Model compounds for light-emitting PPV's: Optical and structural data of substituted oligomers

Phenylene-vinylene oligomers are investigated as model compounds for the luminescent moiety of PPV-type homopolymers or copolymers having oligo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have been substituted at various positions on the vinylene linkages. For several compounds single crystals have been obtained and their structure has been resolved, which allows the spectral response to be related to molecular conformation and packing. Thin film spectra demonstrate that cyano substitution results in a considerable red shift of the emission: a more planar conformation, pi-stacking and, possibly, dipolar interactions cooperate with the electronic effect of the cyano group. The use of a cyano-substituted five-ring compound in a multilayer LED was found to strongly increase the luminescence efficiency.