Excited-State Aromaticity Reversals in Naphthalene and Anthracene

Aromaticity reversals between the electronic ground (S0) and low-lying singlet (S1, S2) and
triplet
(T1, T2, T3) 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
S0, antiaromatic S1 (1Lb), and aromatic S2 (1La) states
in naphthalene are found to resemble the outcomes of fusing together
the respective S0, S1, and S2 shielding
distributions of two benzene rings. In anthracene, 1La is lower in energy than 1Lb, and as
a result, the S1 state becomes aromatic, and the S2 state becomes antiaromatic; the corresponding shielding distributions
are found to resemble extensions by one ring of those around the S2 and S1 states in naphthalene. The lowest antiaromatic
singlet state of either molecule is found to be significantly more
antiaromatic than the respective T1 state, which shows
that it would be incorrect to assume that the similarity between the
(anti)aromaticities of the S1 and T1 states
in benzene, cyclobutadiene, and cyclooctatetraene would be maintained
in polycyclic aromatic hydrocarbons.