Synthesis, electrochemistry, and electroluminescence of novel red-emitting poly(p-phenylenevinylene) derivative with 2-pyran-4-ylidene-malononitrile obtained by the heck reaction

A novel red-emitting polymer, poly{1,4-phenylenevinylene-(4-dicyanomethylene-4H-pyran)-2,6-vinylene-1,4-phenylenevinylene-2,5-bis(dodecyloxy)-1,4-phenylenevinylene} (PM-PPV), was prepared by the Heck coupling reaction between 2-{2,6-bis[2-(4-bromophenyl)vinyl]pyran-4-ylidene}-malononitrile and 1,4-bis(dodecyloxy)-2,5-divinylbenzene. The EL device based on a single-layer structure (ITO/PM-PPV/Al) showed EL emission with a maximum at 652 nm and an external quantum efficiency of 0.004% at 0.12 mA/mm(2), which is higher than that of MEH-PPV (0.0005%, at 0.15 mA/mm(2)) measured under the same conditions. From the electrochemistry and UV-vis spectroscopy, the HOMO and LUMO energy levels of PM-PPV were figured out to be -5.44 and -3.48 eV, respectively, which are lower than those of MEH-PPV at -4.98 and -2.89 eV, respectively. It is concluded that, by lowering the HOMO and LUMO levels in PM-PPV, the injection rates of the holes and electrons are more balanced than in MEH-PPV. To improve the efficiency, the PPV layer was inserted between anode and PM-PPV layer. The device based on bilayer structure (ITO/PPV/PM-PPV/Al) showed EL emission with a maximum at 646 nm and an external quantum efficiency of 0.05% at 0.18 mA/mm(2). The efficiency of the bilayer device was higher than that of the single-layer device by 13 times. The CIE chromaticity coordinates of the bilayer device were x = 0.69 and y = 0.31, which are very close to the CIE chromaticity coordinates (x = 0.67, y = 0.33) of NTSC for the red color.