Cholesteric-nematic phase change

Cholesteric-Nematic Phase Change Effect (CNPC)... 

Figure 3.3 Schematic representation of a cholesteric-nematic phase-change effect (CNPC) LCD. ...

Display devices can also be constructed using the field effect, the cholesteric memory effect and the cholesteric-nematic phase change effect [259, 262]. The recognition of the useful electro-optical properties of liquid crystals has stimulated efforts in synthesis of new mesomorphic materials. Today, more than 6000 compounds are available but an ideal liquid crystal is still elusive. 

There are two types of electro-optic effects which occur with cholesteric materials. The materials which possess negative dielectric anisotropy exhibit the so-called reflective optical storage mode while positive materials undergo field-induced cholesteric-nematic phase changes. 

Emphasis has been placed on nematic materials because of the predominance of the twisted nematic effect in display devices. However, its disadvantages have caused considerable interest in the cholesteric-nematic phase change effect and the dye-phase change device in particular.61>62... 

G. Hauck and H.D. Koswig, Cholesteric-nematic phase change, in Selected Topics in Liquid Crystals Research (edited by H.D. Koswig), Akademie-Verlag, Berlin, pp. 115-139, 1990. 

G. H. Heilmeier and J. E. Goldmacher, Electric-Field-Induced Cholesteric-Nematic Phase Change in Liquid Crystals, J. Chem. Phys., Vol. 51. p. 1258 (1969). 

In the cholesteric-nematic phase change with L/Pq > 1, the wave-vector is given by tt/Pq not tt/L. The experimental rise and decay times are consistent with the theory for the field-induced phase transition. ... 

Cholesteric-nematic phase transition change from negative uniaxiality to positive uniaxiality The electric field is parallel to the helix axis. ... 

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

Cholesteric liquid crystals Historically, the name is derived from cholesterol chiral molecules like the steroids show a certain form of the nematic phase, the cholesteric one. The rigid rods are oriented parallel within virtual layers in one preferred direction (director) the director changes from one single virtual layer to the next continuously, with a certain value creating a helix. The distance between two parallel oriented directors is called the pitch (ca 0.2 pm). 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

The change in chirality obtained by irradiation of 11 was sufficient to provoke the light-induced conversion of cholesteric to nematic phases in liquid crystals of 4/-(pentyloxy)-4-biphenylcarbonitrile doped with 1% w/w... 

In cholesterics, the structure is similar to nematics, but the director rotates in a corkscrewlike fashion along n. Electric-field-induced transitions between the cholesteric and nematic phases are used in the dye phase change display discussed below. 

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