Banana mesogen

This interpretation of the properties exhibited by the major EO-active phases of NOBOW and the rest of the classic banana mesogens posits the existence of four supermolecular diastereomers, at least two of which, the antiferroelectric phases SmCsPA and ShiCaPa, must exist as separate wells on the... 

Shiromo K, Sahade DA, Oda T, Nihira T, Takanishi Y, Ishikawa K, Takezoe H (2005) Finite enantiomeric excess nucleated in an achiral banana mesogen by chiral alignment surfaces. Angew Chem Int Ed 44 1948-1951... 

BIAXIAL NEMATIC PHASES IN LIQUID CRYSTALS CONSISTING OF BANANA MESOGENS... 

Another class of mesogen, which has proven to be a successful candidate for the formation of a biaxial nematic phase, is made up of the so-called bent-core, or banana mesogens. In this kind of mesogen, two rod-like molecules are chemically linked together via a central unit. The molecular structure of such a banana mesogen is schematically given in Figure 5-19. 

Using different experimental methods, two research groups recently independently confirmed phase biaxiality in liquid crystals consisting of banana mesogens. The first group conducted deuterium NMR experiments to unambiguously verify the... 

Figure 8.17 Structure and phase sequence of first banana-phase mesogen, reported by Vorlander in 1929, is given. Liquid crystal phase exhibited by this material (actually Vorlander s original sample) was shown by Pelzl et al.36a to have B6 stmeture, illustrated on right, in 2001. Achiral B6 phase does not switch in response to applied fields in way that can be said to be either ferroelectric or antiferroelectric.

Figure 8.20 Structure and phase sequence of prototypical bent-core mesogen NOBOW (8) are given, along with space-filling model showing one of many conformational minima obtained using MOPAC with AMI force field. With observation by Tokyo Tech group of polar EO switching for B2 smectic phases formed by mesogens of this type, banana LC field was bom. Achiral, polar C2v layer structure, with formation of macroscopic spontaneous helix in polarization field (and concomitant chiral symmetry breaking), was proposed to account for observed EO behavior.

Figure 8.29 Illustration showing how all-anticlinic bilayer smectic should be ferroelectric is given. In case of covalent dimers (bent-core mesogens), equilibrium tilt of director combined with anti clinic layer interfaces in bow plane provides SmCsPF ferroelectric banana structure.

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. 

Mesogen constituted of bent or so-called banana-shaped molecules in which two mesogenic groups are linked through a semi-rigid group in such a way as not to be co-linear. 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

As briefly mentioned in the Introduction, the extensive flow of chirality-related topics has been initiated by the discovery of polar switching in bent-core mesogens [11, 45]. Among the first banana smectic phases B1-B8, at least the B2 [10], B4 [14], and B7 [15] phases can be chiral. The chirality in the B2 phase was... 

Xu JD, Dong RY, Domenici V, Fodor-Csorba K, Veracini CA (2006) C-13 and H-2 NMR study of structure and dynamics in banana B-2 phase of a bent-core mesogen. J Phys Chem B 110 9434-9441... 

Spontaneous formation of chiral LC phases from achiral mesogens—an equivalent to spontaneous resolution of achiral compounds in crystals but in a fluid phase—is a relatively recent phenomenon [9,130-132]. The first compounds discovered with this extraordinary property were the bent-core or banana-shaped mesogens [133,134],... 

Closing this section, a new metathesis-based method for the preparation of strictly alternating copolymers should be mentioned. Alternating diene metathesis polycondensation (ALTMET) was used to prepare a MCLCP consisting of an alternating architecture of a calamitic (linear rod-like core) and a banana-shaped (bend-shape core) mesogen (Fig. 24). 

Figure 3.1 represents the characteristic types of mesogenic molecules. Among them are rods, laths, discs, helices which are more popular for physical investigations and technological applications and also main-chain and side-chain polymers. We may add to this list banana- or bent-shape molecules and dendrimers [4] that recently become very popular. 

Bent-core mesogens are a very special type of mesogen and possess quite distinct properties from the known rod-shaped mesogens. Compared to a rod-shaped LC molecule including one rigid aromatic core and two flexible tails, a bent-core molecule consists of one central bent unit (BU), two rod-like aromatic wings (RWs), and two flexible tails (Fig. 6.2). Hence, bent-core molecules are also called banana or bow-shaped molecules due to their distinct bent shape (Fig. 6.1c). It is known that rod-shaped molecules can rotate freely around their long molecular axes. In contrast. 

Wanga, K. J li, A. Li, H. Yang, Y. Wen, J. Synthesis and mesomorphic properties of resorcyl di[4-(4-alkoxy-2,3-difluorophenyl)ethynyl]benzoate a novel achiral anhferro-electric banana-shaped mesogen. Liq. Cryst. 2001, 28,1705-1708. 

Krishnan, S. A. R. Weissflog, W. PeM, G. Diele, S. Kresse, H. Vakhovskaya, Z. Friedemann, R. DPT and MD studies on the influence of the orientation of ester linkage groups in banana-shaped mesogens. Phys. Chem. Chem. Phys. 2006, 8, 1170-1177. 

Nadasi, H. Weissflog, W. Eremin, A. Pelzl, G. Diele, S. Das, S. Grande, S. Ferroelectric and antiferroelectric banana phases of new fluorinated five-ring bent-core mesogens. J. Mater. Chem. 2002,12, 1316-1324. 

Murthy, H. N. S. Sadashiva, B. K. Banana-shaped mesogens effect of lateral substituents on seven-ring esters containing a biphenyl moiety. lAq. Cryst. 2002, 29, 1223-1234. 

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