Guest-host display

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

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

By far the most important commercial applications of nematic liquid crystals are in the a multi-billion dollar display industry. Cholesteric, or chiral nematic liquid crystals have been used in coloured guest-host displays and in thermography/ther-mochromic applications. 

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

Figure 1. Schematic representations of significant biological functions displayed by host-guest complexation in homogeneous solutions or at membrane surfaces, (a) Separation (e.g., antibody-antigen complex formation), (b) Transformation (e.g., enzymatic reaction), (c) Translocation (e.g., carrier- or channel-mediated transport), (d) Transduction (e.g., receptor-mediated transmembrane signaling).

Among the various LCDs, the three most widely used are (i) dynamic scattering display, (ii) twisted nematic display and (iii) guest-host display (Shanks, 1982). 

Many relatively simple amino- and hydroxyanthraquinone derivatives show a high degree of order in liquid-crystalline systems and are therefore suitable as dichroic dyes for guest-host displays. Several new dyes for this application have been described. For example, 9 is a blue dichroic dye [4],... 

Many different kinds of LCD displays can be constructed, but only twisted nematic (TN), dynamic scattering, and guest-host (GH) displays are presently of commerical importance. 

Fig. I. Schematic diagrams of the contrast versus the applied field for (a) a twisted nematic, (b) a cholesteric guest-host bistable display (Wysocki ei al., 1972 Ohtsuka and Sukamoto, 1973), and (c) a bistable LCD (Boyd et al., 1982).

Fig. 7. (Top) Schematic layout of the TFT in the addressing matrix. Cross-hatched area is the drain pad and the dotted area is the source contact. The gap between the source and drain contact is bridged by the underlying gate electrode (broken line) and the a-Si H semiconductor (not shown). (Bottom) View of the finished matrix. The small dark strip is a-Si H. The bright central square is the drain pad, doubling as the optical reflector of the guest-host display. Note that the transistor completely surrounds the drain pad.

Guest-host (GH) LCDs are coloured displays, whereby the colour and changes in colour are attributable to the absorption of incident light by a dichroic guest dye dissolved in a liquid crystalline host material, usually nematic, and the co-operative reorientation of these dichroic dyes in an electric field, i.e. electro-optical devices. 

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. 

Figure 3.14 Schematic representation of a Heilmeier and Zanoni guest-host liquid crystal display (GH-LCD)...

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

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. 

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