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

Abstract

Abstract 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 formation of the strong two-electron-multicenter bonding. Crystal structure analysis revealed that TOT derivatives show strong self-assembling ability forming one-dimensional columns, which further construct three-dimensional networks by multiple intercolumnar non-covalent interactions due to the absence of bulky substituent groups. Substituent groups at the apexes of the triangular carbon-framework of TOT afford variations of the π-stacking mode in the one-dimensional columns, influencing the magnetic properties and photo-absorptions around the near-infrared region. The electronic effect of the substituent groups also affects the redox potentials of TOT. The peculiarly high stability of TOT neutral radicals and their three-dimensional networks by robust intermolecular interactions achieved in our study are very beneficial for the molecular design of new polycyclic air-stable neutral radicals. Furthermore, we believe that the open-shell electronic structures of neutral π-radicals, which are quite different from those of close-shell molecular systems, will also produce milestones for the exploration of peculiar physical properties and catalytic activity for organic transformation originating from their unconventional electronic-spin nature.