Synthesis and Isolation of a Kinetically Stabilized Crystalline Triangulene

Abstract

The synthesis and isolation of hydrocarbons with a triplet ground state in crystalline forms have been sought in materials science. Triangulene is one of the most famous tripletground-state benzenoid hydrocarbons. Its unique electronic structure and highly symmetric structure have prompted many scientists to synthesize and isolate triangulene and its derivatives, but all attempts so far to isolate them as crystals have been unsuccessful. Herein we report the synthesis and isolation of a kinetically stabilized crystalline triangulene for the first time. The key to success is the introduction of bulky substituents onto the reactive zigzag edges. Its highly symmetric structure was confirmed by X-ray crystallography, and its fundamental properties, including the triplet ground state, were revealed. The achievement here will open the door for the synthesis and isolation of other hydrocarbons with higher spin multiplicity.

Trimesityltriangulene: a persistent derivative of Clar's hydrocarbon

“…Simultaneously to this work, 27 Shintani et al 28 reported the synthesis of a persistent triangulene derivative kinetically stabilized by six substituents (Fig. 1, middle), which allowed its isolation in the solid state and structure confirmation by X-ray crystallography.…”

“…The precise addition of oxidizing agent allows the oxidation to be performed in a stepwise fashion; first a mixture of monoradicals 8a + 8b is formed, then diradical Mes 3 -Tr . This differs from the procedure reported by Shintani et al , 28 where the last step must be performed under reductive conditions. The oxidation method allows formation of pristine triangulene monoradical and its analysis by X-ray diffraction (Fig.…”

“…S10, ESI†), showing the outstanding stability of Mes 3 -Tr in deoxygenated solution. It is noteworthy that Mes 3 -Tr together with the hexasubstituted derivative 28 are rare examples 20,40 of purely organic triplet diradicals, where the triplet state could be detected in isotropic solution at room temperature. The reason might be the scarcity of persistent non-Kekulé diradicals prepared until now.…”

“…Due to high reactivity, the chemistry of non-Kekulé open-shell graphene fragments has until recently 27,28 been limited to ultra-high vacuum and low-temperature conditions. In this work, we presented a persistent derivative of an iconic member of this class of compounds—triangulene—kinetically stabilized by only three substituents, which sterically shield all nine reactive positions.…”

“…The triplet ground state was validated by means of continuous wave, pulse and variable-temperature EPR spectroscopy. The diradical species were generated by means of stepwise oxidation, which allows formation of pristine monoradical species, inaccessible via the alternative reduction 28 protocol. The realization of persistent derivatives of triangulene will motivate the synthesis of its analogs, enabling implementation of open-shell graphene fragments into solution-processed and solid materials.…”