Excited-state aromaticity reversals in norcorrole

Abstract

Aromaticity reversals between the electronic ground state (S₀) and the lowest triplet (T₁) and singlet (S₁) excited states of Ni^II norcorrole (NiNc) and norcorrole (H₂Nc) are investigated by comparing the HOMA (harmonic oscillator model of aromaticity) values at the optimized S₀, T₁ and S₁ geometries, and by analysing the changes in the nucleus-independent chemical shift (NICS) values and in the isotropic magnetic shielding distributions between the S₀ and T₁ states. The results strongly suggest that the antiaromatic features of the S₀ states of the NiNc and H₂Nc molecules, two very similar antiaromatic “internal crosses”, undergo aromaticity reversals upon excitation to T₁ or S₁ and merge with the aromatic peripheries to produce Baird-aromatic systems with 24 π electrons each. Somewhat counterintuitively, the geometries of the fully aromatic T₁ and S₁ states of NiNc and H₂Nc turn out to have larger bowl depths and so are more non-planar than the corresponding S₀ geometries at which both molecules display antiaromatic features.