Relations between the aromaticity and magnetic dipole transitions in the electronic spectra of heterocirculenes
Magnetically induced current densities have been calculated at the second-order Møller–Plesset perturbation theory (MP2) level for seven heterocirculenes and their dicationic and dianionic forms. Calculations of the magnetic dipole transition moments have also been carried out at the algebraic diagrammatic construction (ADC(2)) and the second-order approximate coupled-cluster (CC2) levels. The calculations show that the degree of aromaticity and the size of the magnetic dipole transition moment of the lowest magnetic-dipole allowed excited state are related. We show that neutral heterocirculenes are weakly antiaromatic when the first excited state with a large magnetic dipole transition moment of 10–16 a.u. lies at high energies (∼2.8–3.5 eV). For the dications, this transition often lies at much lower energies. Heterocirculene dications with large magnetic dipole transition moments are strongly antiaromatic. The lowest excited states of the heterocirculene dianions have very small magnetic dipole transition moments implying that they are aromatic.