Electronic inductive and resonance effects of substituents on concerted two-proton-coupled electron transfer between electrogenerated superoxide and hydroquinone derivatives in N,N-dimethylformamide

Abstract

Improving energy conversion technologies is pivotal for reducing the effects of global warming. The use of electron transfer catalysts based on electron carriers found in organisms, such as redox biomimetic quinone systems, is a promising route for truly efficient energy conversion. Herein, we report the density functional theory (DFT)-based analysis of the reaction between electrogenerated superoxide radical anion (O2•−) and methyl- or chloro-substituted benzene-1,4-diol (hydroquinone) derivatives in N,N-dimethylformamide. We investigate the electrochemical scavenging of O2•− by hydroquinone derivatives involving two proton transfers (PTs) and one electron transfer (ET). We show that chloro and methyl substituents promote a proton-coupled electron transfer (PCET) reaction with the effect increasing with the number of substituent groups. Our DFT results demonstrate that the substituent effect promotes either of the PT or ET processes, along a sequential PCET pathway. The electrochemical and DFT analyses of the substituent effect indicate that the concerted two-proton-coupled electron transfer (2PCET) promoted by an increasing number of the substituents is a plausible pathway for the reaction between electrogenerated O2•− and hydroquinone derivatives. In addition, our findings show that the resonance effect is the main driving force for the promotion of 2PCET.