Synthesis, Characterization, and Magnetic Interactions of Carbazole-9-oxyl and its Derivatives

Carbazole-9-oxyl, a thermodynamically stabilized π-conjugated radical, and its derivatives were successfully synthesized via a new synthetic route in two steps starting from 2,2'-dibromobiaryls. These radicals are stable enough to be purified using silica-gel column chromatography, allowing for the isolation of methyl and chloro derivatives. ESR measurements and DFT calculations confirmed that an unpaired electron delocalizes not only on the nitroxide moiety but also on the biphenyl moiety. The delocalization of unpaired electron thermodynamically stabilizes these radicals, and no significant decomposition was observed in solution under air for 24 h. These radicals exhibited weak absorption bands at 500–650 nm and amphoteric redox properties. In crystal, the methyl derivative formed π-dimers and one-dimensional chains, leading to intermolecular antiferromagnetic and ferromagnetic interactions, respectively. This study highlights carbazole-9-oxyl as a promising motif for functional magnetic materials.