Effect of solid-state structure on optical properties of conjugated organic materials

Luminescence from PPV derivatives and related conjugated polymers in the solid state is strongly subject to intermolecular interactions. Model systems of oligomers have been investigated to explore such interactions in more detail. Direct electronic interaction is found in single crystals of a cyano-substituted five-ring oligo(phenylene vinylene), as a result of the π-stacking arrangement of the molecules. Luminescence spectra and PL-decay curves suggest that excimers are formed upon photoexcitation. This model may account for similar observations in films of related polymers. A second example is represented by a three-ring, stilbazole-type molecule, of which thin multilayer films have been obtained by means of the Langmuir-Blodgett technique. In this case, the features of the absorption and luminescence spectra are explained by dipole-(induced) dipole interactions within aggregates.