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

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 (MMCN₄-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 MMCN₄-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.