Air-Stable Cobalt-Semiquinone Radical Complexes on Carbon Nanotubes: A Redox Switch for Anion Response

Surface-bound radicals represent an emerging frontier for controlling the physical and chemical properties of nanomaterials in applications across sensing, electrocatalysis, spintronics, and redox-switchable devices. However, air- and moisture-stable radicals are rare, and harnessing their chemical reactivity for nanomaterial function remains underexplored. Herein, we report the synthesis and characterization of a dianionic cobalt complex featuring a ligand-centered semiquinone radical and outer-sphere pyrene trimethylammonium cations ([Pyr]₂[Co]), which can be noncovalently immobilized on single-walled carbon nanotubes (SWCNTs). The resulting hybrid material (SWCNT-[Pyr]₂[Co]) features molecularly defined surface functionalization, as established by Raman and X-ray photoelectron spectroscopic characterization in combination with electron microscopy. Electron paramagnetic resonance (EPR) spectroscopy revealed an interaction between the paramagnetic cobalt complex and the SWCNT surface, with radical character persisting in the presence of air and moisture for several months. Electrochemical studies showed that the ligand-centered radical in [Pyr]₂[Co] undergoes reversible oxidation at mild potentials, triggering selective chemical reactivity with exogenous cyanide ions (CN^-). This feature, in combination with the environmental stability of the radical, was leveraged to demonstrate proof-of-concept electrochemical CN^- detection using SWCNT-[Pyr]₂[Co]. Overall, our findings establish a remarkably stable radical-nanotube interface and outline a general strategy for constructing hybrid materials with redox-switchable function.