Simple Air-Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water

The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a pi-dimer, and a sigma-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, sigma-dimer in water that converts to the radical anion with the addition of organic solvents or to a pi-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a pi-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or pi-dimer exhibit significantly enhanced cycling profiles. Ester-substituted [3]radialene radical anions dimerize in solutions of water. The dimerization mode is structure specific and can be controlled by using heat, solvent, and salt. These results provide guidance for increasing the capacity of aqueous redox flow batteries containing radialene catholytes. A buildup of sigma-dimer prevents stable charge-discharge cycling. The formation of pi-dimers through the addition of salt increases the stability of the charged species.image