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Updated: Jun 12, 2025

Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
Published on: September 18, 2016
Excited-state aromaticity reversals in norcorrole
Peter B Karadakov1, Edward Cummings1
1Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK. peter.karadakov@york.ac.uk.
Abstract:
Aromaticity reversals between the electronic ground state (S0) and the lowest triplet (T1) and singlet (S1) excited states of NiII 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.
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