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 π-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 π-conjugated
molecules with poly-radical characters and also gives some insights
into molecular magnetism in neutral π-conjugated polycyclic
heteroarenes.