Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem=458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2.

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We present a new synthetic approach to both phenylquinoxaline polymers and low molar mass glasses. A palladium-catalyzed coupling of arylalkynes and bromobenzenes and subsequent oxidation of the triple bonds lead to the corresponding benziles. Reaction with diaminobenzidine yields poly(phenylquinoxalines) (PPQs), whereas the reaction with 1,2-diaminobenzenes leads to low molar mass bis(phenylquinoxalines) (BPQs) and tris(phenylquinoxalines) (TPQs). Both PPQs and TPQs carry tert-butyl or CF3− substituents and are fully soluble in chlorinated hydrocarbons. The starburst TPQs are able to form stable, low molar mass glasses. Cyclic voltammetry reveals that the TPQs have low-lying lowest unoccupied molecular orbitals levels at about −3.6 eV and are attractive as electron-transport materials in organic light-emitting diodes (LEDs). Two-layer LEDs with poly(phenylenevinylene) were fabricated that show a maximum brightness of 450 cd/m2.

Polarized light over a large spectral region is provided by the novel procedure described in this work. The active material of these light‐emitting diodes (LEDs) is formed by two polymer layers that are oriented perpendicularly to each other. The orientation is obtained using the rubbing technique combined with thermal annealing. The Figure represents the electroluminescence (EL) emission from the two‐layer LED and its structure (see also inside front cover).

The electron transport properties of two soluble tris-phenylquinoxalines have been investigated by the time-of-flight technique. The electron mobilities for both compounds approach 10−4 cm2/V s at electric fields of 106 V/cm at room temperature. These are high values for isotropic electron transport materials suitable for use in organic light emitting diodes.

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A number of optical devices such as liquid crystal displays (LCDs) require polarized light. In such displays linearly polarized light is usually made from unpolarized light emitted by a glow-discharge lamp by means of a sheet polarizer in the back of the active LCD layer. However, a polarizer usually absorbs 50 % of the light, which leads to an increased power consumption and is undesirable for use in portable computers and mobile phones.Organic light-emitting diodes (OLEDs) are an alternative light source and they have been extensively developed over the last decade. Currently they attract a lot of interest as a new display technology, [1±5] as the use of organic materials in OLEDs offers the possibility of polarized emission. Generally low molar mass chromophores or polymers in OLEDs are randomly distributed and have no preferential orientation. Alignment of such small molecules and polymers with a large anisotropy of the optical dipole moment is an attractive feature since it leads directly to OLEDs emitting polarized light.Several polarized OLEDs based on different orientation mechanisms have already been described. For low molar mass materials epitaxial evaporation of rod-like emitters, [6,7] and Langmuir±Blodgett films [8,9] have been used. For polymers, stretch alignment [10] turned out to be a suitable method for polarized OLEDs.Liquid crystallinity is known to provide quick access to one-or two-dimensional orientation of chromophores. Thermotropic liquid crystals, including both low molar mass materials and polymers with smectic phases, have been used in electroluminescence (EL) devices. [11±13] A stabilization of the LC state was accomplished either by device operation at elevated temperatures or by quenching the LC state of the polymer into a glass. Grell et al. used a main-chain liquid-crystalline polymer, poly(9,9-di-n-octyl-2,7-fluorene), which was reported to show excellent holetransport mobility, to demonstrate polarized electroluminescence. [14,15] The highest dichroic ratios for polarized electroluminescence (EL k /EL c = 15) were recently reported for poly(9,9-di-(2-ethylhexyl)-2,7-fluorene). [16] From a technological point of view a rather interesting technique for the preparation of polarized OLEDs is rubbing alignment of polymers. Hamaguchi and Yoshino reported on polarized EL from rubbing-aligned liquid-crystalline poly(2,5-di-nonyloxy-1,4-phenylenevinylene) [17±19] whereby a dichroic ratio of 5.3 for photoluminescence was demonstrated. The corresponding ratio of polarized EL was 1.6. To what extent these anisotropies originate from the LC phase of the polymer used is not clear since similar results were obtained from non-liquid-crystalline poly(2,5-dialkoxy-1,4-phenylenevinylenes) and poly(3-alkyl-thiophenes). [19] In this paper we present a simple and effective method utilizing rubbing alignment of segmented precursor poly (p-phenylenevinylene) (PPV) for the preparation of twolayer LEDs showing high dichroic ratios of linearly polarized electroluminescence.For our experiments we used...