Solution-synthesized stable triaza[4]triangulene triradical with a quartet ground state

As a renowned class of open-shell molecules characterized by triangular nanographenic structures and novel magnetic properties with high-spin ground states, triangulenes are attractive targets to both synthetic chemists and molecular magnet researchers. Here, a highly stable aza-derivative of [4]triangulene triradical (1) is achieved, with a quartet ground state (S = 3/2). By virtue of the stability under ambient conditions (τ1/2 ~ 9 days in the solid state and 28 h in an air-saturated solution), thorough structural and magnetic characterizations are performed. Crystallographic analysis confirms that 1 has a planar triangular polycyclic aromatic skeleton embedded with delocalized spins. Concurrently, continuous-wave and pulsed EPR spectroscopies provide unambiguous evidence supporting its high-spin ground state. SQUID measurements reveal impressively large doublet-quartet energy gap (ΔED-Q = 3.18 kcal/mol), indicating very strong intramolecular ferromagnetic spin-spin exchanges (J/kB = +534 K) rendered by the triangulene structure. As an important class of open-shell molecules characterized by triangular nanographenic structures and interesting magnetic properties with high-spin ground states, triangulenes are attractive targets. Here, the authors report a highly stable aza-derivative of [4]triangulene triradical with a quartet ground state.