Anisotropic guest-host interaction

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. 

The chirality interaction tensor Wy is responsible for the interaction of the chiral guest, the chirality of which is described by the chirality tensor Cy, with the anisotropic host Wy = ikLkj- Ly covers the anisotropic host properties. W = Tr JT), is different from zero. The gyki are the orientational distribution coefficients of the guest in the molecular ensemble of the guest-host phase. From the two possible representations to avoid non-diagonal elements in (3.19), the representation in the system of principal axes of the order tensor is chosen instead of the principal axes of the chirality interaction tensor Wy. With Saupe s order parameters the HTP is also a sum of three terms... 

Thus far, a propane molecule is treated as a spherical particle. This approximation is essential when the magnitude of the coupling between host and guest is examined. However, propane is actually anisotropic. Here, we examine more realistic models for propane and ethane together with argon, which are appropriately described by three, two and single LJ interaction sites whose parameters are listed in Table 3[25, 26, 27]. The nonspheri-cal propane consists of three interaction sites, two of which are equivalent and denoted by methyl , and the other is denoted by methylene . The nonspherical ethane consists of two interaction sites, both of which are equivalently denoted by methyl . 

Without interactions with potential host molecules and in diluted solutions to avoid excimeric formations, pyrene presents in solution an intense and anisotropic fluorescence, as well as a high fluorescence quantum yield [34-37], Direct evidence of ground-state interactions of pyrene with potential host molecules is provided by the emission spectra. The vibrational structure of the emission spectrum of pyrene is constituted by five fine peaks, named I, I2, h, I4, and I5 (Fig. 13.2) [38]. An increase of the intensity of peak Ii is observed in polar solvents, while I, is solvent insensitive. Thus, the evolution of the ratio of intensities /1//3 gives information on the evolution of the polarity of the environment close to molecular pyrene, and consequently on the encapsulation of this guest in a host molecular or supramolecular object [39]. This sensitivity of pyrene, and of peri-fused polycyclic aromatic hydrocarbon molecules in general, to the polarity of the environment is a photophysic property that is extensively used to study host-guest interactions [40]. 

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