Nematic Liquid Crystals—Hosts

Analysis of the electro-optical characteristics of GH-LCDs showed that for all types of guest-host chiral nematic phase change effects the contrast is greatest when 

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Nematic Liquid Crystal Hosts of Positive Dielectric Anisotropy... 

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. 

Figure 2.26 Structures of the nematic liquid crystal host matrices MBBA and 5CB, and the structure of the europium(III) complex [Eu(tta)3(phen)]...

Figure 2.27 Room temperature luminescence spectrum of [Eu(TTA)3(phen)] in the nematic liquid crystal host MBBA. The doping concentration was 4% w/w. The excitation wavelength is 396 Reprinted with permission from Binnemans...

Synthesis and Analysis of Nickel Dithiolene Dyes in a Nematic Liquid Crystal Host... 

A series of model nematic liquid crystals (among them oxadiazole derivatives) with transverse dipole moments were used to study the flexoelectric effect in guest-host mixtures with a commercial liquid crystal host <2005CM6354>. 

GH Heilmeier and LA Zanoni, Guest-host interactions in nematic liquid crystals a new electrooptic effect, Appl. Phys. Lett., 13 91-92, 1968. 

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. 

Fig. 14 (a) Chemical structures of the polyphilic dispersion-promoter molecules, (b) Tailor-designed polyphilic molecules promoting CNT dispersion in the nematic host. Pyrene anchoring group (blue), mesogenic CB unit (dark red), flexible hydrocarbon or ethylene oxide spacer (green), and liquid crystal host (light red) [464]. (Reproduced by permission of The Royal Society of Chemistry)... 

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. 

Another approach to obtain luminescent lanthanide-containing liquid crystals is by dissolving a luminescent lanthanide complex in a suitable liquid crystal host matrix. The advantage of this method is that the luminescence and mesomorphic properties can independently be optimised. This allows easier access to nematic lanthanide-containing liquid... 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

H. Qi, B. Kinkead, V.M. Marx, H.R. Zhang, T. Hegmann, Miscibility and alignment eifects of mixed monolayer cyanobiphenyl liquid-crystal-capped gold nanoparticles in nematic cyanobiphenyl liquid crystal hosts. Chem. Phys. Chem. 10, 1211-1218 (2009)... 

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