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

Area of Science:

• Electrochemistry
• Materials Science
• Organic Chemistry

Background:

• Hexacyano[3]radialene radical anion (1) shows promise as a catholyte material for redox flow batteries (RFBs).
• Ester substitution improves solubility and maintains redox properties of radialene radical anions.
• Understanding dimerization is crucial for optimizing catholyte stability and performance.

Purpose of the Study:

• To investigate the reversible dimerization of ester-functionalized hexasubstituted [3]radialene radical anions in aqueous solutions.
• To explore the stimuli-responsive behavior and tristate switching mechanism of these radical anions.
• To correlate dimerization equilibrium with catholyte stability during redox flow battery cycling.

Main Methods:

• Synthesis and characterization of ester-functionalized [3]radialene radical anions (2 and 3).
• Spectroscopic analysis (UV-Vis, EPR) to study dimerization modes (π-dimer, σ-dimer, radical anion).
• Electrochemical studies including galvanostatic charge-discharge cycling in static H-cells.

Main Results:

• Ester-functionalized [3]radialene radical anions exhibit reversible dimerization in water, with modes dependent on substitution patterns.
• A tristate switching mechanism allows toggling between free radical, π-dimer, and σ-dimer states via solvent, temperature, or salinity.
• Triester-tricyano[3]radialene (3) forms a σ-dimer in water, while diester-tetracyano[3]radialene (2) exists as a π-dimer.
• Conditions favoring the free radical or π-dimer significantly enhance catholyte stability during battery cycling.

Conclusions:

• Reversible, stimuli-responsive dimerization of ester-functionalized [3]radialene radical anions is demonstrated.
• The ability to control dimerization states offers a pathway to tune catholyte properties for improved redox flow battery performance.
• This work provides insights into molecular design for advanced energy storage materials.