Recently, antiaromatic compounds with 4n π-electrons have received much attention due to their intriguing properties, which are different from those of conventional aromatic compounds. They are expected to be applicable for ambipolar semiconductors and secondary batteries. Antiaromatic π-systems generally have a closed-shell configuration because of the lowering of the symmetry. Consequently, antiaromatic compounds generally show substantial bond-length alternation. It is quite challenging to investigate the properties of 4n π-conjugated systems upon enhancing their symmetry.

In this study, we have succeeded in preparing Ni(II) tetrabenzonorcorrole TBNc. Interestingly, the introduction of fused benzene rings resulted in bond-length equalization of the π-system of norcorrole NC. Moreover, we found that the enhancement of the symmetry of the norcorrole conjugation significantly decreases the HOMO–LUMO gap, which results in the emergence of a singlet diradical character and significant increase of the antiaromatic character.