Effective Regulation of Organic Radicals within a Radical-Doped Metal-Organic Framework by Post-Treatment

This work reports a rare radical-doped metal-organic framework (MOF) crystal with one-dimensional open channels achieved by coordination-driven self-assembly. X-ray single-crystal diffraction, solid-state UV-vis-NIR diffuse reflectance spectroscopy, and electron paramagnetic resonance analysis were used to clarify its unique structure and why many organic radicals stably exist in the crystal lattice. The study shows that this crystal has strong intermolecular cofacial π-π interactions, a narrow optical bandgap (only 1.47 eV), a low LUMO, and a shallow HOMO. These features allow an inherent charge transfer, leading to the genetation of many organic radicals under ambient conditions. Interestingly, post-treatment can effectively adjust the organic radicals in this MOF. Mechanical grinding quickly reduces the number of organic radicals. Then, soaking the sample in an N,N-dimethylformamide solution with diphenylamine, para-methylaniline, or carbazole (as electron donors), or physically grinding with these amine organic molecules, can recover the reduced radicals to some degree.