Chizu Sekine

Recent progress of high performance polymer OLED and OPV materials for organic printed electronics

Abstract The development of organic printed electronics has been expanding to a variety of applications and is expected to bring innovations to our future life. Along with this trend, high performance organic materials with cost-efficient fabrication processes and specific features such as thin, light weight, bendable, and low power consumption are required. A variety of organic materials have been investigated in the development of this field. The basic guidelines for material design and the recent progress of polymer-based organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) are reported.

Synthesis and properties of high birefringence liquid crystals: thiophenylacetylene and benzothiazolylacetylene derivatives

We have synthesized and evaluated the physical properties of thiophenylacetylene and benzothiazolylacetylene derivatives in order to obtain higher Δn liquid crystals as compared with analogous phenylacetylene-based compounds. These new materials had poorer liquid crystallinity than the phenylacetylene derivatives. Exchanging the phenyl ring for a benzothiazole ring was effective in increasing Δn, whereas a thiophene ring decreased Δn because of the associated reduction in the anisotropic polarizability. These conjugated hetero rings shifted the UV absorption spectra into the visible region such that they are yellow or brownish in colour.

Synthesis and properties of some novel high birefringence phenylacetylene liquid crystal materials with lateral substituents

We have synthesized and investigated new 3-ring phenylacetylene liquid crystals with high birefringence values (Δn) to improve the nematic temperature range while retaining a high optical anisotropy. In the case of modifying the terminal ring, the introduction of branched alkoxy chains, fluorine or methyl groups shifted the nematic phase to lower temperatures. In order to minimize the influence of the substituent on Δn, the incorporation of lateral methyl groups was chosen as the most profitable method for obtaining wide and low temperature nematic phases. With these results in mind, we studied the effect of the position of the methyl group on the physical properties. The most effective methyl position for improving the nematic temperature range was on the central ring. From the standpoint of obtaining high Δn values and low viscosity, the central ring was also the best position.

Novel high birefringence dibenzothiophenylacetylene liquid crystals

We have designed, synthesized and evaluated dibenzothiophenylacetylene homologues as new heterocyclic molecules which were expected to have high δα values and short wavelength UV absorption spectra. The dibenzothiophenyl acetylene homologues exhibited wide nematic phase ranges and had very high extrapolated δn of over 0.6 and directly measured values of 0.5. The compounds with two dibenzothiophenyl rings were colourless, so that we have succeeded in obtaining colourless super-high δn liquid crystals. We have also analysed the contribution of the transition band to the refractive indices and the birefringence. From the analysis, the main origins of δn for compounds 1 and 2 are lower MOs than the HOMO and this results in the compatibility of large δn and lack of colour. The dibenzothiophenylacetylene compounds behaved according to our design expectations.

Synthesis and physical properties of high birefringence phenylacetylene liquid crystals containing a cyclohexyl group

We have synthesized new phenylacetylene-based liquid crystals containing a cyclohexyl or cyclohexylethyl group and evaluated their physical properties in order to develop a range of materials having high value of birefringence. The cyclohexyl-containing compounds exhibited nematic behaviour near room temperature and moderate values of δn of around 0.3. The cyclohexylethyl-containing compounds had a very wide nematic range with a high TNI and very high values of δn of over 0.4. They also exhibited low viscosities. The order parameter was not affected by introducing either a cyclohexyl or a cyclohexylethyl group and the values of δn based on calculated polarizabilities were obtained experimentally.

High Birefringence Phenylacetylene Liquid Crystals with Low Viscosity

Abstract We have synthesized new high Δ n 3-ring phenylacetylene(3PA) liquid crystals which have substituent groups with core center phenyl ring and investigated the effects of substitution on physical properties. Nematic range and viscosity were improved remarkably with keeping high Δ n by introducing trifluoromrthoxy group. The Δ n of 3PAs were also descussed with relationship of polarizability and order parameter.

High birefringence photopolymerizable phenylacetylene liquid crystals

Photopolymerizable three-ring phenylacetylene (3PA) liquid crystalline compounds were synthesized and the physical properties of the monomers and polymers were characterized in the expectation of obtaining high birefringence polymers. 3PA monomers containing acrylate substituents exhibited enantiotropic nematic phases and high δn values of around 0.35. Homogeneous alignment was retained after polymerization and high values of δn (~0.34) for the polymers were obtained. The orientation of the monomers at the photopolymerization temperature was almost fixed, such that δn and the order parameter of the polymers varied with the polymerization temperature.

Highly Anisotropic Molecular Materials for LCD

Abstract Two kinds of novel molecular materials with high optical anisotropy were synthesized and evaluated. Liquid crystalline phenylacetylene derivatives, which have mesogen of a 1,4-Bis(phenylethynyl)benzene, not only showed high δn but also were stable for heating or light. Triphenodioxazine derivatives were highly dichroic in guest-host liquid crystal mixtures, where they also showed vivid color due to a sharp peak in absorption spectra. Hence, these molecules possibly improve visual performance of some reflection type liquid crystal displays (LCDs).

Synthesis and physical properties of novel phenylbenzoate type ferroelectric liquid crystals

Abstract A relationship between molecular structure and physical properties of novel 2- and 3-ring phenylbenzoate derivatives is reported. In these compounds, the chiral carbon is directly linked to the core aromatic ring, or to the methylene unit which is linked to the core aromatic ring. The internal methylene unit plays an important role to exhibit the liquid crystal phases in 2-ring phenylbenzoate.

29.1: Invited Paper: Recent Progress in Light-Emitting Polymers for Full Color OLEDs

We have significantly improved the efficiency, lifetime and hue of red, green, blue (RGB) light-emitting polymers (LEPs) through intensive studies on degradation and charge balance in polymer organic light-emitting diodes (OLEDs). The degradation of LEPs during device operation is shown to result from decreased emissive material fluorescence efficiency and a loss of charge-carrier balance. Laser photolysis results show that low-energy sites are generated in the LEP during driving, leading to the quenching of blue and green emission, but not red emission. Charge carrier balance also shifts due to decreased electron injection compared with hole injection. Taking these degradation mechanisms, we have focused on copolymerization of various functional units to stabilize the photoluminescence (PL) intensity and the balance of charge carriers. In this paper, we present the degradation mechanism and performance of our latest RGB LEPs.