Sei-Hum Jang

DIRECTOR TILTING OF LIQUID CRYSTALS ON PHOTOISOMERIZABLE POLYIMIDE ALIGNMENT LAYERS DOPED WITH HOMEOTROPIC SURFACTANT

Director tilting of liquid crystal (LC) molecules on photosensitive alignment layers has been investigated. The layers, polyimides with photoisomerizable groups in their main chains, were irradiated with unpolarized near-UV (UPUV) light at oblique incidence. The irradiated film was found to align LC molecules homogeneously with a negligible pretilt angle. It was found that the pretilt angle can be generated successfully by introducing surfactant lecithin and decreasing the incidence angle of the UPUV light. The LC alignment and electro-optical properties of the LC cells with the photoalignment layers were demonstrated to be suitable for twisted nematic LC, vertical alignment LC, and surface-stabilized ferroelectric LC devices.

Mixing effect of chelate complex and metal in organic light-emitting diodes

Organic light-emitting diodes using thin film dispersing a hole transport material into a soluble polyimide as a hole transport layer and the sublimed molecular film of a chelate complex as an emissive layer were fabricated. In order to improve the injection of electrons into the emissive layer as well as the durability of devices, we have attempted mixing the chelate complex and metal between the emissive layer and the cathodic electrode. The charge injection of the device with the mixed layer was initialized at an applied voltage of 4.19 V. It was observed from the electroluminescent spectra that the oscillator strength was dramatically enhanced with the applied voltage.

Alignment Mechanism of Nematic Liquid Crystal on Rubbed Polymer Surface Studied by Subsequent Processes of Rubbing and Photoalignment

A novel method for understanding the alignment mechanism was motivated by the texture observation of a nematic liquid crystal (LC) contacted with a photoaligned layer after rubbing. Reorientation of director occurs by subsequent photoalignment to different direction from that forced by rubbing. Moreover, it was found by polarized absorption spectra that the preferential average main chain axis over whole the alignment layer does not change, indicating that the orientation change by photoalignment occurs only at very top surfaces. This experiment without changing surface morphology indicates that the alignment priority for the nematic LC is mainly governed by the anisotropic short-range intermolecular interaction between alignment films and LC molecules and the effect of microgrooves plays a minor role.

Comparative Study of Anchoring Strengths for Photo- and Rubbing-Aligned Liquid Crystals on Photoisomerizable Polyimide Alignment Films

The high-electric-field technique was used to measure the out-of-plane (polar) anchoring strength at the interface between a nematic liquid crystal, pentylcyanobiphenyl (5CB), and a photosensitive polyimide (PI) alignment layer possessing an azo group in the main chain. The anchoring energy of the photoaligned state was found to be rather strong, on the order of 0.1 mJ/m2, being at least half of the anchoring energy on the rubbed surface of the same azo PI. The anchoring energy exhibited a conspicuous peak near the clearing temperature of 5CB, particularly in the photoaligned system, indicating a significant role of molecular motion in anchoring behavior.

Interrupted σ-Bonds in Organic Materials with Colligative Magnetic Properties

Abstract Solutions of tris(2, 6-dimethoxyphenyl)methyl radical (1) and lithium salts form solids showing unusual magnetic behavior. The propeller-shaped radicals, with their tripods of nucleophilic oxygens, are preorganized polydentate ligands. By coordination with small alkali metal cations the radicals may stack, enforcing spin-spin interactions in extended chains of paramagnets. Ab initio calculations on a linear H3C-Li+-CH3 model shows that high-spin (ferromagnetic) electron coupling is favored for a wide range of radical-Li+distances.

Stable Low Voltage Organic Light-Emitting Diode Using Insulating Polymer/Conducting Polymer Blends

Abstract Organic light-emitting diodes (OLEDs) utilizing the blend of doped polyaniline (PANI-DBSA) and aromatic polyimide (PMDA-ODA PI) as a hole transport layer (HTL) were successfully fabricated. Tris(8-hydroxyquinolinato aluminum) (Alq3), indium-tin oxide (ITO), and aluminum (Al) were used as a lumophore, transparent anode, and stable cathode, respectively. The charge injection of the OLEDs with the softbaked and imidized HTL was started at ca. 4 Vdc and 3 Vdc, whereas the turn-on voltage was ca. 5 Vdc for the OLED with the imidized film as a HTL. The electroluminescent (EL) spectra of both OLEDs were almost the same as those of photoluminescence (PL) of the HTL/Alq3 films with marginal shift toward higher energy.