Validation of the triplet ground state in a persistent derivative of Clar’s hydrocarbon

Triangulene is a prototypical non-Kekulé diradical known as Clar’s hydrocarbon. It is the smallest non-Kekulé graphene fragment comprised of six benzenoid rings fused in a triangular shape. In accordance with Hund’s rule, a triplet ground state is predicted for this hydrocarbon. However, because of its high reactivity, synthesis of a stable or persistent derivative of neutral triangulene to validate this prediction has been an elusive goal for almost 70 years. Herein, we report the synthesis and characterization of the first persistent derivative of triangulene, kinetically stabilized by three sterically demanding substituents installed at the most reactive positions in the centers of the zigzag edges. The triplet ground state of this hydrocarbon is unequivocally confirmed by pulse electron paramagnetic resonance spectroscopy and supported by density functional theory. The realization of a long-sought-after persistent triangulene derivative will motivate the synthesis of analogous open-shell graphene fragments as prospective building blocks of high-spin materials for the newly emerging field of organic spintronics.