Electronic structure of linear polyacenes in the SCF–RPA method

Abstract

The excited states of the π-electron system of linear polyacenes are studied with the self-consistent field–random phase approximation (SCF–RPA) scheme. The semiempirical Pariser–Parr–Pople (PPP) model is used. The parametrization is derived by fitting the benzene and it is held fixed throughout the polyacene series. Substantial improvement with respect to Tamm–Dancoff results [J. Chem. Phys. 24, 250 (1956)] for the singlet oscillator strengths is obtained. It is pointed out that the ratio between the experimental oscillator strength of the strong 1B3u transition and the total f-sum rule strength shows a drastic drop for naphthacene and pentacene. It is argued that this behavior is associated with a deep modification of the π-electron correlation structure of these two molecules with respect to the smaller ones. The calculated energies are compared with experimental and Tamm–Dancoff values. The assignments of Meyer et al. [J. Chem. Phys. 56, 801 (1972)] for the triplet excited states of naphthalene and anthracene are confirmed. Theoretical implications of the results are discussed and possible developments and improvement of the theory are indicated.