Competition of Stacking and Piling Improves Molecular Solubility in Electrolyte

Figure Caption: Concentrated BzNSN solutions in acetonitrile: a) Competitive packing motifs; b) Small-angle X-ray scattering revealing aggregates; and c) molecular dynamics snapshots for different BzNSN concentrations .

Scientific Achievement

Organic redox-active molecules (redoxmers) are charge carriers in redox flow cells. Since the energy density of a battery fluid is proportional to concentration of active molecules, high molecular solubility is desirable. However, as the redoxmer solutions become crowded, solute-solute interactions become stronger, opposing high solubility. Here we demonstrate how a small redoxmer molecule, 2,1,3-benzothiadiazole (BzNSN), can achieve exceptionally high solubility through a competition of two packing motifs – N-H bond piling and π-stacking – that introduces disorder that frustrates crystallization in concentrated solutions.

Significance and Impact

Our observation that competing packing motifs improve redoxmer solubility suggests new strategies for designing concentrated all-organic electrolytes for redox flow cells.

Research Detail

  • High BzNSN solubility in acetonitrile arises from competitive molecular interactions of forming face-to-face planar π-stacks or random bonds between N and H atoms in the molecule (N-H piling)
  • Small-angle X-ray scattering (SAXS) indicates these two solute-solute interactions in close competition
  • The solubility limit of ~6 M corresponds to a leveling off of the fraction of molecules in π-stacks
  • Molecular dynamics reveals details of molecular interactions and their effect on solution properties

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DOI: 10.1021/acs.jpcb.0c07647

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