Nematic liquid-crystalline guest-host

Nematic Liquid-Crystalline Guest-Host System in a Switching Experiment... 

Figure 2-19. Chemical stniciures and transition temperatures of the investigated nematic liquid-crystalline guest-host system.

Figure 2-20. Step-scan FTIR spectra of the investigated nematic liquid-crystalline guest host solution before (0.0 ms) and after (19.5 ms) reorientation in the electric field.

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. 

The early studies on electric field poling were performed mainly on the guest-host systems, for instance, DANS dissolved in thermotropic nematic liquid-crystalline polymers [12]. In this guest-host system polar order decays rapidly. The Xzz] value was about 1 pm/V. Therefore, the later investigations on guest-host systems were focused on both increasing the temporal stability as well as the x value. 

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. 

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

An alternative approach to obtain luminescent liquid crystals is to use the host-guest concept a luminescent lanthanide complex is dissolved in a hquid-crystalline matrix. This approach allows one to optimize the luminescence and mesomorphic properties of the liquid-crystal mixture independently. Boyaval et al. investigated the luminescence of lanthanide complexes in a cholesteric hquid crystal mixture (Boyaval et al., 1999, 2001). Birmemans and Moors (2002) showed that a nematic liquid crystal can be an interesting host matrix to study the spectroscopic properties of luminescent lanthanide complexes. These authors doped the /3-diketonate complex [Eu(tta)3(phen)] into the nematic liquid crystals N-(4-methoxybenzyhdene)-4-butylaniline (MBBA) and 4-n-pentyl-4 -cyanobiphet5rl (5CB) and observed narrow-band red photoluminescence with a well-resolved crystal field sphttmg. Later on, these authors extended their studies to lanthanide j8-diketonate complexes emitting in the near-infrared (Ln = Nd, Er, Yb) (Van Deun et al., 2003). 

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