Multifaceted effects of ring fusion on the stability of charged dialkoxyarene redoxmers

Abstract

Due to their almost unlimited scalability, redox flow batteries can make versatile and affordable energy storage systems. Redox active materials (redoxmers) in these batteries largely define their electrochemical performance, including the life span of the battery that depends on the stability of charged redoxmers. In this study, we examine the effects of expanding the π-system in the arene rings on the chemical stability of dialkoxyarene redoxmers that are used to store positive charge in RFBs. When 1,4-dimethoxybenzene is π-extended to 1,4-dimethoxynaphthalene, a lower redox potential, improved kinetic stability, and longer cycling life are observed. However, when an additional ring is fused to make 9,10-dimethoxyanthracene, the radical cation undergoes rapid O-dealkylation possibly due to increased steric strain that drives methoxy out of the arene plane thus breaking the π-conjugation with O 2p orbitals. On the other hand, the planar structure of 1,4-dimethoxynaphthalene may facilitate second-order reactions of radical cations leading to their neutralization in the bulk. Our study suggests that extending the π-system changes reactivity in multiple (sometimes, opposite) ways, so lowering the oxidation potential through π-conjugation to improve redoxmer stability should be pursued with caution.