withdrawing group (EWG)‐substituted BQs exhibit higher first and second reduction potentials compared to electron‐donating group (EDG)‐substituted derivatives. Specifically, –NO 2 shows first and second reduction potentials of 3.24 and 2.58 V, respectively, while –CN exhibits 3.22 and 2.51 V, and –CF 3 shows 3.19 and 2.43 V, respectively, in water. In contrast, EDG‐substituted BQs such as –NH 2 (2.86 and 2.12 V) and –OH (2.91 and 2.05 V) display comparatively lower reduction potentials. These trends reflect the influence of substituents on the electronic structure of the redox‐active quinone core, where EWGs reduce electron density and stabilize the reduced species, thereby increasing the reduction potential, whereas EDGs increase electron density and lower the reduction potential. Furthermore, coordination with Lewis acids and their decomposition products significantly enhances the reduction potentials of BQ. The calculated first and second reduction potentials are 6.07 and 3.95 V for BF 3 , 5.82 and 3.71 V for BCl 3 , and 6.35 and 4.28 V for AlF 3 , respectively. These results indicate that metal–Lewis acid interactions can substantially modulate the electrochemical properties of quinone systems. Unlike previous studies focusing primarily on single‐electron reduction processes, this work provides a unified DFT framework that simultaneously evaluates substituent effects, solvent polarity, and Lewis acid coordination on both first and second reduction processes of BQ derivatives.