Harald Bock

Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates

Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Ultrathin films of homeotropically aligned columnar liquid crystals on indium tin oxide electrodes

We report the achievement of very thin films (thickness of about 50 nm) of thermotropic columnar liquid crystal in homeotropic (columns normal to the interface) orientation on indium tin oxide (ITO) electrodes. The face-on alignment of the discotic compound has been obtained by thermal annealing without any intermediate coating between the mesophase and the ITO substrate. Such a columnar mesophase alignment is thus shown on a substrate of technological interest in open supported thin film reaching the thickness range suitable for organic photovoltaic devices.

An all-columnar bilayer light-emitting diode

Abstract The first organic light-emitting diode with both an electron-rich columnar liquid crystal as hole transport material and an electron-deficient fluorescent columnar liquid crystal as electron transport material is presented. Red fluorescence is observed above 10 V from the configuration indium–tin oxide (ITO)/hexabutoxy-triphenylene/tetraethyl perylenetetracarboxylate/Al. The excellent charge transport properties of columnar phases are thus exploited for light-emitting diodes. Both organic materials exhibit their columnar liquid crystal phase well above room temperature and the underlying ordered solid columnar phase is used in the diode. The I ( V ) and L ( V ) characteristics are very reproducible from device to device, with an emission peak at 620 nm and an FWHM of 80 nm, a current rectification ratio of about 30, I ∼ V 2 at low voltages and I ∼ L ∼ V 7 at higher voltages. Initial lifetime measurements are encouraging.

Fluorescent columnar liquid crystalline 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes

The title compounds were recently demonstrated to be useful as luminescent electron transport materials in organic light emitting diodes. Here, we present studies of the fluorescence of the homologues with chain lengths between 2 and 10 carbon atoms. Dilute solutions of the investigated compounds show absorption in the range between 410 and 490 nm and fluorescence between 475 and 555 nm. However, the columnar phase of the pure compounds exhibits fluorescence at considerably larger wavelength (550-650 nm) and gives larger fluorescence lifetimes. This behaviour can be explained by the formation of excimers in the mesophase. The phase transitions to the liquid crystal state are associated with a distinct change of the fluorescence intensity.

Twisted Polycyclic Arenes by Intramolecular Scholl Reactions of C3-Symmetric Precursors

With the aim of opening an efficient access to large and sterically crowded polycyclic arenes as well as improving insight into the geometrical preferences of the Scholl reaction, a versatile synthesis strategy has been developed to form a family of flexible yet strongly crowded substrates for multiple dehydrocyclizations. Their intramolecular Scholl reactions lead with high selectivity either to considerably twisted species where the initial C3 symmetry is maintained, or to strongly rearranged products where the formation of multiple [6]helicene fragments is avoided by the formation of unusual hexa[7]circulene moieties under loss of the C3 symmetry.

Face-on Oriented Bilayer of Two Discotic Columnar Liquid Crystals for Organic Donor−Acceptor Heterojunction

In this work, we report the achievement of a homeotropically (or face-on) oriented bilayer formed by a pair of discotic materials designed with specific properties: selective solubility, adjusted transition temperatures, and room temperature hexagonal columnar liquid-crystalline phase. The homeotropic orientation of the bilayer which is only a few hundred nanometers thick is performed by solution-processed deposition followed by thermal annealing in the geometry of open supported films and is evidenced by X-ray scattering. This represents the first proof of principle of an organic heterojunction based on two oriented columnar liquid crystal layers.

Highly Twisted Arenes by Scholl Cyclizations with Unexpected Regioselectivity

Lets twist! The Scholl reaction with quinquephenyl derivatives has been shown to have an unexpectedly strong preference for forming twisted, helicene aromatic polycycles, instead of their flat counterparts. This tendency is so strong that it will overcome even severe steric hindrance, and the procedure can be used in the efficient synthesis of hexa-tert-butylhexabenzotriphenylene from a simple biaryl starting material (see scheme).

Order induced charge carrier mobility enhancement in columnar liquid crystal diodes.

Discotic molecules comprising a rigid aromatic core and flexible side chains have been promisingly applied in OLEDs as self-organizing organic semiconductors. Due to their potentially high charge carrier mobility along the columns, device performance can be readily improved by proper alignment of columns throughout the bulk. In the present work, the charge mobility was increased by 5 orders of magnitude due to homeotropic columnar ordering induced by the boundary interfaces during thermal annealing in the mesophase. State-of-the-art diodes were fabricated using spin-coated films whose homeotropic alignment with formation of hexagonal germs was observed by polarizing optical microscopy. The photophysical properties showed drastic changes at the mesophase-isotropic transition, which is supported by the gain of order observed by X-ray diffraction. The electrical properties were investigated by modeling the current-voltage characteristics by a space-charge-limited current transport with a field dependent mobility.

Absorption and luminescence spectra of electroluminescent liquid crystals with triphenylene, pyrene and perylene units

New derivatives of triphenylene, perylene, and pyrene are described, some of which form columnar mesophases. The absorption spectra are investigated both experimentally and theoretically. The spectra calculated using the density functional tight binding (DFTB) theory are in good agreement with experimental results. The investigated compounds show photoluminescence of violet-blue (triphenylene), yellow-green (pyrene), and orange-red (perylene) colours. In addition, electroluminescence is observed in thin films of these compounds between a positively charged ITO electrode and a negatively charged aluminium electrode. The brightness of the electroluminescence decreases in the order perylene > pyrene > triphenylene. Threshold voltages below 20 V and a luminance up to 100 cd m-2 were observed.

Morphology of open films of discotic hexagonal columnar liquid crystals as probed by grazing incidence X-ray diffraction

The structure and the orientation of thermotropic hexagonal columnar liquid crystals are studied by grazing incidence X-ray diffraction (GIXD) for different discotic compounds in the geometry of open supported thin films. Whatever the film deposition mode (either spin-coating or vacuum evaporation) and the film thickness, a degenerate planar alignment with the liquid crystalline columns parallel to the substrate is found. However, if a specific thermal process is applied to the liquid crystal film, homeotropic anchoring (columns normal to the interface) can be stabilized in a metastable state.