The effect of anion complexation on the aromatic properties of aromatic and antiaromatic porphyrinoids

Abstract

The effect of anion complexation on magnetically induced current densities and excitation energies of antiaromatic molecular rings has been investigated by calculations on expanded antiaromatic porphyrinoids including orangarin, rosarin, amethyrin and on a theoretically predicted strongly antiaromatic hydrocarbon ring. Magnetically induced current densities and the lowest vertical excitation energies have been calculated at the density functional theory (DFT) and time-dependent DFT (TDDFT) levels using the M06-2X functional. Similar calculations have been performed on sapphyrin, cyclo[6]carbon and rubyrin, which are aromatic expanded porphyrinoids. The calculations show that anion complexation weakens the strength of the ring currents and the degree of (anti)aromaticity of the studied porphyrinoids and the antiaromatic hydrocarbon ring, because electronic charge is transferred from the anion to the molecular ring. The anion complexation weakens the calculated ring-current strength susceptibility of the antiaromatic porphyrinoids by 5–7 nA T⁻¹ (25–30%), by 6–16 nA T⁻¹ (21–48%) for the aromatic porphyrinoids, and by 8 nA T⁻¹ (27%) for the antiaromatic hydrocarbon ring, whereas the current-density pathways remain the same for most molecules. Calculations on the lowest excited states show that the electronic excitation transfers electron density from the anion to the molecular ring. An antiaromatic heterocyclic molecular ring with five inner NH moieties that was constructed from the antiaromatic hydrocarbon ring was found to have a large Cl⁻ complexation energy of 67.6 kcal mol⁻¹.