Interaction of Electrons and Holes in a Molecular Crystal

A classical electrostatic model is used to compute the energy of an anthracene crystal containing an electron and a hole, each localized on specific sites. The calculation is carried out for a large number of sites and gives the effective potential hypersurface of interaction of the charges. The model includes real, finite charge distributions and anisotropic polarizabilities and is carried out to terms in induced-dipole—induced-dipole interactions, which are quite significant. The energy of the charge-transfer state is estimated as 3.84 eV, 0.39 eV above the experimental value. The model gives the energy of two infinitely separated charges as 5.77 eV, 1.85 eV above the reported threshold for intrinsic photoconduction. The energy of (dressed) charge—charge interaction is non-Coulombic for charges in different a, b planes, but is Coulombic if both charges lie in the same a, b plane, in which case the computed effective dielectric constant in the a, b plane is 1.25.