pH Controls Charge Localization in Redox-Active Ladder Polymers

Organic mixed ionic-electronic conducting polymers (OMIECs) are versatile active materials for applications in transistors, energy storage, and bioelectronics. To meet the varied demands of these technologies, the chemical structure of an OMIEC is often designed with the goal of modifying the localization of added charge, modulating the energetics of frontier orbitals, and altering the degree of charge transfer to charge compensating species. Here, we show that the redox behavior of the archetypal ladder OMIEC, poly(benzimidazobenzophenanthroline) (BBL), is fundamentally modulated by the pH of the electrolyte, even under neutral to basic conditions where protons were previously assumed to not participate in redox processes. Through a combination of electrochemical characterization, operando Raman spectroscopy, ab initio simulations and electrochemical modeling with a multicomponent regular solution framework, we untangle BBL's redox mechanism. Our results reveal the competitive formation of proton-coupled and salt cation-coupled redox states, each possessing distinct characteristics. Notably, we find that proton-coupled redox dominates at neutral pHs, challenging the prevailing view that BBL is reduced to its salt-compensated bipolaronic form in this pH regime. Using a modified Pourbaix diagram, we illustrate how the balance between a proton-coupled and salt cation-coupled form of BBL can be continuously tuned via pH and applied potential. These findings highlight the complexity of multiphase coexistence and nontrivial effect of pH in controlling the redox properties of n-type ladder OMIECs, paving the way to understand and ultimately control a wide range of aqueous electrochemical reactions.