Dichroic dyes, guest-host effect

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

Absorption anisotropy (dichroism) in liquid crystals takes place either due to the presence of the short wavelength (< 400 nm) oscillator, usually coinciding with the direction of its long molecular axis, or due to the impurities, such as dichroic dyes ( guests ) dissolved in liquid crystals ( hosts ). We will consider the second case as the most important for practical applications ( guest-host effect [18]). 

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]. 

The guest host effect realized in polymers on addition of a dichroic dye and untwisting of the cholesteric helix open up certain prospects for the construction of solid optical elements based on LC polymers such as dichroic polaroids and selective filters. The important possibility of obtaining ferroelectric polymeric LC materials was recently demonstrated in [67]. 

Dichroic dye molecules absorb light in an anisotropic way and show different colors in different directions. If such dyes are dissolved in a liquid crystal, they form a guest-host-type interaction, and are oriented by the host liquid crystal molecule. Application of an electric field will reorient the liquid crystal and dye this effect is used for liquid-crystal displays. 

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. 

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