UC Santa Barbara

Molecules with reversible, redox-switchable properties have potential application in many fields, including catalysis, electronics, waste remediation, and energy storage. To further this field, a detailed understanding of the relationship between chemical structure and electronic properties must be developed for different systems. Here, we first examine this relationship for phthalocyanines, a well-studied class of macrocyclic ligand. We first discuss generalized synthesis and characterization of a library of soluble, metal phthalocyanine species, and then proceed to investigate the electronic character of this macrocyclic ligand in extreme detail with modern experimental and computational methods, and discuss a fundamental discovery regarding a terminal manganese nitride phthalocyanine (EtOPcMnN) that is the first reported molecular species capable of reversibly switching between aromatic, non-aromatic, and anti-aromatic states. We then discuss possible reasons for the extreme stability of EtOPcMnN, and discuss more generalized conclusions pertaining to the electronic character of other phthalocyanines and related aromatic macrocycles.

Further studies leverage the unusual stability of EtOPcMnN for energy storage applications, revealing a potential use as a symmetric redox-flow battery charge carrier. Electrochemical kinetics and battery measurements are performed, revealing that EtOPcMnN can function as a 2-electron symmetric charge carrier capable of functioning as both anolyte and catholyte in a redox flow battery. A new architecture for redox-flow batteries is also developed, in which a redox-active “sediment” is utilized in lieu of a traditional solution-state charge carrier, and represents a possible strategy for increasing charge carrier loading beyond the solubility limit while preventing the high viscosity typically associated with slurry-based systems.

Finally, the redox-switchable properties of another class of ligand, carboranes, are examined for application as reversible chelating agents. It is demonstrated with a carborane phosphine species that redox events can be accessed to induce a C-C bond scission which results in a structural distortion of the carborane that can be utilized as a “molecular switch” capable of changing the bond angle of the phosphine ligands, resulting in a convenient electrochemical switch for the capture and release of metal ions. We demonstrate a proof-of-concept system in which uranyl can be electrochemically captured and released from biphasic systems in a reversible manner, and discuss the application of such systems for new nuclear waste remediation strategies.