Redox‐Modulated σ‐Bond Reactivity in a Radical‐Anion Dimer

ABSTRACT Functional materials, such as stimuli‐responsive polymers, require molecules that change their properties in response to environmental changes or external cues, but finding well‐behaved structures that exhibit large stimuli‐responsive property changes remains challenging. Here, a class of redox‐active, water‐ and air‐tolerant, substituted‐trimethylenecyclopropane radical dimers was synthesized that participate in dynamic covalent chemistry (DCC) and multielectron transfer. Density functional theory (DFT) was used to design methylmalonate‐tetracyano trimethylenecyclopropane (MMCN 4 ‐CP) radical‐anions with localized spin densities that dimerize upon formation. The resulting sigma (σ)‐dimers are structurally characterized by x‐ray diffraction, while their solubilities are tuned by the counterion choice. Dimerization occurs via carbon‐carbon bond formation in aqueous solutions or organic solvents by electrochemical or chemical oxidation, respectively. The σ‐dimers are redox‐active, undergoing dimer cleavage and formation of neutral [3]radialenes, following oxidation. The dimers are also broken at high temperatures in solution to yield radical anions. Furthermore, they are mechanically severed during ball milling, resulting in a dramatic color change to blue in the solid state as the radicals are liberated. We prepared modified MMCN 4 ‐CP dianions with pendent alcohol groups to incorporate these dimers into polyurethanes. The polyelectrolytes exhibit a mechanochromic response upon application of force and can be electrochemically modulated in thin film devices.