First excited singlet state aromaticity of macroheterocycles

The ground and first excited singlet and triplet states of porphine, tetraoxa-isophlorin, and tetraoxa[8]circulene have been studied in terms of aromaticity. The magnetically induced ring-currents have been calculated at the density functional theory and ab initio (CASSCF) levels of theory to assess aromaticity. Based on the analysis of the expression for the ring-current density in the first excited singlet state, a spectroscopic criterion for antiaromaticity in the S₁ state has been proposed. The antiaromatic S₁ state is expected for molecules with low-lying magnetic dipole-allowed electronic transitions S₁ → S_n, where the transition energy is less than 1.0 eV.