Electro-optic , chiral molecule

This situation changed dramatically in 1996 with the discovery of strong electro-optic (EO) activity in smectics composed of bent-core, bowshaped, or banana-shaped achiral molecules.4 Since then, the banana-phases exhibited by such compounds have been shown to possess a rich supermolecular stereochemistry, with examples of both macroscopic racemates and conglomerates represented. Indeed, the chiral banana phases formed from achiral or racemic compounds represent the first known bulk fluid conglomerates, identified 150 years after the discovery of their organic crystalline counterparts by Pasteur. A brief introduction to LCs as supermolecular self-assemblies, and in particular SmC ferroelectric and SmCA antiferroelectric LCs, followed by a snapshot of the rapidly evolving banana-phase stereochemistry story, is presented here. 

Many other interesting examples of spontaneous reflection symmetry breaking in macroscopic domains, driven by boundary conditions, have been described in LC systems. For example, it is well known that in polymer disperse LCs, where the LC sample is confined in small spherical droplets, chiral director structures are often observed, driven by minimization of surface and bulk elastic free energies.24 We have reported chiral domain structures, and indeed chiral electro-optic behavior, in cylindrical nematic domains surrounded by isotropic liquid (the molecules were achiral).25... 

The situation is more favourable for smaller chiral molecules. Computational advances in quantum chemistry have been to the interpretation of ROA what the development of laser technology and of electro-optics was to its measurement. Over a period of about two decades [31-39] the ab initio computation of ROA spectra has matured from a somewhat haphazard exercise to a reliable tool for determining absolute configurations, and solution conformations. 

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

The molecule is a liquid crystalline polymer with chiral smectic C phase forming parts attached as side chains. The field required to switch the direction of polarization of the polymer is very low (0.3 MVm ). There is a lot of interest in liquid crystalline ferroelectric polymers, because of their possible use for fast-switching electro-optical devices. More information about ferroelectric liquid crystals can be found in references [36,37]. 

The chemists from Orsay have synthesised chiral compound p-decyloxybenzy-lidene-p -amino-2methylbutylcinnamate (DOBAMBC), Fig. 13.3. Indeed, in the temperature range 95-117°C, this substance showed a linear electro-optical effect characteristic of a pyroelectric phase. The effect was observed in thick home-otropically oriented layers. Due to chiral stmeture of DOBAMBC molecules, the SmC phase had a spiral structure with the helical axis perpendicular to the limiting glasses. Fig. 13.4a. Under a microscope the preparation showed a conoscopic cross typical of a uniaxial phase, and, upon application of the in-plane electric field E, ... 

Since a lyotropic lamellar phase with tilted and chiral molecules has the same polar C2 point group symmetry as the thermotropic SmC phase, the important question emerges, whether the lyotropic SmC phase might also show ferroelec-tricity and polar electro-optic switching like its thermotropic counterpart. 

In this thesis Johanna Bruckner reports the discovery of the lyotropic counterpart of the thermotropic SmC phase. By means of polarizing optical microscopy. X-ray diflfraction and electro-optic experiments she firmly establishes aspects of its stmcture and elucidates its fascinating properties, among them a pronounced polar electro-optic effect, analogous to the ferroelectric switching of its thermotropic counterpart. The helical ground state of this new lyotropic phase raises the fundamental question of how chiral interactions are communicated across layers of disordered and achiral solvent molecules which are located between adjacent... 

Most frequently, a planar orientation of CLCs is obtained by the rubbing technique. However, in cells coated with polyamide (without rubbing), a new cholesteric state, called an amorphous cholesteric structure, is formed [82]-[85]. The surface, covered by polyamide, orients the molecules parallel to the surface, but without any preferable direction in the surface plane. Thus, a structure with random orientation of liquid crystal molecules, and a helical axis oriented normal to the surfaces, is obtained (Figure 6.2(e)). All parts of the amorphous cholesteric structure with medium chirality rotate the polarization plane by the same angle. The whole structure can be considered as optically active. In this case, the angle of rotation is independent of the direction of the polarization plane of the incident light. This is the basic property used for electro-optic application of the amorphous cholesteric structure vrith medium chirality. Characteristic features of this effect are ... 

Soon after the initial discovery of ferro-electricity in chiral smectic LCs it was predicted that, if the helix of an SmC phase were suppressed by surface forces in very thin layers between two glass electrodes, then this would pin the molecules in their positions and allow switching between two energetically equivalent polarization directions, thereby giving rise to an electro-optic memory effect [22]. This is the basis of the electro-optic display device called the surface stabilized ferroelectric liquid crystal... 

Lahiri T, Pal Majumder T (2012) The effect of cross-linked chains of polymer network on the memory states of polymer stabilized ferroelectric molecules. Polymer 53 2121-2127 Lee K, Suh SW, Lee SD (1994a) Fast linear electro-optical switching properties of polymer-dispersed ferroelectric liquid crystals. Appl Phys Lett 64 718 Lee K, Suh SW, Lee SD, Kim CY (1994b) Ferroelectric response of polymer-dispersed chiral smectic C liquid crystal composites. J Korean Phys Soc 27(1) 86... 

Maradudin AA, Montroll EW, Weiss GH, Ipatava P (1971) Theory of lattice dynamics in the harmonic approximation. Academic Press, New York March N (1992) Electro Density Theory of atoms and molecules. Academic Press Marcott C, Havel HA, Hedlund B, Overend J, Moscowitz A (1979) A vibrational rotational strength of extraordinary intensity. Azidomethemoglobin A. In Mason SF (cd) Optical activity and chiral discrimination, Reidel, Dordrecht, p 289... 

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