Frenkel exciton model of optical absorption and photoluminescence inα-PTCDA

The optical properties of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) in the monoclinic $\alpha$ phase are investigated with a model allowing for the transfer of the electronic excitation between different molecular sites in the crystal, resulting from the interactions between the molecular higher occupied to lower unoccupied molecular orbital transition dipoles. Due to the large extension of the molecules with respect to the intermolecular distances, the calculation has to account for the delocalization of the transition charge density over the carbon and oxygen atoms involved in both orbitals. Applying these microscopic ingredients to a calculation of the dispersion of the Frenkel excitons, it can be shown that the dielectric tensor is related to excitons at the $\Gamma$ point of the Brillouin zone and the photoluminescence to vertical recombination starting from the minimum of the Frenkel exciton dispersion at the surface of the Brillouin zone. The calculated results agree both with the extinction coefficient measured on polycrystalline thin films and with the dominating low-temperature photoluminescence decay channel observed on single crystals of $\alpha$-PTCDA.