3D global aromaticity in a fully conjugated diradicaloid cage at different oxidation states

A conjugated diradicaloid cage has been synthesized and its aromaticity was investigated. The neutral compound and the dication have dominant monocyclic conjugation pathways and both are aromatic (the former following Huckel's rule and the latter Baird's rule). The tetracation ([6n + 4] pi-electrons) exhibits global 3D antiaromaticity whereas the hexacation ([6n + 2] pi-electrons) exhibits global 3D aromaticity and has high D-3 symmetry. Aromaticity is a vital concept that governs the electronic properties of pi-conjugated organic molecules and has long been restricted to 2D systems. The aromaticity in 3D pi-conjugated molecules has been rarely studied. Here we report a fully conjugated diradicaloid molecular cage and its global aromaticity at different oxidation states. The neutral compound has an open-shell singlet ground state with a dominant 38 pi monocyclic conjugation pathway and follows the [4n + 2] Huckel aromaticity rule; the dication has a triplet ground state with a dominant 36 pi monocyclic conjugation pathway and satisfies [4n] Baird aromaticity; the tetracation is an open-shell singlet with 52 pi-electrons that are delocalized along the 3D rigid framework, showing 3D global antiaromaticity; and the hexacation possesses D-3 symmetry with 50 globally delocalized pi-electrons, showing [6n + 2] 3D global aromaticity. Different types of aromaticity were therefore accessed in one molecular cage platform, depending on the symmetry, number of pi-electrons and spin state.