The Synthesis, Optoelectronic Properties And Structure Defects Of CN-PPV

Electroluminescence (EL) devices based on organic thin layers have attracted much attention because of their academic interest and potential application as large-area light-emitting displays. It is known that the performance enhancement of the light-emitting device is dependent on not only the device configuration, but also importantly, the employing of top-quality materials. Endless improving of the material structure to perfect their properties is a long-term task for material scientists. Since the first report of polymer light-emitting diodes based on poly(p-phenylenevinylene) (PPV) by Burroughes et al., a variety of conjugated polymers have been reported to exhibit electroluminescence. Electroluminescence in conjugated polymers is well known to be generated by recombination of electrons injected into the conduction band and holes in the valence band to form singlet excitons. For the conjugated polymers investigated so far, electron injection has proved to be more difficult than hole injection. Much effort has been made toward the preparation of high electron-affinity polymers which may provide more efficient electron injection properties. The introduction of strong electron affinity groups, e.g. cyano groups, onto the vinylene moiety of PPV backbone has proved to be an effective way to lower the lowest unoccupied molecul arorbital (LUMO) of a polymer and therefore enhanced the electron injection ability. With the enhancement of electron injection of luminescent polymers, airstable metals such as aluminum can be employed without the loss of electroluminescent efficiency. It is widely known that the defects in the polymer backbone can affect the properties of the device. The main structural defects in PPV-type conjugated polymers involve the "tetrahedral"structural segments and the cis-double bone configurations. However, in contrast with plentiful investigations for the structural characteristics of PPVs, the research on the CN-PPVs is relatively lacking. Actually, there may be abundant cis-segment in CN-PPV backbone because the cis-trans isomerization in CN-PPV should be the same as or more active than PPVs. To the best of our knowledge, no assured spectral evidence of the cis-segment in CN-PPV backbone have been reported. In order to achieve a better understanding about the structure-property relationship of CN-PPV, we investigated the spectral feature of model compound 8a and its stereoisomers 8b and 8c by 1H NMR spectral technique. According to aborative analysis for 1H NMR spectra of three model compounds and through the comparison of 1H NMR spectra between model compounds and polymer MEH-CN-PPV, all NMR signals of MEH-CN-PPV were assigned, especially for the signals relative to cis-vinylene conformation, which provided a method to quantificationally determine the contents of trans-and cis-vinylene in MEH-CN-PPV backbone. According to the ratio between the cis-vinylene signal and trans-vinylene signal, the content of the cis-vinylene could be estimated to be 15% in MEH-CN-PPV. This large cis-vinylene content came from the rapid photochemical...