Trioxotriangulene: Air- and Thermally Stable Organic Carbon-Centered Neutral π-Radical without Steric Protection

To stabilize organic neutral radicals, which are usually very unstable chemical species in air atmosphere, “steric protection” is the most general and indispensable method. Based on the design of electronic-spin structure of polycyclic carbon-centered π-radicals, we have for the first time realized a peculiarly stable neutral π-radical without bulky substituent groups, 4,8,12-trioxotriangulene (TOT), whose decomposition point is higher than 240 °C in the solid state under air. This remarkably high air-stability as a neutral radical is achieved by spin-delocalization originating from the symmetric and expanded π-electronic structure of TOT. The oxo-functionalities also highly contribute to the high stability through electronic-spin modulation, where the largest electronic spin density located at the central carbon atom further decreases the spin densities of the peripheral carbon atoms. In the solution state, TOT is in the equilibrium between the monomer and highly symmetric π-dimer, as stabilized by the f...