Application to PTCDA and MePTCDI crystals

In this subsection, we apply the relations obtained above for calculations of the energies of edge excitons in MePTCDI and PTCDA crystals. As we already mentioned, these crystals are quasi-one-dimensional. The distance between the molecules within the one-dimensional stacks is 0.34 nm for MePTCDI (45) and 0.337 nm for PTCDA (46)-(48). 

This distance is small in comparison with other lattice constants and also small in comparison with the size of molecules. This causes strong interactions of the 7r-electron systems within the stacks and a very weak interaction in the other directions. Due to these strong interactions in the stack, we can expect that in such crystals the qualitative difference between Frenkel and CT excitations becomes smaller, their energies approach each other and their strong mixing determines the nature of the lowest energy states (40), (41). 

To compare the energies of edge and lowest energy bulk states in MePTCDI and PTCDA, we use for these crystals the fitting parameters obtained in (41) to explain the frequency dependence of the polarization ratio in absorption spectra of MePTCDI crystals. These fitting parameters are collected in Table 12.1. 

Using these data (without any new fitting parameter) we obtain the parameters a, / , A and M, which we need for the calculation of energies of edge and lowest energy bulk states in PTCDA and MePTCDI crystals. 

For each crystal only one solution fulfills Z 1 and therefore we have only one edge state. Using relation (12.65), we can now obtain the energy of edge states. These energies and the energy of the lowest bulk state 7 are collected in Table 12.2. 

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