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

Luminescence from PPV derivatives and related conjugated polymers in the so lid state is strongly subject to intermolecular interactions. Model systems of oligomers have been investigated to explore such interactions in more d etail. Direct electronic interaction is found in single crystals of a cyano -substituted five-ring oligo(phenylene vinylene), as a result of the pi-sta cking arrangement of the molecules. Luminescence spectra and PL-decay curve s suggest that excimers are formed upon photoexcitation. This model may acc ount for similar observations in films of related polymers. A second exampl e is represented by a three-ring, stilbazole-type molecule, of which thin m ultilayer films have been obtained by means of the Langmuir-Blodgett techni que. In this case, the features of the absorption and luminescence spectra are explained by dipole-(induced) dipole interactions within aggregates.