Regensburg 2007 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 10: Organic semiconductors

HL 10.3: Talk

Monday, March 26, 2007, 15:15–15:30, H14

The colours of perylene pigments: Interference between Frenkel excitons and charge transfer (CT) states — •L. Gisslen and R. Scholz — Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, D-85748 Garching

The oscillator strength of optical transitions in molecular crystals arises mainly from neutral excitations of the molecules, resulting in Bloch waves known as Frenkel excitons. However, if the underlying transitions are close to resonant with CT states, both types of crystal excitations are mixed via electron and hole transfer, resulting in complex absorption lineshapes. As a basis for our exciton model including Frenkel and CT states, we determine the deformation patterns in the relaxed excited state and in anionic or cationic molecules with density functional methods. The electron and hole transfer parameters are deduced from the splitting of the Kohn-Sham orbitals in a stacked dimer. The sign and size of these transfer parameters depend on the geometric arrangment of the stack neighbours, so that the interference of Frenkel and CT transitions may result in a strong modulation of the absorption lineshape. As the relative energetic arrangement of Frenkel and CT states is difficult to calculate with high precision, we treat the difference E_F−E_CT as the only free fit parameter of the model. From an analysis of five perylene derivatives [PTCDI, PTCDA, DiMe-PTCDI, N,N’-bis(3,5-xylyl)perylene-3,4:9,10-bis(dicarboximide), and N,N’-bis(2-phenylpropyl)perylene-3,4:9,10-bis(dicarboximide)] we derive a monotonous dependence of the energetic difference between Frenkel and CT states on the length of the stacking vector.