Title: Stable Cyclopropenium-Based Radicals
Description: Stable radicals have enjoyed widespread use in a variety of fields including synthetic chemistry, materials chemistry, energy storage, and biochemistry. This thesis outlines our investigations of cyclopropenium-based stable radicals and their application as redox mediators, redox-active ligands, catalysts, and materials for energy storage. The first chapter gives a brief overview of the use of radicals in synthetic chemistry. The principles that govern the stability of radicals is discussed and notable examples are highlighted. The second section of the first chapter reviews the aromatic platforms that have been developed by the Lambert group and how they might be converted into stable radical species. The second chapter details our study of 2,3-diaminocyclpropenones as stable radicals. These electron rich cyclopropenium derivatives undergo facile oxidation to yield a radical cation species. The origin of the stability of this oxygen-centered radical was elucidated by density functional theory calculations and analysis of the crystal structure.

Diaminocyclopropenones were also found to be effective neutral L-type ligands in Ce(IV) complexes. EPR and UV-VIS experiments revealed that these complexes exhibited reversible homolytic dissociation of their diaminocyclopropenone ligands. The third chapter describes the use of trisaminocyclopropeniums as catholytes for nonaqueous redox flow batteries. A newly designed trisaminocyclopropenium structure could be accessed in large quantities and showed long lasting stability in its oxidized state. A new composite polyionic material was developed for use as a membrane suitable for organic solvent and high voltages. Cycling in combination with a perylenediimide anolyte yielded a 1.7 V battery that exhibited excellent coulombic efficiency and capacity retention. Using a spiro-bis(phthalimido) anolyte afforded a battery with an open circuit voltage of 2.8 V. The fourth chapter details how our battery studies with trisaminocyclopropenium radical dications led us to discover their photoinduced reactivity.

We developed an electrophotocatalytic platform using trisaminocyclopropeniums as a species capable of being activated by both photochemical and electrochemical energy. The excited state oxidation potential of the doubly activated species was found to be +3.33 V, which was capable of effecting oxidative coupling reactions using both arenes and ethers as substrates. Density functional theory calculations and spectroscopic experiments revealed that the photoreactivity was due to a SOMO-inversion event. The trisaminocyclopropenium radical dication could be prepared on scale via direct electrolysis and subsequently used in high throughput screening.