Ki-Jong Han

Bright pure blue emission from multilayer organic electroluminescent device with purified unidentate organometallic complex

Multilayer organic electroluminescent devices (OELDs) were fabricated with highly pure 2-(2-hydroxyphenyl)benzoxazolato lithium (LiPBO), which was obtained through stepwise purification process, as a blue emission layer. The ionization potential of the carefully purified LiPBO was ∼5.82 eV. The multilayer OELD with a hole-blocking layer (HBL) emitted almost pure blue light with the CIE color coordinate of x=0.15 and y=0.08. However, the emission color was redshifted when an electron-transporting layer (ETL) was introduced instead of the HBL. The device with both the HBL and the ETL showed stable and bright blue emission above 14 000 cd/m2 with the color coordinate of x=0.15 and y=0.11, even though the color purity was slightly poorer than that with only the HBL.

Dendritic fullerenes (C60) with photoresponsive azobenzene groups

Fullerene-cored dendrimers bearing up to eight photoisomerizable azobenzene groups in the periphery have been synthesized as potential photoswitches and spectroscopically characterized.

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.

Hole-transporting polyimide for organic electroluminescent display ☆

Abstract The thermal stability of newly synthesized hole-transporting polyimide, poly[N,N′–diphenyl–N,N′–bis(4–aminobiphenyl)–(1,1′–biphenyl)–4,4′–diamine pyromellitimide] (PMDA-DBABBD PI), via vapor deposition polymerization was investigated with the aid of the capacitance–temperature (C–T) measurement technique. Prior to the examination of the complete organic electroluminescent device (OELD), the single layer devices with the individual materials including tris(8-hydroxyquinolinato) aluminum (Alq3), N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB), copper phthalocyanine (CuPc), and PMDA-DBABBD PI were subjected to the C–T measurement. The relaxation temperatures of the single layer devices with Alq3, TPD, NPB, CuPc, and PMDA-DBABBD PI were 180, 76, 110, 125, and more than 200°C, respectively. The OELD with PMDA-DBABBD PI and Alq3 as a hole-transporting layer and emissive layer was not relaxed up to 150°C, while those containing CuPc/TPD and NPB thin films were catastrophically damaged at ca. 76 and 110°C, respectively. The OELD with the small organic hole-transporting molecule has almost the same relaxation temperature as the single layer device with the respective molecules. The rectifying and electroluminescent characteristics disappeared for the annealed OELD with the small organic hole-transporting molecules, whereas the OELD with the hole-transporting polyimide did still show the rectifying behavior with the green light emission even though the current density and the light intensity became largely reduced.

Stabilization of the Charge-Separated States of Covalently Linked Zinc Porphyrin-Triphenylamine-[60]Fullerene

Spectroscopic, redox, computational, and electron transfer reactions of the covalently linked zinc porphyrin-triphenylamine-fulleropyrrolidine system are investigated in solvents of varying polarity. An appreciable interaction between triphenylamine and the porphyrin pi system is revealed by steady-state absorption and emission, redox, and computational studies. Free-energy calculations suggest that the light-induced processes via the singlet-excited porphyrin are exothermic in benzonitrile, dichlorobenzene, toluene, and benzene. The occurrence of fast and efficient charge-separation processes ( approximately 10(12) s(-1)) via the singlet-excited porphyrin is confirmed by femtosecond transient absorption measurements in solvents with dielectric constants ranging from 25.2 (benzonitrile) to 2.2 (benzene). The rates of the charge separation processes are much less solvent-dependent, which suggests that the charge-separation processes occur at the top region of the Marcus parabola. The lifetimes of the singlet radical-ion pair (70-3000 ps at room temperature) decrease substantially in more polar solvents, which suggests that the charge-recombination process is occurring in the Marcus inverted region. Interestingly, by utilizing the nanosecond transient absorption spectral technique we can obtain clear evidence about the existence of triplet radical-ion pairs with relatively long lifetimes of 0.71 mus (in benzonitrile) and 2.2 mus (in o-dichlorobenzene), but not in toluene and benzene due to energetic considerations. From the point of view of mechanistic information, the synthesized zinc porphyrin-triphenylamine-fulleropyrrolidine system has the advantage that both the lifetimes of the singlet and triplet radical-ion pair can be determined.

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.

Convenient Synthesis of N-Hydroxysuccinimide Esters from Carboxylic Acids Using Triphosgene

Abstract A simple and convenient method for the synthesis of N-hydroxysuccinimide ester is developed using triphosgene as an acid activator. Several aromatic and aliphatic N-hydroxysuccinimide esters are prepared from their corresponding carboxylic acids at room temperature in good yields in a rapid process using triphosgene. Some of the major advantages are mild conditions, good yields, and easy operation.

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.

An efficient one-pot synthesis of N-methoxy-N-methylamides from carboxylic acids

Abstract Several N-methoxy-N-methylamides were prepared by the reaction of the corresponding carboxylic acids with N,O-dimethylhydroxylamine hydrochloride at room temperature using trichloromethyl chloroformate in the presence of triethylamine in excellent yields.

Synthesis and absorption properties of zinc-phthalocyanines with photoresponsive azobenzene groups

The phthalocyanines (1) and (2) with eight photoisomerizable azobenzene groups have been synthesized and spectroscopically characterized. These macrocycles exhibit partial trans-cis isomerzation properties upon irradiation of UV/VIS light.