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 difference between the lowest Frenkel exciton and the nearest-neighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these effects for crystalline N,N'-dimethylperylene-3,3,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic 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 fitting parameters, we demonstrate that this model call explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different 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 different from that of the Frenkel excitons. (C) 2000 Elsevier Science B.V. All rights reserved.