Switching of Liquid Crystalline Phases

The cholesteric screw sense was measured by the Grandjean-Cano method, which showed that M-cis-17a and P-trans-17b give cholesteric phases with opposite handedness. Observations of the switching behavior of 17 in a large number of LC materials, 

]pps is related to the anisotropy factor gx ([e.e.]pps = gl/2), both Pm and gx must be sufficiently large to enable detection of a cholesteric phase.1941 

2 (a) Photochromism, Molecules and Systems in Studies in Organic Chemistry 40, Durr, FI. Bouas, H. Laurent, H. Eds, Elsevier Amsterdam, 1990 (b) Organic Photochromes El tsov, A. V. Ed. Plenum Press New York, 1990  

4 (a) Photoreactive Materials for Ultrahigh Density Optical Memory, Irie, M. Ed. Elsevier Amsterdam, 1994 (b) Willner, I. Rubin, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 367 (c) Gomez-Lopez, M. Stoddart,J. F. Bull. Chim. Soc. Belg. 1997, 306, 493 (d) Balzani, V. Seandola, E. Supramolecular Photochemistry Ellis Horwood New York, 3993. 

Stereochemistry of Organic Compounds, Wiley, New York, 1994, see also Cahn, R.S. Ingold, C. K. Prelog, V. Angew. Chem., Int. Ed. Engl. 1966, 5, 385. 

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S. Abraham, V.A. Mallia, K.V. Ratheesh, N. Tamaoki, S. Das, Reversible thermal and photochemical switching of liquid crystalline phases and luminescence in diphenylbutadiene-based mesogenic dimers. J. Am. Chem. Soc. 128, 7692-7698 (2006)... 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

A conglomerate in real liquid crystalline phases was first observed in the smectic phase of a rod-shaped mesogen with two stereogenic centers in its tail [42], We used a racemic mixture which was supposed not to electrically switch. Evidence for conglomerate formation was provided by clear electro-optic switching and texture observation under a polarizing microscope domains with stripes, which themselves display fine stripes. These stripes are tilted in two different directions with respect to the primary stripes. This is a still very rare example now that fluid soft matter is known to resolve spontaneously into a three-dimensional conglomerate. 

In a different approach, Bruce and coworkers 210) described the preparation of liquid-crystalline derivatives of a N,C,N-Pt (II) Imninophore (Fig. 18). Interestingly, they foimd that emission in the liquid-crystal phase is characteristic of the monomeric complex, while excimer-like emission normally characterizes nonliquid-crystalline analogues. They showed that the emission of pure films is responsive to both method of preparation and tribological stimulation so that it is possible to switch in a controllable manner between monomer- and excimer-like states. 

D. Pijper, M.G.M. Jongejan, A. Meetsma, B.L. Feringa, Light-controlled supramolecular helicity of a liquid crystalline phase using a helical polymer functionalized with a single chiroptical molecular switch. J. Am. Chem. Soc. 130, 4541 552 (2008)... 

Based on the netpoints, which determine the permanent shape, there are three categories of SMPs. The permanent shape can be fixed either by chemical crosslinks (category A) or by hard domains associated to a Ti, (category B) or to a 7 (category C) as physical netpoints. The reversible netpoints related to the temporary shape can be photoreversible chemical links (e.g., formed by dimerization of cinnamic acid), or switching domains associated to a 7 or 7, or a liquid crystalline phase transition. 

Keith, C. Reddy, R. A. Baumeister, U. Hahn, H. Lang, H. Tschierske, C. Continuous transition from antiferroelectric to ferroelectric switching liquid crystalline phases in two homologous series of bent-core mesogenic dimers based on carbosilane spacer units. J. Mater. Chem. 2006,16, 3444-3447. 

Generally, the photoisomerization of azobenzenes is both thermally and optically reversible cis-io-trans isomerization can be driven with irradiation at visible wavelengths or by thermal relaxation. In a bulk LC material, this can result in an isotropic-to-liquid crystalline phase transition. Thus, light can be applied at different wavelengths to control the switching between the ordered LC and disordered isotropic states in both directions, as shown in Figure 7.4. This is very important for photonic applications of PLCP materials. 

One core chiral system that shows dramatic amplification of its chiral structure is the substituted helicenes of Katz and coworkers [83]. In essence, this research cuts the helix into a number of six-helicene subunits that self-assemble (Figure 10). Only when these subunits, which look like lock washers, are prepared in optically pure form the material associates into supramolecular helical columns [84]. The assemblies have been synthesized with different amounts of substitution around the exterior. Depending on the helicenes substitution, the material exhibits hexagonally ordered soft-crystalline [84] or liquid-crystalline phases [85]. The liquid-crystalline versions of these molecules switch when electric fields are applied to neat and solution-phase samples and have been characterized as a dielectric response [85-87]. Upon association, these materials have enormous changes in their CD intensities and optical rotations [74]. In addition, this supramolecular chirality also significantly enhances the second-order nonlinear optical behavior of these materials in Langmuir-Blodgett films [88]. 

Keith C, Reddy RA, Tschitaske C (2005) The first example of a liquid crystalline side-chain polymer with bent-cme mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem Commun 7 871-873 Keith C, Dantlgraber G, Amaranatha Reddy R, Baumeister U, Tschierske C (2007) Ferroelectric and antiferroelectric smectic and columnar liquid crystalline phases framed by silylated and non-silylated molecules with ilurainated bent Cotes. Chtan Mater 19 694-710... 

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