Thermodynamically Stabilized π-Conjugated Nitroxide with SOMO-HOMO Energy Level Inversion.

Organic radicals with SOMO-HOMO energy level inversion (SHI) have attracted attention not only because they formally violate the Aufbau principle, but also because they offer the potential to enhance radical stability and realize unconventional magnetic and photophysical properties. In particular, SHI radicals exhibiting significant spatial overlap between the SOMO and HOMO are especially fascinating. However, their development has been hindered by their low stability and the lack of a concrete molecular design principle. Herein, we report the synthesis and isolation of a thermodynamically stabilized π-conjugated nitroxide radical with SHI electronic configuration. The SHI radical, designed by introducing electron-donating diphenylamino groups at carbon atoms with large HOMO coefficients but negligible SOMO coefficients, exhibited unique properties such as the generation of a triplet-ground-state diradical cation upon electrochemical oxidation and spin-center transfer induced by protonation and hydrogen-bonding interactions. By utilizing these properties, multiple NIR chromisms, including electrochromism, halochromism, and thermochromism, were achieved. These findings establish a molecular design principle for stable SHI radicals with significant SOMO-HOMO overlap and provide a platform for developing functional organic radical-based materials responsive to multiple external stimuli.