Abstract:

Electrochemical CO₂ capture with redox-active organic molecules is often limited by oxygen sensitivity because the reduced states responsible for capture can also transfer electrons to O₂, leading to parasitic carrier reoxidation and the formation of reactive oxygen species. This challenge affects both quinones, which bind CO₂ directly after reduction, and flavins, whose proton-coupled redox chemistry can drive aqueous pH swings. Biological systems tune both cofactors through precisely positioned hydrogen bonds and electrostatic interactions, motivating analogous molecular designs. Prior work showed that carefully designed hydrogen-bond donor–acceptor interactions can shift quinone reduction potentials to less-negative values without proportionally weakening CO₂ binding. Related studies of hydroxy-substituted anthraquinones, however, demonstrated that rigid, built-in donors can restrict CO₂ binding. We therefore examined whether a flexible, nonconjugated hydroxymethyl donor, covalently held near the quinone core, could stabilize reduced states while preserving CO₂ affinity. Electrochemical, spectroscopic, and computational studies show that the hydroxymethyl group stabilizes the reduced quinone through inductive and hydrogen-bonding effects, while chlorination shifts reduction potentials anodically with only modest changes in CO₂-binding affinity. As a complementary exploratory study, this design principle was extended to water-soluble flavin derivatives bearing charged and hydrogen-bonding side chains. Their electrochemical, acid–base, and spectroscopic properties were evaluated to assess how these modifications influence proton-coupled redox behavior and their potential for aqueous pH-swing capture. Together, these studies examine how bio-inspired molecular environments can tune direct CO₂ binding and proton-coupled capture pathways.