The invention relates to a liquid-crystalline medium based on a mixture of polar compounds having negative dielectric anisotropy (Δe), which contains at least one compound selected from the group of compounds of the formula I, II A and II B and at least one compound of the formula I* in which R0, R1, R2, R1*, X1, X2, A1, A1*, A2*, Z1, Z1*, Z2*, L1*, L2*, p, q, v, m and m* are as defined in Claim 1, and to the use thereof for an active-matrix display based on the ECB, VA, PS-VA, FFS, PALC or IPS effect.

Liquid Crystalline Medium

This critical review begins with a brief, but essential, introduction to the special nature of liquid crystal materials, their peculiar properties, and their commercial applications, followed by an introductory insight into the remarkable nature of the fluoro substituent, and its fascinating influence on the properties of organic compounds. However, the main focus of the review is to discuss the enormous amount of exciting research on fluorinated liquid crystals that has been reported. The small size of the fluoro substituent enables its incorporation into all types of liquid crystal, including calamitic, discotic, banana, lyotropic, and polymers, without ruining the liquid crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity, confers many fascinating, and often remarkable, modifications to melting point, mesophase morphology and transition temperatures, and the many other very important physical properties, such as dielectric anisotropy, optical anisotropy, and visco-elastic properties. There are many different positions within a liquid crystal structure where a fluoro substituent can be located, including (i) a terminal position, (ii) within a terminal chain, as a semi-fluorinated or as a perfluorinated chain, or as one fluoro substituent at a chiral centre, (iii) as part of a linking group, and (iv) a lateral position in the core section. Such variety enables the interesting and advantageous tailoring of properties, both for the fundamental purposes of establishing structure–property relationships, and for materials targeted towards commercially-successful liquid crystal display applications.

Fluorinated liquid crystals – properties and applications

At least two antiferroelectric liquid crystalline phases were discovered in MHPOBC. These phases appear below the usual ferroelectric Sm C* phase. Because of the alternation of the molecular tilt directions as well as the dipole orientations in successive layers, the optic axis is along the layer normal. This strong stabilization along the layer normal brings about the so-called third stable state responsible for the tristable switching. The antiferroelectric structure was strongly supported by selective reflections in oblique incidence; a full-pitch band does not appear in the antiferroelectric phases, while it does appear in the ferroelectric phase.

https://iopscience.iop.org/article/10.1143/JJAP.28.L1265/pdf

Antiferroelectric Chiral Smectic Phases Responsible for the Trislable Switching in MHPOBC

Since the development of the first white organic light-emitting device (OLED) in 1993, twenty years have passed. The power efficiency and lifetime of this white OLED were reportedly only <1 lm W−1 and <1 day, respectively. However, recent rapid advances in material chemistry have enabled the use of white OLEDs for general lighting. In 2012, white OLED panel efficiency has reached 90 lm W−1 at 1000 cd m−2, and a tandem white OLED panel has realized a lifetime of over 100 000 hours. What is more important in OLEDs is to shed clear light on the new design products, such as transparent lighting panels and luminescent wallpapers. These fascinating features enable OLEDs as a whole new invention of artificial lighting. In this review, we would like to overview the recent developments of white OLED, especially three key elemental technologies related to material chemistry: (1) low operating voltage technology, (2) phosphorescent OLED technology and (3) multi-photon emission (MPE) device technology.

Development of high performance OLEDs for general lighting

The use of a certain class of liquid crystal materials that exhibit a smectic C phase allows the production of a bistable liquid crystal display element. Such bistable display elements promote the use of matrix addressing for liquid crystal based elements in a display.

Smectic liquid crystals

The phase, designated as Sm CA*, which shows tristable switching was investigated in C8H17O–--COO–-COO*CH(CH3)C6H13 (MHPOBC) by means of thermal analyses, a miscibility test and microscope observation. The phase transition from Sm C* to Sm CA* is not observable by AC calorimetry but small peaks comparable to the Sm A-Sm C* transition are discernible in DSC. According to the miscibility test of racemic MHPOBC with a standard compound, this new phase was found to exist between Sm C* and Sm I* and not to be miscible with Sm Bhex. Therefore, the assignment of Sm CA* to one of the known phases is ruled out, which means that Sm CA* is a totally new phase. The difference between the pure enantiomer and the racemate is also pointed out.

https://iopscience.iop.org/article/10.1143/JJAP.28.L1261/pdf

Novel Phases Exhibiting Tristable Switching

We have demonstrated new and efficient electron transporting (ET) and hole blocking (HB) materials for organic light-emitting devices (OLEDs). The bipyridyl-substituted oxadiazole derivatives can form a stable glassy state with glass transition temperatures greater than 100 °C. The bipyridyl-substituted oxadiazole derivatives have excellent ET, HB and electron accepting ability and also exhibit electrical operation durability. Bipyridyl-substituted oxadiazoles are a promising candidate for ET and HB materials for OLEDs.

Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials

Chiral smectic C compounds having an ester group in the central core, an optically active tail group (alkoxy or alkoxycarbonyl), and an electronegative substituent (halogen or cyano) ortho to the chiral tail were synthesized. The effect of the ortho substitution on the characteristics, especially on spontaneous polarization (Ps), was examined. When the chiral tail is a 1-methylheptyloxy group the lateral substituent increases Ps relative to the unsubstituted analogue. In the case of 1-methylheptyloxycarbonyl tail group, however, the lateral substituent decreases Ps relative to the unsubstituted. These effects of lateral substituents are explained in terms of chemical structure of a single molecule, i. e., by taking statistical weight of conformers and the associated electric dipole moments into account.

Chiral smectic C liquid crystals having an electronegative substituent ortho to the chiral tail group - a study of a factor determining the magnitude of spontaneous polarization

Chiral smectic compounds having a high response rate and being superior as a ferroelectric liquid crystalline material, and chiral smectic mixtures containing the same are provided, which compounds being expressed by the formula ##STR1## wherein X represents --CH 2 --, --O-- or --CO--; n represents 0, 1 or 2; when X represents --CH 2 --, Y represents R--, R--CO--, ROCO--, ##STR2## wherein R represents an alkyl group having 1 to 18 carbon atoms, and when X represents --O-- or --CO--, Y represents R; and * represents a symbol indicating an optically active carbon atom.

Liquid crystalline biphenyl derivatives and mixtures thereof

Novel ferroelectric liquid crystalline compounds having a superior stability and chiral, smectic liquid crystalline compositions containing at least one kind of the same are provided, which compounds are expressed by the general formula ##STR1## wherein ##STR2## represents 1,4-phenylene group ##STR3## or 1,4-trans-cyclohexane group ##STR4## R*, an optically active alkyl group; m=o, 1 or 2; n=1 or 2; X, a linear chain or branched alkyl group or alkoxy group, each having 1 to 18 carbon atoms; and when ##STR5## represents ##STR6## m=1; and n=1, X represents a linear chain or branched alkyl group having 1 to 18 carbon atoms or a linear chain alkoxy group having 11 to 18 carbon atoms.

Kenji Furukawa

Papers

Novel Phases Exhibiting Tristable Switching

Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials

Chiral smectic C liquid crystals having an electronegative substituent ortho to the chiral tail group - a study of a factor determining the magnitude of spontaneous polarization

Liquid crystalline biphenyl derivatives and mixtures thereof

Liquid crystalline compounds and mixtures thereof