Dielectric theory of weak charge-transfer crystals. II. Polarization energies

Polarization energies and charge-quadrupole energies are calculated for a single excess charge in the complexes of anthracene with PMDA (pyromellitic dianhydride) and TCNB (1,2,4,5-tetracyanobenzene) using different sets of molecular polarizabilities from part I. Comparison with experimental crystal ionization energies and photoconductivity band gaps allows preferred polarizabilities to be identified. Relaxation energies of about 0.8 eV per charge are deduced in anthracene-PMDA and 0.4 eV in anthracene-TCNB, consistent with strong electron-phonon coupling. Polarization, Coulomb and charge-quadrupole energies are calculated for charge-transfer configurations with a hole on anthracene plus an electron on the acceptor (heteromolecular CT), on another anthracene (homomolecular CT), or symmetrically split between two acceptors (split CT). In anthracene-PMDA split CT configurations have energies comparable with heteromolecular CT while in anthracene-TCNB the split ones lie somewhat higher. In both crystals the lowest homomolecular CT configurations lie about 1 eV above the lowest heteromolecular ones and should interact strongly with them. This interaction could reconcile the calculated CT transition energies with observed values some 0.5 eV lower.