Competitive Pi-Stacking and H-Bond Piling Increase Solubility of Heterocyclic Redoxmers

Abstract

Redoxmers are organic molecules that carry electric charge in flow batteries. In many instances, they consist of heteroaromatic moieties modified with appended groups to prevent stacking of the planar cores and increase solubility in liquid electrolytes. This higher solubility is desired as it potentially allows achieving greater energy density in the battery. However, the present synthetic strategies often yield bulky molecules with low molarity even when they are neat and still lower molarity in liquid solutions. Fortunately, there are exceptions to this rule. Here, we examine one wellstudied redoxmer, 2,1,3-benzothiadiazole, which has solubility ∼5.7 M in acetonitrile at 25 °C. We show computationally and prove experimentally that the competition between two packing motifs, face-to-face π-stacking and random N−H bond piling, introduces frustration that confounds nucleation in crowded solutions. Our findings and examples from related systems suggest a complementary strategy for the molecular design of redoxmers for high energy density redox flow cells.

Insights into the Nanostructure, Solvation, and Dynamics of Liquid Electrolytes through Small‐Angle X‐Ray Scattering

“…In our recent collaborative work, the packing motifs of a well‐studied redoxmer, 2,1,3‐benzothiadiazole, were investigated by SAXS and MD simulation. [ 62 ] This redoxmer is a small, symmetric, planar molecule that have the anomalously high solubility in electrolyte solutions. We show computationally and prove experimentally that competition between two packing motifs, face‐to‐face π‐stacking and random NH bond piling, introduces frustration that confounds nucleation in crowded solutions.…”

Experimental Protocols for Studying Organic Non-aqueous Redox Flow Batteries

“…26 Often, MD is used to compute radial distribution functions and to simulate small-angle X-ray scattering (SAXS) spectra. 10,19 These provide molecular-level insights into the bulk solvation structure of the solute, which can be used to understand solubility trends of different ROM species in non-aqueous solution. 10 To include another critical design parameterthe stability of ROMsseveral methods based on first-principles calculations have been proposed.…”

“…10,19 These provide molecular-level insights into the bulk solvation structure of the solute, which can be used to understand solubility trends of different ROM species in non-aqueous solution. 10 To include another critical design parameterthe stability of ROMsseveral methods based on first-principles calculations have been proposed. 17,18 Perhaps the simplest approach is to investigate whether a molecule is susceptible to undesirable side reactions (e.g., ring-opening, de/re protonation, bond dissociation or formation) upon reduction/ oxidation.…”

Balancing high energy density and chemical stability in redox flow batteries with symmetric tetrazines

“…To date, many organic molecules have been evaluated as anolytes (negative charge carriers) and catholytes (positive charge carriers) in NRFBs. 14,15,16,17 Among the anolytes, Nheterocyclic aromatic compounds have been prominent due to their reversible reductions, including viologens, 18,19,20,21,22,23 Nsubstituted phthalimides, 24,25,26,27,28,29,30 2,1,3benzothiadiazoles, 31,32,33,34 pyridiniums, 35,36,37,38,39,40 and bipyrimidines. 41 Despite these successes, the structural variety within these anolytes is limited due to the stringent requirements posed by the application (i.e., combining low redox potentials and high solubility while being exceptionally stable in all states of charge).…”

“…Other creative approaches to solve this problem include adding ionic moieties to attenuate π-stacking through electrostatic repulsion or introducing H-bonding interactions to outcompete πstacking. 33 A most unusual property of s-tetrazines is that due to π-orbital symmetry, π-stacking is not favourable (based on crystal structures 44,45,46 ). As a result, we predicted that stetrazines would exhibit high solubility in polar solvents even as low molar mass materials.…”