Reentrant behavior in nematic liquid crystals is described by the treatment of the system as a mixture of monomers and dimers interacting through attractive induced soft interactions as well as hard-core repulsions It is shown that the induced forces between dimers, monomers, and between monomers and dimers, respectively, under suitable conditions of temperature and density lead to a smectic layer structure with a period equal to the length of the dimer However, with decreasing temperature or increasing density, repulsive steric forces, due to the unfavorable packing of the dimers in the smectic planes, may take over and thus favor the nematic phase again In agreement with experiment the best conditions to get a reentrant nematic phase are predicted for the ratio of the length of the dimer to that of the monomer between 13 and 14 The predictions of the model are in good agreement with currently available experimental data In principle, the model applies also to discotic liquid crystals

Microscopic one-particle description of reentrant behavior in nematic liquid-crystals

Two large pentakis(phenylethynyl)phenyl alkyl ethers, without side chains as is usual for discotic systems, were synthesized in palladium catalysed one pot coupling reactions. These low molecular weight and non-liquid-crystalline electron donor compounds were doped with 2,4,7-trinitrofluorenone under the formation of charge transfer complexes. The induction of two types of enantiotropic mesophases (i.e. nematic and hexagonal columnar phases (Nc and Dho)) was observed. These charge transfer-induced mesophases were characterized by polarization microscopy, differential scanning calorimetry, and X-ray analysis; phase diagrams of the binary systems were also recorded as well as the hexagonal columnar to nematic columnar transition as a function of the length of the disturbing alkoxy group. It is considered possible now to study low molecular mass Nc phases by tailor-made two component mixtures on the basis of our easily accessible disc-shaped multiynes.

Charge transfer induced nematic columnar phase in low molecular weight disc-like systems [1]

Induction and variation of discotic columnar phases through doping with electron acceptors

This paper reviews the present status and art of liquid crystal displays. It discusses the fabrication and operational principles of dynamic scattering, twisted nematic, guest-host, dye phase change, tunable birefingence, thermally addressed smectic A mode, etc, displays. The potentialities of applications of these displays are examined critically. Twisted nematic mode displays, which capture over 99% of today's LCD market, is discussed in detail. Molecular enginnering, materials, and fabrication processed have been described. Addressing of liquid crystal displays, multiplexing and criteria for high level multiplexing are also discussed in detail. The use of nonlinear active devices (like varistor, MIM, TFT) for high level multiplexing are pointed out.

Liquid Crystal Displays

Recent developments in understanding structural and material properties of ferroelectric liquid crystals are reviewed with emphasis on their relevance for electro-optic effects and device applications. In only a few years the material situation has changed from one characterized by an almost complete lack of technically useful materials to one, not yet adapted to technical needs, but beginning to offer interesting, commercially available ferroelectric mixtures. The present device limitations can, however, be tracked down, not so much to the principal non-availability of materials as to the lack of understanding of which material properties are desirable and needed, in particular to permit simple and efficient matrix addressing. Thus we have centered our discussion to properties of prime importance for device dynamics in this review, which also contains a general discussion of material properties based on the molecular structure, as well as an assessment of the present industrial development.

Material Properties of Ferroelectric Liquid Crystals and Their Relevance for Applications and Devices

Non-linear behaviors of nematic-isotropic transition temperatures and the appearance of the “solid-like” phase found in the mixtures between MBBA and 4-cyano-4-n-pentylbiphenyl (CPB)1 are now found to be quite general among many other nematogens. The cyano-group containing nematogens do not have to be biphenyl derivatives, nor do the “donner” type molecules need to be limited to Schiff Bases and azo-type compounds. This type of phase diagram has also been obtained in mixtures of CPB with phenyl benzoates, azoxy and tolan nematogens. Other cyano-group containing nematogens, such as substituted 4′-cyanophenyl benzoates and 4′alkoxybenzylidene-4-aminobenzonitriles gave rise to behavior identical with the above nematogens. A detailed study on the phase diagram of mixtures containing 4-cyano- 4′-n-octyloxybiphenyl and 4′n-hexyloxy-phenyl-4-n-butyl blenzoate has yielded further insight into the nature of the “solid-like” phase. The smectic thermal stability of the alkoxycyanobiphenyl decreased sharply ...

Induced Smectic Mesomorphism by Incompatible Nematogens

A series of optically active aromatic Schiff's bases containing p-alkoxy and p-acyloxy groups were prepared and their mesomorphic properties were studied. Chirality was achieved by introducing an asymmetric carbon atom into the hydrocarbon chain of either the alkoxy or acyloxy group. Thus, 4-active-alkoxybenzylidene-4′-aminophenyl acylates, 4-active-alkoxy-benzylidene-4′-aminobenzonitriles and bis-(4′-active-alkoxybenzylidene)-2-chloro-1,4-phenylenediamines were synthesized. Most of the compounds exhibited both smectic and cholesteric mesomorphism. Several compounds showed only cholesteric behavior. For example, S(+)-4-methoxybenzylidene-4′-aminophenyl p-methylpentanoate was cholesteric between 41 and 78°C. By comparison, the racemic mixture of R and S enantiomers of this compound was nematic between 36 and 78°C.

Liquid Crystals VIII. The Mesomorphic Behavior of Some Optically Active Aromatic Schiff's Bases

: Research on materials and electro-optic response measurements of certain cholestericnematic mixtures was performed in an effort to develop an optical storage display. Mixture of selected compounds from the series, p- alkoxybenzylidene-p'-aminophenyl acylates, were prepared and one ternary mixture (NH-1) which exhibited nematic behavior from 25 to 105C was found. A group of optically active benzylidene aniline derivatives having alkoxy and acyloxy groups with asymmetric carbon atoms was prepared, and the compounds were found to exhibit cholesteric mesomorphism. A simulated, airborne ground location marker was constructed with a cell containing this material. In actual operation this device would provide the pilot or navigator of an aircraft with a map presentation of his past and present position as determined by two ground navigation stations.

Chan S. Oh

Papers

Induced Smectic Mesomorphism by Incompatible Nematogens

Liquid Crystals VIII. The Mesomorphic Behavior of Some Optically Active Aromatic Schiff's Bases

Liquid Crystal Systems for Electro-Optical Storage Effects