Hiroki Iwanaga

Oligothiophene dyes for guest-host liquid crystal displays

Three types of donor-acceptor-substituted bithiophene and terthiophene compounds were studied as dichroic dyes, which were dissolved in liquid crystals. These compounds were found to have large absorption coefficients, high dichroic ratios, excellent hues, and to be very attractive for guest-host-type liquid crystal displays. They also showed good solvatochromic behaviour in liquid crystal and cyclohexane solutions. These properties were found to be closely related to the solubilities of these compounds in liquid crystals. One donor-acceptorsubstituted bithiophene was found to behave as a polarity indicator in various kinds of liquid crystal.

The Molecular Structures and Properties of Anthraquinone-Type Dichroic Dyes

Abstract Relations between the molecular structures and properties of anthraquinone-type dichroic dyes were studied. Newly developed asymmetric dyes with thiophenyl groups are highly soluble in fluorinated liquid crystals and the dichroic ratios of these dyes are high (about 10). However, ortho-position (thiophenyl)-substituted dyes have lower solubilities and dichroic ratios than other position-substituted dyes. The properties of anthraquinone dyes with anilino groups are much lower than those of dyes with thiophenyl groups. These results can be strongly related to the flexibility of substituents. We newly established a hypothesis concerning dyes in liquid crystal solvent. In liquid crystals, the dye has a suitable conformation to adjust to liquid crystal phase. In this conformation, the solubilities and dichroic ratios are increased because the interactions between the dye molecules and liquid crystal molecules are strengthened. Flexible substituents are considered to easily form a suitable conformation and realize excellent properties.

Highly Soluble Anthraquinone Dyes with CF3-Groups for Guest-Host Liquid Crystal Displays

Anthraquinone dyes possessing thiophenyl substituents with a trifluoromethyl (CF3)-group at the meta position were synthesized for the first time. Among these dyes, only asymmetrical compounds were highly soluble in fluorinated liquid crystals even at low temperatures, and their solubilities were higher than the corresponding non-CF3-substituted ones.

Study of Molecular Structures and Properties of Europium(III) Complexes with Phosphine Oxides by NMR Analysis

The 31P-NMR spectra of europium complexes with β-diketones and phosphine oxides in CDCl3, DMSO-d6, toluene-d8 with dissolved fluorinated polymers, and fluorinated solvent were investigated as well as the temperature dependence of these spectra. When a mixture of europium complex 1 and 1.0 molar equivalent of trioctylphosphine oxide and triphenylphosphine oxide was dissolved in a fluorinated solvent, europium complex 2 with two types of phosphine oxides was formed at room temperature. In CDCl3 and toluene-d8 solutions, however, this complex was identified only at low temperatures. This difference is thought to be due to the differences in the speed of ligand exchanges. In DMSO-d6 solution, no signal corresponding to the europium complex with phosphine oxides was observed. This implies that the major portion of phosphine oxides is not coordinated with europium ions in DMSO-d6 solution. The fluorescence intensity of each europium complex in a solvent was correlated to the solvent and the 31P-NMR spectrum. It was found that the fluorescence intensity was the highest in the fluorinated solvent.

Solubility in Fluorinated Medium and Thermal Properties of Europium(III) Complexes with Phosphine Oxides

Relationships between molecular structures of three types of europium complexes and solubility in hexane and fluorinated media were investigated. The solubility of europium complexes with β-diketones (type-1) was correlated to the substituents of β-diketones. Introducing bulky substituents was effective for enhancing solubility. On the other hand, europium complexes with β-diketones and phenanthroline (type-2) were scarcely soluble in either solvent. Among europium complexes with β-diketones and phosphine oxides (type-3), complex 16 which has bulky substituents at β-diketones and two different phosphine oxides, was found to be highly soluble even in a fluorinated medium. Thermal properties of europium complexes were correlated to the molecular structures and solubility. ΔHf of 16 is characteristically small compared with similar type-3 complexes. The transparent fluorescent solid in which complex 16 was dissolved plays an important role as a fluorescent layer of light-emitting diodes (LEDs) for realizing large luminous flux (over 870 mlm when 20 mA of direct current was applied to the ultraviolet (maximum wavelength: 402 nm) LED chip).

Emission Properties, Solubility, Thermodynamic Analysis and NMR Studies of Rare-Earth Complexes with Two Different Phosphine Oxides

The paper proposes novel molecular designs for rare-earth complexes involving the introduction of two different phosphine oxide structures into one rare-earth ion. These designs are effective for improving solubility and emission intensity. Additionally, the complexes are indispensable for realizing high performances in LEDs and security media. The thermodynamic properties of Eu(III) complexes are correlated with the solubility. Correlations between coordination structures and emission intensity were explained by NMR analysis. The luminous flux of red LED devices with Eu(III) complexes is very high (20 mA, 870 m lumen). A new white LED has its largest spectra intensity in the red region and a human looks much more vividly under this light.

Relation between Molecular Structures of Dichroic Dyes and Their Solubilities in Fluorinated Liquid Crystals

A molecular design rule for dichroic dyes with a high solubility in fluorinated liquid crystals has been investigated. Molecular interactions and a concept of motional segments proportional to molecular weight were introduced to explain the deviation from an ideal solution. A semi-empirical equation was obtained: log C=-20.75 (ΣΔHf/T - ΣΔSf)- 0.013(Eh - Ep)+2.32, where C represents a saturated solubility in wt% at T (K). ΣΔHf (kJ kg-1) represents the sum of the enthalpies of fusion and of phase-transitions from T to melting point. ΣΔSf represents the sum of the entropies. Calculated Eh represents a hydration energy. Ep is the hydrophobic energy determined from calculated 1-octanol log P. Log C is mainly determined by the interaction between dye molecules. Bulky, rigid and asymmetric structures give a high solubility. Large electrostatic interaction between a dye molecule and liquid crystal molecules increases solubility.

Correlations among molecular structures, solubilities in fluorinated media, thermal properties and absorption spectra of anthraquinone dichroic dyes with phenylthio and/or anilino groups

Attractive correlations among molecular structures, solubilities in fluorinated media, thermal properties and absorption spectra of several anthraquinone dichroic dyes with phenylthio and/or anilino groups were investigated. These dyes are classifiable into three types from the viewpoint of correlations between thermal properties and solubilities. The first group consists of hardly soluble anthraquinone dyes with the same phenylthio groups at positions 1 and 5. They have large enthalpy of fusions (94–127 kg/kJ) which induce strong interactions among dye molecules. The second group consists of dyes with two different phenylthio groups at positions 1 and 5. They have smaller enthalpy of fusions (around 60 kg/kJ) and can be solved in the cases of some combinations of phenylthio groups. The third group consists of dyes with two different phenylthio groups and one of them has a trifluoromethyl group at a meta position. They have very low enthalpy of fusions (2.8–22.8 kg/kJ) and their solubilities are very large even at low temperatures. The difference between the solubility in fluorinated liquid crystals and in fluorinated solvents suggests that dyes in liquid crystal phase have specific solvation forms that are different from those in isotropic phase.

Color Design and Adjustment of Dichroic Dyes for Reflective Three-Layered Guest-Host Liquid Crystal Displays

ABSTRACT Relationships between properties of dichroic dyes and performances of three layered Guest-Host Liquid Crystal Displays (GH-LCDs) with subtractive color mixing of yellow, magenta and cyan were studied by color mixing simulation. In the case of using the dyes we developed, it was found that the color reproduction areas were as much as 1.6 times larger than in the case of using only commercially available dyes. Performances of our prototype reflective three-layered GH-LCDs were high (the luminous reflectance of the white state was 43%, and the contrast was 5.3), indicating that three-layered GH-LCDs are practical for the display of full-color images.

P-73: Reflective Color LCD Composed of Stacked Films of Encapsulated Liquid Crystal (SFELIC)

A reflective color liquid crystal display (LCD), which dose not consume power for backlighting, is a key device for portable information equipment. A novel structure for three-layered guest-host (GH) liquid crystal displays (LCDs) has been proposed. The structure, called stacked films of encapsulated liquid crystal (SFELIC), has no intermediate substrates, and is expected to realize low cost and high brightness while eliminating parallax.