On biexcitons in organic crystals

If molecules of a crystal have no inversion center, the static dipole moments of a molecule in the ground and in the excited states are usually not equal to zero. In this case and in the dipole-dipole approximation for the expression (15.16) can be reduced to the interaction energy of dipoles Ps and Ps  

For dipoles of the order of 5 Debye and nearest distance of molecules of the order of 5 A the energy M s, is of the order of 1000 cm-1 which can be large or of the order of the width of the exciton bandwidth. In such crystals we can expect the existence of biexciton states. The theory of biexcitons in layered organic anthracene type crystals has been developed by Spano et al. (23), (24). 

The first observation of biexciton states in a molecular structure has been performed by Chakrabarti et al. (25) for the ID 77-aggregate of an epitaxially grown trivalent-halogen-phthalocyanine with a unique packing arrangement of the monomers with nearest distance 4 A. The distinct photoinduced absorption to a continuum of two-exciton states and to a bound biexciton state was observed. The value of the exciton-exciton interaction for this structures was estimated to be equal to ss 0.3 eV. 

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