Fully Fused Quinoidal/Aromatic Carbazole Macrocycles with Poly-radical Characters

While the chemistry of open-shell singlet diradicaloids has been successfully developed in recent years, the synthesis of pi-conjugated systems with poly-radical characters (i.e., beyond diradical) in the singlet ground state has been mostly unsuccessful. In this study, we report the synthesis and isolation of two fully fused macrocycles containing four (4MC) and six (6MC) alternatingly arranged quinoidal/aromatic carbazole units. Ab initio electronic structure calculations and various experimental measurements indicate that both 4MC and 6MC have an open-shell singlet ground state with moderate tetraradical and hexaradical characters, respectively. Both compounds can be thermally populated to high-spin excited states, resulting in weak magnetization at room temperature. Our study represents the first demonstration of singlet pi-conjugated molecules with poly-radical characters and also gives some insights into molecular magnetism in neutral g-conjugated polycyclic heteroarenes.