Design of ferroelectric liquid crystals

D. M. Walha. M. a Roe, N. A. dark. R. Shao. K. M. Johnson. M. O. Robinson. J. Y. Un. and D. Doroski. An approach to the design of ferroelectric liquid crystals with large second order electronic nonlinear optical susceptibility. MoL Cryst Liq. CrysL J98 51 (1991). 

L. Dinescu, K.E. Maly, R.P. Lemieux, Design of photonic liquid crystal materials synthesis and evaluation of new chiral thioindigo dopants designed to photomodulate the spontaneous polarization of ferroelectric liquid crystals. J. Mater. Chem. 9, 1679-1686 (1999)... 

Figure 12 The Petschek-Wiefling design for ferroelectric liquid crystals. The incom-patibflity between the A and C blocks can result in microphase separation of the coronal blocks thus aligning the electric dipole residing on the middle, rodflke, B block. The aligned state is depicted in (a) and the unaligned situation, with mixed coronas, in (b). (Adapted from Ref. 29.)...

This volume of Topics in Stereochemistry could not be complete without hearing about ferroelectric liquid crystals, where chirality is the essential element behind the wide interest in this mesogenic state. In Chapter 8, Walba, a pioneering contributor to this area, provides a historical overview of the earlier key developments in this field and leads us to the discovery of the unique banana phases. This discussion is followed by a view of the most recent results, which involve, among others, the directed design of chiral ferroelectric banana phases, which display spontaneous polar symmetry breaking in a smectic liquid crystal. 

Applications of cyclotriveratrylenes and cryptophanes to the design of materials for optoelectronics are still in the area of prospective research. We only mention here works dealing with ferroelectric liquid crystals and three-dimensional charge transfer salts. 

Walba, D.M. Slates, S.C. Thurmes, W.N. Clark, N.A. Handschy, M.A. Supon, F. Design and synthesis of a new ferroelectric liquid crystal family. Liquid crystals containing non-racemic 2-alkoxy-l-propoxy unit., 1. Am. Chcm. Soc. 1986, 108 (17). 5210-5221. 

Walba et a/. synthesized a low molar mass ferroelectric liquid crystal specially designed for second-order nonlinear optics that showed a second harmonic coefficient 1X22) of 0.6 0.3 pmA in the chiral smectic C (SC ) phase. The nonlinear optics-chromophores aligned in the direction of the polarization, perpendicular to the long axis of the molecules were later successfully varied. This can be illustrated as follows ... 

R. Zhang and H. Hua, Design of a polarized head-mounted projection display using ferroelectric liquid-crystal-on-silicon microdisplays, Appl. Opt. 47, 2888 (2008). 

Walba, D. M., Keller, P., Parmar, D. S., Clark, N. A., and Wand, M. D., Design and synthesis of new ferroelectric liquid crystals. 9. An approach to creation of organic polymer thin films with controlled, stable polar orientation of functional groups, J. Am. Chem. Soc., Ill, 8273-8274 (1989). 

The trend to segregation has been used in the design of ferroelectric materials with a small positive or even negative temperature dependence of the layer thickness [136]. Considering this trend to segregation, Tour-nilhac et al. [137,138] have designed poly-philic liquid crystals, which consist of two fluorinated moieties, an ordinary alkyl part and the core. Compound 2 [137] is atypical... 

Finally, doping surface-stabilized ferroelectric liquid crystals with charge-transfer complexes, for example, tetramethyltetra-thiafulvalene/octadecyltetracyano-1,4-qui-nodimethane, has been utilized to improve their bistability [37] Ions from the CT complex form an internal electric field in reverse to the applied pulse. By applying this phenomenon, a ferroelectric liquid crystal cell with perfect bistability and inverted memory characteristics was designed [37]. Ono and Nakanowatari developed a method for the determination of the internal electric field and studied a TCNQ doped ferroelectric liquid crystal [38]. 

In other words, molecular design and the synthesis were carried to express various characteristics for the required purposes that depend on the specific application of the ferroelectric liquid crystal. For example, ferroelectric liquid crystals with large spontaneous polarization, a large tilt angle with small temperature dependence, a specific viscosity, a small rotational viscosity of the molecules along their long axis, a chiral smectic C phase over a wide temperature range, a characteristic anisotropy of the dielectric constant, a suitable phase sequence, a suitable helical pitch, photochemical stability, and so on have been newly synthesized [3-5]. 

Here, I would like to go off on a tangent about the 1st international ferroelectric liquid crystal conference. One of the most important moments for the discovery of antiferroelectrie liquid crystal was the presentation by Kenji Furukawa (Chisso Corp.) on this conference [47]. The designated SmY phase exists at the low-temperature side of the SmC phase of MHPOBC it has an abnormally small dielectric constant compared to the ferroelectric phase, indicating a threshold during the direct current switching by an electric field. Thus, experimental facts pointed to an antiferroelectrie phase rather than a ferroelectric phase as early as 1987. 

Figure 2-2 shows the situation when n is parallel to the smectic layer normal k. This is similar to the SmA phase of calamitic liquid crystals, except that now the layers are polar. This difference is designated by adding the letter P (for polar) to SmA. In this case one can have two distinct situations, the layer polarization P can be either parallel or antiparallel in the subsequent layers corresponding to ferroelectric (SmAP ) or antiferroelectric (SmAP ) subphases. Here and later... 

Since the early designs described in the previous section, there has been httle change in the principles, namely continuous light source with two choppers or one chopper and one ferroelectric liquid crystal (or similar device), or pulsed hght source with or without chopper. However, technically each component of the microscope has sustained large improvement. In this sechon, we briefly outline this technical evoluhon closely following the review article by R. E. Conally and J. A. Piper [30]. 

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Keith, C. Reddy, R. A. Hahn, H. Lang, H. Tschierske, C. The carbosilane unit as a stable building block for liquid crystal design a new class of ferroelectric switching banana-shaped mesogens. Chem.Commun. 2004,18, 1898-1899. 

In the early development of liquid crystals, for the most part, the study of small molecular systems dominated the field because of the close link between molecular design and commercial applications. However, it is only in the last 20 years that materials with unusual, and often hybrid structures have been investigated for their liquid-crystalline behavior. As noted, phasmidic materials, which have molecular structures that are part-disc part-rod, were found to exhibit both columnar and smectic phases. More recently, molecular systems having bent-rod-like structures have been investigated and found to exhibit a wide range of novel phases, many of which were found to be ferroelectric or antiferroelectric (without molecular chirality) due to the reduced symmetry of their mesophase structures. 

By mixing a chiral liquid-crystal compound with its optical antipode, systems possessing arbitrary values of the enantiomeric excess can be designed. If a chiral compound shows smectic-C and smectic- I phases, the racemate, i.e., the 1 1 mixture of the two antipodes, also exhibits these phases but the ferroelectric properties of the smectic-C phase and the electroclinic effect in the smectic- phase are lost. This offers the unique possibility to study a given system with and without ferroelectridty or with a variable markedness of its ferroelectric properties. 

Can liquids in which the constituents are dipoles be ferroelectric For instance, if we could make a colloidal solution of small particles of the ferroelectric BaTi03, would this liquid be ferroelectric The answer is no, it would not. It is true that such a liquid would have a very high value of dielectric susceptibility and we might call it superparaelec-tric in analogy with the designation often used for a colloidal solution of ferromagnetic particles, which likewise does not show any collective behavior. An isotropic liquid cannot have polarization in any direction, because every possible rotation is a symmetry operation and this of course is independent of whether the liquid lacks a center of inversion, is chiral, or not. Hence we have at least to diminish the symmetry and go to anisotropic liquids, that is, to liquid crystals, in order to examine an eventual appearance of pyroelectricity or ferroelectricity. To... 

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