Absorption guest-host effect

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). 

The electro-optical measurements were carried out using an electro-optical cell described by Talroze, et al. which was mounted on a heating stage of a polarizing microscope, MIN-8. A photo-element was used in conjunction with a recorder to monitor the transmitted light intensity. The film thickness was about 12 pm, electric field intensity was 10 V/cm. The pleochroic dye for studying the guest host effect had an absorption band at 505 nm. 

Most of the electrooptic effects are based on light scattering or on light absorption by polarizers or by dissolved dyes. Electrooptic effects which belong to the second group are tunable birefringence, the twisted nematic effect and the guest-host effect. 

As the previous sections have shown, nematic polymer liquid crystals may be oriented by surface forces and in electric fields. It has been shown recently that such field-induced changes in orientation may also be used to orient pleochroic dyes through the guest-host effect. In such an effect either guest dyes dissolved in a nematic polymer " host or side-chain dye moieties in a nematic copolymer system (where A is a nematic moiety and B is a dye in Fig. 2b) undergo a cooperative realignment as the nematic director responds to the applied field. Since the pleochroic dye has its absorption transition... 

It has been shown that moderate laser powers may be used to induce textural changes in both nematic and smectic polymer systems. However, in order to achieve laser writing on an acceptable timescale with low pulse energies, the polymer should be doped with a dye, as in the guest-host effect, to increase the absorption at the wavelength of the writing laser. It has been demonstrated that laser writing may be... 

Absorption anisotropy (dichroism) in liquid crystals takes place either due to the presence of the short wavelength (< 400 nm) oscillator, usually coinciding with the direction of its long molecular axis, or due to the impurities, such as dichroic dyes ( guests ) dissolved in liquid crystals ( hosts ). We will consider the second case as the most important for practical applications ( guest-host effect [18]). 

FIGURE 4.26. The guest-host effect in a nematic Uquid crystal. The orientation of dye molecules and absorption spectra in polarized light (a) no field present (b) the field exceeds the threshold level (P is the polarizer). 

Coles et al. were the first to investigate the dye guest host effect in a FLC polymer (60). The high-polarization polyacrylate 14 [31] (see above), containing 4% of a blue azo dye (M 483 from Mitsui Toatsu Chemical, Inc.X gave the absorption spectra displayed in Figure 41. From the absorptioo parallel and perpendicular to the direction of molecular orientation (diiector). an order parameter of 0 J7,... 

Incorporation of a pleiochroic dye and elimination of the analyzer results in voltage-controlled optical absorption, the guest-host effect. ... 

Kaino and co-workers [255] have investigated the effect of polymer polydis-persity on electro-optic materials properties. No dependence on polydispersity was observed for guest host materials but for Disperse Red chromophores covalently attached to monodisperse polystyrene weaker absorption tails were observed. This result suggests that chromophore-chromophore interactions are modified by the polymer host. 

Ag° atoms isolated in the cubo-octahedral site of rare gas solids. The observation of multiple structure on the 2P 2S absorption and large red spectral shifts for the 2P - 2S emission of site I entrapped Ag° atoms, indicates that the guest-host interactions are markedly different for the 2S and 2P states and can be explained in terms of site I relaxation effects, using a vibronic coupling model similar to that described in detail for Ag° atom rare gas cage complexes (5). 

When comparing the laser threshold energy values it is apparent that the most promising candidates for low-threshold operation are guest-host systems. Here, the self-absorption of the material matrix is effectively separated from the spectral region of the emission. The doped systems containing DCM2 or the stilbene... 

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. 

Barto, R.R., Jr., P.V. Bedworth, C.W. Frank, S. Ermer, and R.E. Taylor. 2005. Near-infrared optical-absorption behavior in high-beta nonlinear optical chromophore-polymer guest-host materials. II. Dye spacer length effects in an amorphous polycarbonate copolymer host. J Chem Phys 122 234907 1-14. 

A further effect connected to the absorption of light was reported very recently by Janossy tt al. They found in a commercial nematic guest-host mixture optical reorientation at anomalously low power levels. E.g. in a homeotropic sample at normal incidence the optical Freedericksz threshold was around 1.5 mW in... 

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