Study Of Photochemical And Photophysical Properties In CN-PPV

Electroluminescence (EL) devices based on organic thin films have attracted much attention because of their academic interest and potential application as large-area light-emitting displays. 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. 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 for the balance of electron and hole injection. CN-PPV is the first reported semiconducting polymers with high electron affinity. The incorporation of electron-withdrawing cyano groups into the vinylene moiety lowered the lowest unoccupied molecul orbital (LUMO) and therefore enhanced the electron injection ability of polymer. Now, CN-PPV and its derivatives are one of promising electroluminescent polymers with good electron injection ability.The introduction of cyano groups on vinyle not only lowers the orbital energy of PPV but also have a great effect on the photochemical and photophysical properties of CN-PPVs. Most of CN-PPV derivatives preserve the trans-vinyle and the twisted configuration because of the steric interactions between the cyano and the neighboring group. The intramolecular motions of the twisted structure cause the complicated photochemical processes c.a. cis-trans isomerization to occur easily and thus decrease the fluorescence efficiency of CN-PPVs in solution. And CN-PPVs prefer to possess face-to-face stacking with strongπ-πinteractions due to the high electron affinity of the cyano group and therefore exhibit the quenching of luminescence in solid state due to the formation of less emissive species such as excimers. Base on these problems of CN-PPV, this thesis includes three section:First, because vinylene moiety with cyano substituent are very unstable and easily isomerized in solution under the excitation of light, the easy photochemical cis-trans isomerization could lead to the appearance of cis-vinyle in the backbone of CN-PPVs and its derivatives. In order to achieve a better understanding about the structure-property relationship of CN-PPV, we synthesized MEH-CN-PPV and its model compounds and investigated the photochemical cis-trans isomerization of the model compound. According to 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. According to the intensity of the signals relative to cis-conformation and that to trans-conformation in the 1H-NMR spectrum, the content of the cis-conformation is estimated to be 15% and that of the trans-conformation is 85% in MEH-CN-PPV. These cis-vinylenes should be ascribed to the easy photoisomerization of cyanovinylene during the experimental process, such as synthesis, reprecipitation and characterization process. The abundant cis-vinylene defects and its weak luminescence should depress the electroluminescence of the MEH-CN-PPV.Second, in order to prevent the appearance of cis-vinylene defects in the backbone of CN-PPVs and theπ-πinteractions in solid state, we synthesized EHODPCNPPV with phenyl group as the side chain. This polymer exhibits poor electroluminescent properties, although the bulky phenyl groups restrain the formation of excimers. Moreover, we synthesized the model compound DPCNPV-1 and DPCNPV-2. The NMR spectra, IR spectra and crystal data indicate these compounds are very unstable and easily occur intramolecular photocyclization reactions to form the 9,10-dihydrophenanthrene structure. Because EHODPCNPPV, DPCNPV-1 and DPCNPV-2 preserve o-vinylbiphenyl structure withα-substituent cyano group, the steric interactions between the cyano and the neighboring phenyl group lead to the twisted structure and are beneficial to the occurrence of the intramolecular photocyclization reactions. This photochemical reactions include two-stage mechanism for product formation: photocyclization to yield the unstable 8a,9-dihydrophenanthrene followed by a thermal intramolecular hydrogen shift to yield 9,10-dihydrophenanthrene. The poor luminescence of EHODPCNPPV, DPCNPV-1 and DPCNPV-2 is ascribed to the nonradiated photochemical process.Third, we designed and synthesized three CN-OPVs with phenyl group as the side chain and the cyano group substituted atβ-position of vinylene. The fluorescence of these molecules in solution is very weak because of easy nonradiated photoisomerization. However, these molecules in crystals exhibit high fluorescence efficiency and aggregation-induced emission (AIE) property, The crystal data indicate the hydrogen-bonding network prevent the free twisting motions around the double bonds and nonradiated photoisomerization process. The high-quality crystals of these molecules with high fluorescence efficiency reveal amplified spontaneous emission (ASE) with the low threshold value, especially for CNDPASDB show a threshold value about 30KW/cm2, which is one of the best reported crystals with ASE. And these materials with high fluorescence efficiency in solid state can be used for the fabrication of non-doping OLEDs. The non-doping devices based on CNDPASDB exhibit excellent orange emission with luminescence yield about 6.0cd/A, which indicate these AIE compounds are excellent electroluminescent materials.