Electronic spectrum of anthracene: An ab-initio molecular orbital calculation combined with a valence bond interpretation

The properties of the two lowest excited singlet states of anthracene, S₁(¹B_1u) and S₂(¹B_2u), and the first triplet state, T₁(³B_1u), were calculated at the CIS/3-21G level of theory. The main structural changes are described, and the calculated vibrational levels are compared with available experimental data. The assignment of the two-photon observed transition at about 28 000 cm^-1 to the S₂ state is confirmed, as well as some recent argon matrix vibronic bands (Wolf, J.; Hohlneicher, G. Chem. Phys. 1994, 181, 185). The calculation suggests some changes in the vibrational assignment, and correlation with the ground state vibrational modes shows that one b_2u mode undergoes a large frequency increase upon electronic excitation to S₂. This change is similar to that observed for Kekulé-type modes of benzene, naphthalene, and their derivatives. It is compatible with the notion that the π electrons of these molecules tend to distort the molecule to a Kekulé structure.