Excited-State Aromaticity Reversals in Naphthalene and Anthracene

[Image: see text] Aromaticity reversals between the electronic ground (S(0)) and low-lying singlet (S(1), S(2)) and triplet (T(1), T(2), T(3)) 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(0), antiaromatic S(1) ((1)L(b)), and aromatic S(2) ((1)L(a)) states in naphthalene are found to resemble the outcomes of fusing together the respective S(0), S(1), and S(2) shielding distributions of two benzene rings. In anthracene, (1)L(a) is lower in energy than (1)L(b), and as a result, the S(1) state becomes aromatic, and the S(2) state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S(2) and S(1) states in naphthalene. The lowest antiaromatic singlet state of either molecule is found to be significantly more antiaromatic than the respective T(1) state, which shows that it would be incorrect to assume that the similarity between the (anti)aromaticities of the S(1) and T(1) states in benzene, cyclobutadiene, and cyclooctatetraene would be maintained in polycyclic aromatic hydrocarbons.