The lowest energy Frenkel and charge-transfer excitons in quasi-one-dimensional structures: application to MePTCDI and PTCDA crystals

We consider the exciton states in quasi-one-dimensional organic crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy di€erence between the lowest Frenkel exciton and the nearestneighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these e€ects for crystalline N ,N 0-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA). To model the exciton states, we use a Hamiltonian which includes the mixing of Frenkel excitons with several vibronic levels and charge-transfer excitons. With appropriate ®tting parameters, we demonstrate that this model can explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show di€erent polarization ratios for the various absorption peaks. This polarization behavior is discussed as a qualitative proof for the varying contribution of the charge-transfer excitons, which have a transition dipole direction di€erent from that of the Frenkel excitons. Ó 2000 Elsevier Science B.V. All