Efficient poly( p-phenylenevinylene) derivative with 1,2-diphenyl-2′-cyanoethene for single layer light-emitting diodes

Abstract

A novel luminescent polymer, poly(2-methoxy-5-[6′-{1-cyano-1′-phenyl-2-(4-phenoxy)-ethene}]-hexyloxy-1,4-phenylenevinylene- alt-2,5-bis-(dodecyloxy)-1,4-phenylenevinylene) (MCN-PPV), was synthesized by the Heck coupling reaction. Electron withdrawing pendant, 1,2-diphenyl-2′-cyanoethene (CNST), is separated from the main chain via linear 1,6-hexamethylenedioxy chain. The band gap of the polymer figured out from the UV-Vis spectrum was 2.10 eV and the maximum of photoluminescence (PL) appeared at 582 nm. These values are similar to those of poly(2-methoxy-5-ethylhexyloxy-1,4-pnenylenevinylene) (MEH-PPV) (2.12 eV and 580 nm). The maximum wavelength of electroluminescence (EL) of the device based on single layer structure (ITO/MCN-PPV/Al) appeared at 582 nm, which is similar to that of MEH-PPV (583 nm). Relative PL quantum yield of MCN-PPV film is 2.5 times higher than that of MEH-PPV film. In the PL and EL spectra, emission from CNST pendants was not observed. The presence of CNST pendant did not affect the EL and PL maximum of the main chain of MCN-PPV. The HOMO and LUMO energy levels of the main chain figured out from the cyclic voltammogram and the UV-Vis spectrum are −4.99 and −2.89 eV, respectively, which are similar to those of MEH-PPV (−4.94, −2.82 eV). The estimated HOMO and LUMO energy levels of the pendant are −6.23 and −3.07 eV, respectively. A single layer EL device based on MCN-PPV has an efficiency of 0.114 cd/A at a current density of 63.0 mA/cm 2, which is significantly higher than that of MEH-PPV (5.93×10 −3 cd/A at a current density of 65.8 mA/cm 2) measured under the same conditions. The energy levels figured out from optical and electrochemical data strongly supports that the CNST pendants have hole-blocking as well as electron-transporting properties.