Investigation of the Redox Chemistry of Anthraquinone Derivatives Using Density Functional Theory

Application of density functional calculations to compute electrochemical properties such as redox windows, effect of substitution by electron donating and electron withdrawing groups on redox windows, and solvation free energies for ∼50 anthraquinone (AQ) derivatives are presented because of their potential as anolytes in all-organic redox flow batteries. Computations suggest that lithium ions can increase (by ∼0.4 V) the reduction potential of anthraquinone due to the lithium ion pairing by forming a Lewis base-Lewis acid complex. To design new redox active species, the substitution by electron donating groups is essential to improve the reduction window of AQ with adequate oxidative stability. For instance, a complete methylation of AQ can improve its reduction window by ∼0.4 V. The quantum chemical studies of the ∼50 AQ derivatives are used to derive a relationship that connects the computed LUMO energy and the reduction potential that can be applied as a descriptor for screening thousands of AQ derivatives. Our computations also suggest that incorporating oxy-methyl dioxolane substituents in the AQ framework can increase its interaction with nonaqueous solvent and improve its solubility. Thermochemical calculations for likely bond breaking decomposition reactions of unsubstituted AQ anions suggest that the dianions are relatively stable in the solution. These studies provide an ideal platform to perform further combined experimental and theoretical studies to understand the electrochemical reversibility and solubility of new quinone molecules as energy storage materials.