Irradiation smectic phase

An even more dramatic example of the potential lack of selectivity afforded to the Norrish II reactions of ketones by supposedly very ordered systems than that described in the 76 systems is provided by neat samples of the mesomorphic alkanophenones (81) [278]. These molecules are capable of existing in nematic and smectic B mesophases (see Figure 16) as shown in Scheme 42. The instability of the monotropic smectic B phase of 81a and smectic B phase of 81b did not allow their photoreactions to be examined these smectic phases became solids soon after the initiation of irradiation. 

Thin films of the photoreactive liquid crystals containing 0.5 wt % of initiator (10-biphenyl-4-yl-2-isopropyl-9-oxo-9H-thioxanthen- 10-ium hexafluor-phosphate) were polymerized in the smectic phases by irradiation of blue semiconductor laser (X = 405 nm). Polymerization of the oxetane moiety proceeded via cationic intermediates. The carrier transport characteristics of the polymerized thin films were studied by the time-of-flight technique. 

Tamaoki and coworkers reported some chiral azobenzenes and diphenylbutadiene-based LC dimers 13-15 exhibiting light-induced phase transition behavior starting from smectic phases [26, 51-55]. For example, dimers 15, which were synthesized by connecting a cholesteryl group with 1,4-diphenylbutadiene unit, were found to experience isothermal phase transition from SmA to N and further to the isotropic state upon UV light irradiation at 366 nm [26]. 

The phase transitions starting from the smectic phase were also observed in several doped systems [57-60]. The smectic LC host 8CB, doped with chiral molecules and azobenzene compounds, showed the phase transition from smectic to cholesteric due to the photo-induced isomerization of azobenzene molecules, and the prolonged irradiation drove the phase transition further to the isotropic phase. Matsui et al. also reported a chiral smectic C (SmC )-cholesteric (N ) phase sequence in azo-dye-doped ferroelectric LCs [57]. 

In some cases, it has been demonstrated that the presence of the less-ordered phase in a two component system can magnify the apparent reactivity of the solute. For example, the increased yield (relative to that from irradiation in a model isotropic solvent) of dimerization products from irradiation of ethyl 4-methoxycinnamate in a smectic liquid crystal was attributed to reaction occurring mainly in a solute-enriched isotropic component of a biphasic system, and not to anisotropic ordering of the reactants in the smectic phase as might have been predicted [356], Samori and co-workers have proposed that the (bimolecular) quaternization rearrangement reaction of allyl-p-dimethyl-aminobenzenesulphonate in smectic phases is assisted by phase separation phenomena, in the sense that a solute-rich isotropic phase acts as a reservoir for the reactant, which must diffuse into the smectic phase in order to react [357]. This has a clear analogy in phase transfer catalysis. 

Fig. 6.23 The thermo-optic effect of smectic-cholesteric LC mixture. The transmission variations are caused by mixing of cholesteric LC. By the laser beam irradiation on to a portion of the cell, the light scattering is observed in the smectic phase, and it rranains in the honeycomb-H focal conic texture. It can be applied to the display of static figures. Sm smectic phase, Ch cholesteric phase. Is isotropic phase...

Fig. 6.25 The electro-thermo-optic effect of smectic-cholesteric mixture COB CN = 90 10. The light transmission is greatly decreased by the electric field application [91]. The light scattering centers are created in the smectic phase by the irradiation on to a part of the LC cell, and they are erased by the electric field application and become clear. This characteristic can be applied to a dynamic display. M mesophase, I isotropic phase...

However, the monotropic nematic phase of 81a was sufficiently stable to allow its Norrish II reactions to be examined. Similarly, Norrish II product ratios from irradiation of the smectic B phases of 81c and 81d were easily measured. Although the packing of molecules in the solid phases of the 81 homologues is unknown, inferential evidence supports their being layered also. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

In summary, chiral smectic-C phases lack a center of symmetry. Hence they can be used as materials for second-order nonlinear optics [120-124], and possess piezoelectric and pyroelectric properties. Pyroelectric measurements have been performed on LC polymers [125] as well as on LCEs [126-128]. Irradiation of an FLCE sample with light usually leads to a temperature increase resulting in a pyroelectric signal [129]. More interesting are systems in which dye molecules like azobenzenes lead to a shift of the phase transition temperature upon isomerization [19]. 

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