The search for new
redox-active organic materials (ROMs) is essential
for the development of sustainable energy-storage solutions. In this
study, we present a new class of cyclobuta[b]quinoxaline-1,2-diones
or squaric acid quinoxalines (SQXs) as highly promising candidates
for ROMs featuring exceptional stability and high redox potentials.
While simple 1,2- and 1,3-squaric acid amides (SQAs), initially reported
by Hünig and coworkers decades ago, turned out to exhibit low
stability in their radical cation oxidation states, we demonstrate
that embedding the nitrogen atoms into a quinoxaline heterocycle leads
to robust two-electron SQX redox systems. A series of SQX compounds,
as well as their corresponding radical cations, were prepared and
fully characterized, including EPR spectroscopy, UV–vis spectroscopy,
and X-ray diffraction. Based on the promising electrochemical properties
and high stability of the new ROM, we developed SQX-functionalized
polymers and investigated their physical and electrochemical properties
for energy-storage applications. These polymers showed remarkable
thermal stability well above 200 °C with reversible redox properties
and potentials of about 3.6 V vs Li+/Li. By testing the
galvanostatic cycling performance in half-cells with lithium-metal
counter electrodes, a styrene-based polymer with SQX redox side groups
showed stable cycling for single-electron oxidation for more than
100 cycles. These findings render this new class of redox-active polymers
as highly promising materials for future energy-storage applications.