Dyes—Guests

Dichroic dyes of positive and negative dichroism are required for various configurations of the Heilmeier and Zanoni as well as the White and Taylor GH-LCDs with positive and negative contrast. They should be chemically, photochemically and electrochemically stable, as well as exhibit a high order parameter, a high dichroic ratio and good solubility in the host nematic matrix. 

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Guest-Host Mode LCD Systems. Guest-host hquid crystal display systems consisting of dichioic dyes (guest) and hquid crystal media... 

HJ Coles, GA Lester, and H Owen, Fluorescent dye guest-host effects in advanced ferroelectric liquid crystals, Liq. Cryst., 14 1039-1045, 1993. 

Figure 16.6 Schematic representation of the energy transfer process between OPV-poly(propylene imine) dendrimer and a dye guest molecule [84]...

Electrically switchable host for dichroic dyes Guest-host and dyed phase change displays... 

Figure 9.6 Schematic coiifiguialions and operating principles of two absorption-type transflective LCDs with (a) nematic phase LC (host) and dye (guest) mixtures, and (b) cholesteric phase LC (host) and dye (guest) mixtures. Zhu 2006. Reproduced with permission from IEEE.

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]. 

FIGURE 6.12 Structure formulas of the dyes (guests) [38], Eosin Y (E ), Eluorescein (E ), 5-(and-6)-carboxynaphthofluorescin (N ) and Rose Bengal (R ). (Reprinted from Ref [38] with permission of Elsevier. Copyright 2002.)... 

Figure 38. Schematic operation of the White-Taylor dye guest-host chiral nematic electrooptic cell. In (a) for zero applied field the axis of each focal-conic domain is random in the x, y plane, as therefore is the dye, using homeotropic surface alignment. In (b) the texture is planar for the zero field state and therefore the dye spirals around the z direction. In (c) the focal conic (a) or planar (b) transition to homeotropic nematic has taken place above the threshold voltage V,], (WT). The black ellipses represent the dyes in the chiral nematic matrix.

Coles et al. were the first to investigate the dye guest host effect in a FLC polymer (60). The high-polarization polyacrylate 14 [31] (see above), containing 4% of a blue azo dye (M 483 from Mitsui Toatsu Chemical, Inc.X gave the absorption spectra displayed in Figure 41. From the absorptioo parallel and perpendicular to the direction of molecular orientation (diiector). an order parameter of 0 J7,... 

R J. Coles, H. F. Olceson, G. Scherowsky, and A. Schliwa, Ferroelectric polymer liquid crystals II The dye guest host effect. Moi Cryst Liq. Cryst Lett 7 (4) 125 (1990). 

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