Tuning of Redox Behavior and Fluorescence of Cyano-containing Oligophenylenevinylenes
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PPV, and its derivatives, the attachment of electron-withdrawing groups on the vinyl group, particularly cyano substituents, facilitates electron injection, and results in an increase of luminescence quantum efficiency [9]. The processible poly(cyanoterephthalylidene) (CN-PPV) was synthesized by Knoevenagel condensation polymerization. Careful control of reaction conditions is required to avoid side reactions, such as Michael additions (and consequently crosslinking), that may lead to polymers with inferior EL properties. Therefore, there remains a strong challenge to design and synthesize stable and highly electron-deficient polymers. In this paper, we report on a series of cyano-containing distyrylbenzenes (3-6) which have the electron-withdrawing groups functionalized on the phenylene ring via the Wittig-Horner reaction. The goal of this work is to find the correlations between structure and property that may associate with their fluorescence emissions. The effect of substitution on the absorption and emission behaviors was also examined through a systematic changing of acceptor strength on the aromatic ring. The suitability of these organic chromophores as promising repeat units for light-emitting polymers (LEPs) will be examined by incorporating them into polyfluorenes. The unsubstituted compound 1 and the cyano-tetrephthalylidene 2 were also synthesized for comparison. SYNTHESIS AND CHARACTERIZATION
Chart 1 CN NC
2
1 CN
CN NC CN
NC
NC
4
3 NC
NC
CN
CN
NC NC
5
CN
6
Chart 1 presents a series of CN-containing compounds 3-6, which were arranged according to the descending order of transition energies of light emission. Compounds 3 and 4 were made by the Wittig-Horner condensation between 1,4-bis(2,5dicyanobenzalphosphonate) and 4-tert-butylbenzaldehyde, 4-cyanobenzaldehyde, respectively. Further chemical reduction of the cyano-groups on compound 3 by diisobutylaluminumhydride (DIBAL) afforded a monoaldehyde and a dialdehyde depending on the stoichiometric ratios between 3 and DIBAL. Condensation of the
resulting aldehydes with malononitrile gave compounds 5 and 6 in almost quantitative yield. Compound 4 was insoluble in common organic solvent, such as tetrahydrofuran (THF), chloroform, acetone, methylene chloride, 1,2-dichloroethane (DCE), due to its symmetrical and rigid structure. However, the result obtained from elemental analysis verified the composition of the desired compound. Nevertheless, compounds 1-3 and 5-6 could be purified by recrystalization from acetone/ethanol after a brief column chromatography. The stereochemistry of the expected trans olefinic linkage was verified by the coupling constant of the vinylene protons in 1H NMR (J = 15-17 Hz) and by the vibration bands in IR (970 cm-1). ELECTROCHEMICAL PROPERTIES Table 1 lists the values of onset potentials of the model compounds from electrochemical study. Unsubstitued parent compound 1 exhibited two reversible oxidation waves, corresponding to the successive generation of the cation radical and dication. However, no reduction was
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