Phosphine Oxide‐Functionalized Terthiophene Redox Systems

Main group systems capable of undergoing controlled redox events at extreme potentials are elusive yet highly desirable for a range of organic electronics applications including use as energy storage media. Herein we describe phosphine oxide‐functionalized terthiophenes that exhibit two reversible 1e− reductions at potentials below −2 V vs Fc/Fc+ (Fc=ferrocene) while retaining high degrees of stability. A phosphine oxide‐functionalized terthiophene radical anion was synthesized in which the redox‐responsive nature of the platform was established using combined structural, spectroscopic, and computational characterization. Straightforward structural modification led to the identification of a derivative that exhibits exceptional stability during bulk 2 e− galvanostatic charge–discharge cycling and enabled characterization of a 2 e− redox series. A new multi‐electron redox system class is hence disclosed that expands the electrochemical cell potential range achievable with main group electrolytes without compromising stability. Although main group redox systems are attractive as next‐generation energy storage media, achieving high cell voltages via extreme redox couples typically compromises their long‐term stabilities. We show that phosphine oxide functionalization can render terthiophenes robust two‐electron acceptors at very low potentials, thus opening fundamentally new parameter space in the search for stable main group redox systems with high cell voltages.