MODEL COMPOUNDS FOR LIGHT-EMITTING PPVS - OPTICAL AND STRUCTURAL DATAOF SUBSTITUTED OLIGOMERS

Phenylene-vinylene oligomers are investigated as model compounds for t he luminescent moiety of PPV-type homopolymers or copolymers having ol igo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have bee n substituted at various positions on the vinylene linkages. For sever al compounds single crystals have been obtained and their structure ha s been resolved, which allows the spectral response to be related to m olecular 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 inter actions cooperate with the electronic effect of the cyano group. The u se of a cyano-substituted five-ring compound in a multilayer LED was f ound to strongly increase the luminescence efficiency.