This paper reports the effects of polydispersity in conjugation length on polymer light-emitting diode (PLED) device performance. For this study a di-alkoxy substituted pentamer of p-phenylenevinylene (5PV) was blended with a small amount of a lower band gap poly(p-phenylenevinylene) (PPV) or a small amount of a di-alkoxy substituted p-phenylenevinylene type nonamer (9PV). We also fractionated two different di-alkoxy substituted oxadiazole-PPVs and blended the high molecular weight, lower band gap fraction, into a matrix of the lower molecular weight, higher band gap, polymer. It was found that incorporation of a small amount of the low band gap material significantly degraded PLED device performance. To further test the significance of polydispersity we synthesized a low molecular weight narrow polydispersity PPV. Devices made with this PPV were about 2 orders of magnitude better in LED external quantum efficiency when compared to a high molecular weight, higher polydispersity PPV of identical structure. These findings are significant in that they point to the importance of controlling the polydispersity in conjugation length for obtaining efficient PLEDs. They also indicate the importance of controlling polydispersity when comparing one polymer structure to another.

Four novel two-dimensional conjugated poly(p-phenylenevinylene) (PPV)-based molecules have been synthesized, characterized, and evaluated for use in light-emitting diodes (LEDs). These novel molecules all contain a tetra-substituted central phenyl ring, but the length, chemical structure, and placement of the arms is varied in order to tailor their hole and electron transport properties. The materials are all solution-processible, maintain conjugation through the arms and central core, and exhibit evidence of two- and possibly three-dimensional charge delocalization. The first molecule contains four phenylenevinylene arms with solubilizing octyloxy-substituted units. Two of the molecules have two phenylenevinylene arms and two oxadiazole arms that are placed ortho or para with respect to each other. The fourth molecule contains four cyano-phenylenevinylene arms with solubilizing octyloxy groups at the ends of each arm. As a class these molecules all have large Stokes shifts in films, although the propensity for π−π stacking varies between the molecules. The oxadiazole-containing molecules show promise for applications in single-layer LEDs.

Three-component condensation of 4-piperidinones (7), 5-pyrazolones (8), and malononitrile (4) proceeds chemically and electrochemically and is a convenient one-step means of synthesis of substituted 6-amino-5-cyanospiro-4-(piperidine-4')-2H,4H-dihydropyrazolo[3,4-b]pyrans (12). The electrochemical reactions proceed under milder conditions and with yields 12-15% greater than those of the reactions catalyzed by chemical bases.