Novel poly(p-phenylenevinylene)s with an electron-withdrawing cyanophenyl group

Novel poly(p-phenylenevinylene) derivatives with an electron-withdrawing cyanophenyl group on the polymer backbone, poly[2-(2′-ethylhexyloxy)-5-(4′-cyanophenyl)-1,4-phenylenevinylene] (PEHCNPV) and poly[2-dimethyloctylsilyl-5-(4′-cyanophenyl)-1,4-phenylenevinylene] (PSi8CNPV), were synthesized via the Gilch polymerization. These polymers were completely soluble in common organic solvents and showed good thermal stability up to 400°C. Surprisingly, they showed very high glass transition temperatures (above 180°C), indicating that electroluminescence (EL) devices constructed from these polymers should have good thermal stability. The presence of the electron-withdrawing cyanophenyl group lowered the HOMO and LUMO energy levels of PEHCNPV and PSi8CNPV relative to those of common PPV derivatives. Light-emitting diodes (LEDs) with the configuration ITO/PEDOT/polymer/LiF/Al were fabricated using the novel polymers. The LED devices based on PEHCNPV and PSi8CNPV exhibited maximum electroluminescence at 546 and 513 nm, respectively. In particular, the LED device containing PSi8CNPV emitted pure green light with a CIE (Commission Internationale de L'Eclairage) chromaticity coordinate of (0.330, 0.599), which is very close to that of the standard green (0.30, 0.60) used for high definition television (HDTV), a maximum external quantum efficiency of 0.67%, and a maximum brightness of 2900 cd/m².