Twisted guest-host effect

Most of the electrooptic effects are based on light scattering or on light absorption by polarizers or by dissolved dyes. Electrooptic effects which belong to the second group are tunable birefringence, the twisted nematic effect and the guest-host effect. 

Unlike the twisted nematic displays which require an external reflector because of the external polarizers, the reflector for a guest-host display can be located inside the cell in direct contact with the liquid crystal layer. The internal reflector completely eliminates the parallax and shadow effects that are encountered in displays with external reflectors, and allows full use to be made of the intrinsically wider field of view available to the guest-host effect. An internal reflector can also improve the uniformity of the layer thickness because thicker cell plates can be employed without introducing parallax effects. 

A color difference ratio allows us not only to evaluate the usual luminance contrast (4.59), but also to evaluate a color purity and the color hue that makes the human eye very sensitive to the variation of CDR. Reference [170] demonstrates the increase by a factor of three of the number of distinguished rows of the display when evaluating by the colorimetry methods. This effect may be understood by the fact that the human eye is more sensitive to the color contrast than to the luminance one, as the hue and the chroma both contribute to the former. Color differences between on and off display states could be optimized by a special choice of polarizers [170-172], concentration of a dichroic dye (guest) in a liquid crystal matrix (host) [166, 173], using the reflective screens [173], etc. Colorimetric evaluations are very useful in the quality control of both the guest host and twist displays [172, 174]. 

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

The different electro-optic phenomena have been classified into those that involve only dielectric forces and those that depend upon the interaction of conduction and dielectric torques. The field-effect phenomena possess several common properties. The resistivity of the materials may be as high as chemically practical, i.e., p 10 ohm-cm. For the induced birefringence, twisted nematic, and guest-host color switching effects, the threshold voltages are less than 3 or 4... 

Further, two thick rigid polarizing films spoil device flexibility to be unnecessary. Therefore, guest-host twisted liquid crystal devices with dichroic dyes [7] were fabricated to absorb the all-polarization-angle incident light. In this device, a nematic liquid crystal with low birefringence is introduced, and optical rotation effect is suppressed in the twisted liquid crystal layer. The contrast ratio of the display is inferior, but the guest-host nematic liquid crystal of twist orientation is suitable for simple text display. 

Such twisted nematic phases are called induced cholesteric solutions and - as schematically outlined in Fig. 4.6-9 - enantiomers cause countercurrently twisted structures. As discussed by Korte and Schrader (1981) this effect offers the potential of sensitively characterizing the chirality of small amounts of optically active compounds. There are no restrictions as to the type of chirality, and the experiments can advantageously be based on infrared spectroscopy. The application of induced cholesteric solutions was later reviewed again by Solladie and Zimmermann (1984). The host phase is the more twisted the more of the optically active guest compound is dissolved. Quantifying the twist by the inverse pitch z and the concentration by the molar fraction x, the ability of a chiral. solute to twist a given nematic host phase is characterized by the helical twisting power (HTP Baessler and Labes, 1970). For small values of a this quantity P is defined by the relation... 

Figures 2-5 present structures of the complexes taken from the Monte Carlo simulations. The illustrated structures are the last ones from the different simulations. Though their choice is effectively random, they are good representaticsis of typical structures sampled during the calculations. Relatively little motion was chtained for the host exc t for torsional changes for the ei t methojQ groips. However, the guests rattled in the cavity with some twisting and translations of 2-3 A.

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