Excited-State Aromaticity Reversals in Naphthalene and Anthracene.

Aromaticity reversals between the electronic ground (S) and low-lying singlet (S, S) and triplet (T, T, T) states of naphthalene and anthracene are investigated by calculating the respective off-nucleus isotropic magnetic shielding distributions using complete-active-space self-consistent field (CASSCF) wavefunctions involving gauge-including atomic orbitals (GIAOs). The shielding distributions around the aromatic S, antiaromatic S (L), and aromatic S (L) states in naphthalene are found to resemble the outcomes of fusing together the respective S, S, and S shielding distributions of two benzene rings. In anthracene, L is lower in energy than L, and as a result, the S state becomes aromatic, and the S state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S and S states in naphthalene. The lowest antiaromatic singlet state of either molecule is found to be significantly more antiaromatic than the respective T state, which shows that it would be incorrect to assume that the similarity between the (anti)aromaticities of the S and T states in benzene, cyclobutadiene, and cyclooctatetraene would be maintained in polycyclic aromatic hydrocarbons.