Model calculation of the optical properties of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films

Abstract

The marked difference between the optical absorption of dissolved PTCDA monomers and crystalline films is analysed in terms of the deformation of an effective internal vibrational mode of the molecule and transfer of Frenkel excitons between different molecular sites in the crystal. It is shown in detail that the coupling between equivalent molecules in different crystal unit cells, especially among stack neighbours, dominates over exciton transfer between the two different molecules in the crystal basis. The first type of interaction leads to a pronounced deformation of the expected Poisson progression over subsequent vibrational levels while the latter is responsible for a small Davydov splitting and, consequently, different absorption lineshapes along different crystal axes.