Host-guest effect

A third phenomenon depending solely upon dielectric forces is the guest-host or electronic color-switching interaction in which guest pleochroic dyes are incorporated within nematic host materials. The dyes have different absorption coefficients parallel and perpendicular to their optical axes. As illustrated in Fig. 6, the dye molecules can be oriented by the liquid crystal. With zero field, the liquid crystal is in the uniform parallel orientation and the dye mole- 

This effect results from the Prederiks transition, but it is observed in liquid crystals doped with dyes and is, therefore, worthy of an independent treatment. The liquid crystalline matrix (the host) is subjected to the influence of a field the purpose of the dye (the guest) is to enable the effect to be seen. 

Handbook of Liquid Crystals D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess, V. Vill Copyright WILEY-VCH Verlag GmbH. 1998 

The ordering of probe molecules in liquid crystals was in fact known much earlier than 1968. It is widely used to determine various parameters of the solute and solvent liquid crystal using nuclear magnetic resonance (NMR), electron spin resonance (ESR), ultraviolet (UV), visible, and other spectroscopie teehniques. After the pioneering work of Saupe and Englert in 1963 [8], the NMR spectroscopy of molecules oriented in liquid crystals became very important in structural chemistry, as it provides the only direct method for precise determination of the molecular geometries in liquid phase. In addition to structural and confor- 

The performance of a guest-host LCD is greatly dependent on the dye parameters (such as UV stability, solubility, order parameter, absorption, etc.), the host liquid crystal properties (such as viscosity, dielectric anisotropy, birefringence, order parameter, temperature range, stability, etc.), and the compatibility of the dye and the host [12-17, 42-58]. 

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HJ Coles, GA Lester, and H Owen, Fluorescent dye guest-host effects in advanced ferroelectric liquid crystals, Liq. Cryst., 14 1039-1045, 1993. 

One of the manifestations of orientational effect in LC polymers is presented by a so called guest-host effect, which is well-known for low-molecular liquid crystals. 

In the case of LC polymers, the polymeric matrix performs as a host, while the guest is a dye, whose molecules are elongated in shape, and the absorption oscillator is parallel (or perpendicular) to the big axis of the molecule 65,163-165>. The experiments investigating guest-host effect in nematic polymers with dichroic dyes covalently attached to the polymer 163) (type I) and mechanically incorporated65) (type II) reveal the possibility to obtain regulated color indicators (see page 60). 

This paper deals with the study of the mechanism of orientation process, the guest-host effect and the elec-trohydrodynamic (EHD) instabilities in LC comb-like polymers with azomethine and cyandiphenyl mesogenic groups. 

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