Regioisomeric Engineering for Multicharge and Spin Stabilization in Two-Electron Organic Catholytes

Developing multicharge and spin stabilization strategies
is fundamental
to enhancing the lifetime of functional organic materials, particularly
for long-term energy storage in multiredox organic redox flow batteries.
Current approaches are limited to the incorporation of electronic
substituents to increase or decrease the overall electron density
or bulky substituents to sterically shield reactive sites. With the
aim to further expand the molecular toolbox for charge and spin stabilization,
we introduce regioisomerism as a scaffold-diversifying design element
that considers the collective and cumulative electronic and steric
contributions from all of the substituents based on their relative
regioisomeric arrangements. Through a systematic study of regioisomers
of near-planar aromatic cyclic triindoles and nonplanar nonaromatic
cyclic tetraindoles, we demonstrate that this regioisomeric engineering
strategy significantly enhances the H-cell cycling stability in the
above two new classes of 2e– catholytes, even when
current strategies failed to stabilize the multicharged species. Density
functional theory calculations reveal that the strategy operates by
redistributing the charge and spin densities while highlighting the
role of aromaticity in charge stabilization. The most stable 2e– catholyte candidate was paired with a viologen derivative
anolyte to achieve a proof-of-concept all-organic flow battery with
1.26–1.49 V, 98% capacity retention, and only 0.0117% fade/h
and 0.00563% fade/cycle over 400 cycles (192 h), which is the highest
capacity retention ever reported over 400 cycles in a multielectron
all-organic flow battery setup. We anticipate regioisomeric engineering
to be a promising strategy complementary to conventional electronic
and steric approaches for multicharge and spin stabilization in other
functional organic materials.