Liquid crystalline host

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Photochemical modulation of the helical screw sense and pitch of a cholesteric phase was achieved with the combination of a nematic liquid crystalline host and an optically active photoresponsive guest as illustrated in Scheme 25.[92] Doping of 4 -(pentyloxy)-4-biphenylcarbonitrile 41 with P-trans-17b (1 wt%) converts the nematic phase into a cholesteric phase. 

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. 

Fig. 6 (a) Host and guest molecules used in the glycoluril-based materials, (b) Computer-modeled representation of the liquid-crystalline host-guest complex between porphyrin %% and four molecules of Host 21. (c) Modeled structure of the arrangement of dimers in the lamellar plastic crystal formed by 23, (d) Modeled structures of the 1 1 host-guest complexes between 23 and MDB (top) and between 23 and DBA (bottom). (View this art in color at www.dekker.com.)... 

FIGURE 5.30 Top Unidirectional rotation of molecular motor 41 in a liquid crystalline host, and associated HTPs. Bottom Colors of 41 doped LC phase (6.16 wt% in E7) in time, starting from pure (P,P)-trans-4 upon irradiation with >280 mn light at room temperature, as taken from actual photographs of the sample. The colors shown from left to right correspond to 0,10, 20, 30,40, and 80 s of irradiation time, respectively. Reprinted with permission from Reference 63. Copyright 2002 National Academy of Sciences. 

The introduction of dye molecules into the liquid crystalline host does not change the majority of the properties of the host, provided that not too much dye is introduced (not more than 1-2%). The N I transition temperature of the liquid crystal, the viscous and elastic properties, the electrical conductivity (provided the dye is not ionic and does not contain ionic impurities), the dielectric permittivities, (provided the dye molecule does not have a large dipole moment), and even the refractive indices all remain the same. The only significant change in the properties of the crystal is the appearance of absorption bands in the visible region of the spectrum and a slight increase in viscosity [151]. 

Topological defects are often needed to equilibrate an ordered system. There are two different possibilities here. First, the defects can occur to relieve intrinsic ( bulk ) frustrations (for instance, between the twist and layered structures in TGB phases). Second, the defects can occur simply because the system is bounded or because there are foreign inclusions, such as colloidal particles or droplets in a liquid crystalline host. 

Another example of a side-chain supra-raolecular polymer is prepared from poly(4-vinylpyridine) and H-bonding side chains [32-35]. In this case, mesogenic side-chain groups are directly attached to the polymer backbone and liquid-crystalline polymers such as 19 are formed by the noncovalent interaction [33]. This molecular design has been used to incorporate functional molecules into liquid-crystalline, host-guest polymeric systems [32]. 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

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