Absorption Properties of Highly Symmetrical Complexes

In the following section, step by step a qualitative picture is formed describing the impact of intermolecular interactions on the absorption and luminescence of organic conjugated chains. The present calculations do not distinguish between dimers and aggregates (for which the wavefunctions of adjacent chains interact in the ground state, due to, for instance, solid-state effects) and excimers (where overlap occurs only upon photoexcitation) [29]. 

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57]. Table 4-1 collects the INDO/SCI-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

For large interchain separations (8 A R 30 A), the LCAO coefficients of a given molecular orbital are localized on a single chain, as intuitively expected. The lowest excited state of these dimers results from a destructive interaction of the two intrachain transition dipole moments, whereas a constructive interaction prevails for the second excited state. This result is fully consistent with the mo- 

Interchain distance Excited state Transition energy (eV) Intensity (arb. units) Cl description 

In a regime of strong interaction between the chains no optical coupling between the ground state and the lowest excited state occurs. The absence of coupling, however, has a different origin. Indeed, below 7 A, the LCAO coefficients start to delocalize over the two chains and the wavefunctions become entirely symmetric below 5 A due to an efficient exchange of electrons between the chains. This delocalization of the wavefunction is not taken into account in the molecular exciton model, which therefore becomes unreliable at short chain separations. Analysis of the one-electron structure of the complexes indicates that the 

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