Absorbance guest-host effect

Fig. 11.23 Guest-host effect. Field-off (a) and field-rai (b) cell configurations and absorbance spectra for a nematic liquid crystal doped with a dye having elraigaled molecules shown by small black spherocylindras...

Another way to achieve uniform illumination is to lower the optical density of the sample by increasing the effective molar volume. Solid solution of an absorbing guest in a transparent host in either a normal crystal or a molecular inclusion compound can achieve this end so can using a pure crystal of a much larger molecule which includes the chromophore of interest [27]. While it is harder to reach high dilution with the latter approach, it has the virtue that the initial compound is a pure crystal and thus better suited for definitive X-ray investigation. 

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. 

Further, two thick rigid polarizing films spoil device flexibility to be unnecessary. Therefore, guest-host twisted liquid crystal devices with dichroic dyes [7] were fabricated to absorb the all-polarization-angle incident light. In this device, a nematic liquid crystal with low birefringence is introduced, and optical rotation effect is suppressed in the twisted liquid crystal layer. The contrast ratio of the display is inferior, but the guest-host nematic liquid crystal of twist orientation is suitable for simple text display. 

In particular, a spectroscopic analysis of films presenting the SPS/chloro-form 8 clathrate phase [120] has shown that when chloroform is absorbed in the crystalline phase, a significant perturbation of its vibrational spectrum takes place. This effect mainly involves the 1219 cm ( -c-a) peak, which clearly shows a fine structure in the form of an unresolved component at a lower wavenumber (1210cm ), and is not present in the spectrum of the isolated molecule (vapor phase) and in the case of chloroform sorbed in the amorphous phase of SPS [120], Hence, it is likely to be related to host-guest molecular interactions. 

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