Topology-Enabled Stabilization of Imidazolyl Radicals in a Tetrahedral Metal-Organic Cage for Oxygen Sensing.

Molecular oxygen is a highly efficient radical scavenger, yet the development of radical-based oxygen sensors remains challenging due to the intrinsic instability of organic radicals under ambient conditions. Herein, we report a tetrahedral metal-organic cage (XM1) that enables the generation, stabilization, and partially recoverable chemical switching of imidazolyl radicals under mild ultraviolet irradiation. Upon UV exposure, homolytic C-N bond cleavage within the imidazolium units generates nitrogen-centered imidazolyl radicals, as evidenced by EPR spectroscopy (g = 2.004) and mass spectrometric identification of selectively cleaved imidazole-containing fragments. The accumulation of radicals gives rise to a pronounced photoinduced color change and a characteristic absorption band at 620 nm. Owing to their high reactivity toward molecular oxygen, the cage-stabilized radicals are rapidly quenched, resulting in instantaneous decolorization. Control experiments demonstrate that analogous mononuclear or single-arm complexes fail to stabilize persistent radicals, highlighting the critical role of cage topology in suppressing radical-radical recombination through spatial separation. This work establishes supramolecular topology as an effective strategy for stabilizing reactive open-shell species and introduces a chemically specific, radical-mediated paradigm for oxygen sensing.