Fig. 1: Synthetic library of bispyridinium electrolytes.

a, Comparison between classical diradical hydrocarbons and bispyridinium diradicals. b, Schematic representation of bispyridine cores with a variety of redox modulators (1–9) and their fully alkylated analogues, the subject of this work (10–19). BiPy refers to classical 4,4′-bipyridine. c, Linear correlations obtained between DFT-calculated redox potential values and tabulated experimental meta Hammett constant values (σ_m)²⁷ for R-groups introduced to pyridinium nitrogens. Trendlines indicate least-squares linear fits obtained for first and second redox events. d, Plot of solubility versus first reduction potential for compounds 10–19. Filled circles indicate electrochemically reversible compounds. Empty circles indicate electrochemically irreversible compounds. The dashed line (grey) indicates the lowest reduction potential reached by a bispyridinium RFB electrolyte: substituted or unsubstituted. The shaded region indicates compounds with reduction potentials below those of any bispyridinium electrolyte featuring an unsubstituted core reported so far. e, Voltammograms for compounds 10–19 ordered by their respective singlet–triplet gap (E_ST) values. Potentials are referenced to the SHE. Previously reported compounds 10, 11 and 17 were included as positive controls for classical viologen as well as extended homocyclic and heterocyclic bispyridiniums, respectively^8,22,23. For clarity, chloride counterions are omitted in all cases.