M. Jandke

Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene)

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.

Oxadiazoles and phenylquinoxalines as electron transport materials

Abstract The synthesis of a number of star-shaped oxadiazoles and phenylquinoxalines is described. These low molar mass compounds are able to form stable glasses with glass-transition temperatures up to 248 °C. The materials have been tested as electron transport layers in light-emitting devices (LEDs) together with poly(1,4-phenylene vinylene) (PPV). The two-layer LEDs show an improved quantum yield and brightness compared to PPV-monolayer LEDs.

Electron transport in starburst phenylquinoxalines

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.

On the cytotoxity of oxidized phytosterols isolated from photoautotrophic cell cultures of Chenopodium rubrum tested on meal-worms Tenebrio molitor

Abstract Cell injury—as evoked by oxidative stress, insect attack or ageing—causes lipid peroxidation of polyunsaturated acids, e.g. linoleic acid. Hydroperoxides (LOOH) so produced transform fatty acids, terpenes and sterols to epoxides. We describe a bioassay to determine the cytotoxity of sterol oxidation products by measuring the mortality of meal-worms ( Tenebrio molitor ) after injection of diluted test compounds. LOOH was tested in concentrations from 10 −5 M up to 10 −2 M and developed recognizable mortality (25%, 10 −2 M, 23 hr). A direct relation between activity and concentration was observed. 5,6-Epoxides of phytosterols and derived 3,5,6-trihydroxysteranes have been isolated from Chenopodium rubrum cell cultures. Using 5,6-epoxycholesterol and 3,5,6-trihydroxycholestane as standards of known bioactivity, the toxicity of epoxides of sitosterol and stigmasterol, as well as the trihydroxy compounds thereof, was checked. The phytosterol derivatives are by a factor of five less active than the corresponding cholesterol oxidation products. 5,6-Epoxides and 5,6-chlorohydrins showed half of the activity of the corresponding 3,5,6-trihydroxyphytosterols. These developed highest cytotoxity (40% mortality in 5 × 10 −3 M solution after 23 hr). The activity of 5,6α-and 5,6β-epoxides was found to be equal. Metabolism and proposed biological function of oxidized phytosterols is discussed.

A study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy

Abstract The polymer poly(9,9′-dioctyl fluorene) (F8) can be easily driven between a number of different structural phases by different thermal or solvent treatments. The phases which we have identified include (i) as-spin-coated, (ii) semi-crystalline, (iii) glassy, (iv) with an extended intrachain conformation. Atomic force microscopy has been used to investigate the surface structures associated with the different phases of the polymer and different solvents used to cast the film. Raman spectroscopy has been performed on these samples to understand how the polymer conformation changes with the phases. Variations in the relative intensity of the peaks and shifts in energy are observed. We have compared the F8 spectrum with a trimer of fluorene with butyl side chains and acrylate/hydroxy endgroups attached to the fluorene main chain via hexyl spacers. We identify a very small frequency dispersion in the vibrational modes connected to an increase of the conjugation length. Therefore, the assignment of the vibrational modes of the trimer is very important for understanding the F8 spectrum.

Long-range energy transfer of singlet and triplet excitations in dye-doped tris(phenylquinoxaline)

In order to obtain efficient organic light-emitting diodes (OLED) it is necessary to funnel both singlet and triplet excitons generated by electroexcitation to the emitting dye molecule. Energy transfer plays thereby a decisive role. We have studied mechanisms and kinetics of the energy transfer process involving singlet and triplet excitations in a donor–acceptor system suitable for use as active layer in OLEDs. Fluorescent and phosphorescent tris(phenylquinoxaline) served as donor and a red emitting platinum porphyrin as acceptor molecule. The results of our investigations show that efficient energy transfer from the singlet and triplet excited state of the donor to the acceptor molecules takes place by combined long-range dipole–dipole interaction and diffusion. Due to the very different oscillator strengths of radiative transitions from the singlet and triplet excited state to the singlet ground state the rate of energy transfer of triplet excitons is several orders of magnitude slower than that of si...

Polarized electroluminescence from photocrosslinkable nematic fluorene bisacrylates

In this paper the monodomain-alignment of new photocrosslinkable fluorene bisacrylate model compounds and oligomers is reported. The orientation of the nematic LC- phase is preserved by photopolymerization in an oriented LC- network. Monodomain orientations of crosslinked fluorene bisacrylates are investigated by UV-VIS and photoluminescence spectroscopy comparing different alignment layers. OLEDs were fabricated using oriented fluorene bisacrylates and polyfluorenes leading to state-of-the-art polarization ratios for electroluminescence of up to 25 and brightnesses of about 250 Cd/m2.

Novel starshaped molecules based on fluorene

A series of starshaped molecules with different cores such as benzene, triphenylbenzene and triphenylamine and fluorene-moieties as side arms were synthesized by suzuki cross-coupling. These low molecular weight compounds form stable glasses and are easily obtained in high purity by column chromatography and sublimation. Variation of the alkyl chain length in the 9 position of the fluorenes and changing the core have a great influence to their glassforming abilities and the glass transition temperature, which range from 57 °C to 118 °C. All starshaped molecules show blue emission, which can be fine-tuned by the choice of the cores. A first two-layer device with the configuration ITO/NPB/TFPB-Bu/Ca was prepared by vacuum evaporation and shows an external quantum efficiency of 1.2 % at 10 V.

Polarized electroluminescence from rubbing-aligned poly(p-phenylenevinylene)

Abstract In this paper the rubbing-alignment of a poly( p -phenylenevinylene) (PPV) precusor polymer is described. Highly oriented films have been prepared leading to a dichroic ratio of 18 for photoluminescence corresponding to an order-parameter S PL =(PL ∥ −PL ⊥ )/(PL ∥ +PL ⊥ ) of 0.9. Two-layer LEDs with the configuration ITO/PPV/Poly(phenylquinoxaline) (PPQ)/Al that have been fabricated show dichroic ratios for electroluminescence (EL ∥ /EL ⊥ ) of 12 and a maximum brightness of 200 cd/m 2 .

Ultraviolet photoelectron spectroscopy on new heterocyclic materials for multilayer organic light emitting diodes

Electronic band structure of three heterocyclic model compounds: 5,12-dihydro-quinoxalino-2,3-b-phenazine; 7,18-dihydro-5,7,11,16,18,20,22-octa-aza-nonacene; and 1,3,5-Tris[(3-phenyl-6-tri-fluoro-methyl)-quinoxaline-2-yl]benzene was studied by ultraviolet photoelectron spectroscopy. We also determined their ionisation potential, between 5.3 and 6.0 eV, suggesting that these materials can efficiently act as electron transporting layer in organic light emitting devices (OLEDs).