The electronic structure of polyacenes: Naphthalene through pentacene

The new ’’spectroscopic’’ CNDO/S2 model parametrization, constructed to describe the electronic excitation spectra of conjugated hydrocarbons, is applied to determine the electronic structure of naphthalene, anthracene, naphthacene (tetracene), and pentacene. The occupied and virtual orbital eigenvalue spectra are evaluated. Comparison of the former with observed ultraviolet photoemisiion (UPS) spectra reveals that the predicted energies of the highest-energy two or three filled orbitals are in quantitative (ΔE≲0.1 eV) correspondence with these spectra. The ordering of the π-electron levels agrees with earlier assignments, and the deeper levels (both π and σ) are in qualitative (ΔE≲0.5 eV) correspondence with the UPS spectra. The energies and nature of both singlet and triplet transition states are evaluated using configuration-interaction analyses of appropriate dimensionality. All three singlet levels for each molecule agree with observed optical spectra to within ΔE?0.1 eV. The ordering and splittings of the triplet levels correspond well with the available experimental assignments.