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Abstract
The unsubstituted [n]radialenes are a unique class of alicyclic hydrocarbons with radiating alkenes. These compounds are typically difficult to handle and polymerize rapidly. A number of stable radialene derivatives that exhibit reversible electron transfer events are known and are candidates for use in energy storage applications. Specifically, substituted trimethylenecyclopropane dianions, a subclass of hexasubstituted [3]radialenes, are stable in water, exhibit reversible redox processes, and can be synthetically modified. These compounds are thus attractive for use in redox flow batteries (RFBs). Hexacyano-[3]radialene disodium is investigated as a pH 7 aqueous organic catholyte. The dianion and radical anion are stable in air and aqueous solutions at neutral pH. Systematic introduction of asymmetry via step-wise synthesis leads to enhanced solubility and higher capacity retention during galvanostatic cycling. An aqueous flow cell comprising a disester-tetracyano [3]radialene catholyte, sulfonated-methyl viologen as the anolyte, and a cation exchange membrane provides an operating Vcell = 0.9 V, 99.609 % coulombic efficiency, and minimum capacity fade over 50 cycles. Additionally, derivates modified with polyethylene glycol (PEG) units were synthesized in an effort to increase water solubility. A monosubstituted (monoglycol) [3]radialene dipotassium salt, 2K+[C6(CN)5(EG)]2- (EG = Ethylene glycol), was synthesized and structurally characterized. It retained its redox activity in aqueous solutions. Longer glycol chains are also being synthesized to increase solubility.