We present numerical studies of one- and two-photon excited states ordering in a number of polycyclic aromatic hydrocarbon molecules: coronene, hexa-peri-hexabenzocoronene, and circumcoronene, all possessing D6h point group symmetry versus ovalene with D2h symmetry, within the Pariser-Parr-Pople model of interacting π-electrons. The calculated energies of the two-photon states as well as their relative two-photon absorption cross-sections within the interacting model are qualitatively different from single-particle descriptions. More remarkably, a peculiar role of molecular geometry is found. The consequence of electron correlations is far stronger for ovalene, where the lowest spin-singlet two-photon state is a quantum superposition of pairs of lowest spin triplet states, as in the linear polyenes. The same is not true for D6h group hydrocarbons. Our work indicates significant covalent character, in valence bond language, of the ground state, the lowest spin triplet state and a few of the lowest two-photon states in D2h ovalene but not in those with D6h symmetry.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry