Mesogens, bent-core

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.

Stimulated by the Tokyo Tech results, several groups began studying similar bent-core mesogens. At the European Conference on Liquid Crystals, held in Zakopane, Poland, in March of 1997, Heppke et al. confirmed the Tokyo Tech results,46 and Weissflog et al. did as well, with the important exception that the Halle group found antiferroelectric behavior in the B2 phase.47... 

Figure 8.21 Symbolic bow-and-arrow-based representations of bent-core mesogen structures used in this discussion are given.

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

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.

Also mesogens with more complex shapes, such as, for example, those with bent aromatic cores (bent-core mesogens [47]), star mesogens [48], or cone-like... 

Bent-Core Mesogens with Perfluorinated Segments... 

Fig. 66 (a) Examples of bent-core mesogens with semiperfluorinated terminal chains [348-351] and (b) models of the organization of the molecules in polar smectic phases... 

Special effects on mesogenic self-assembly have also been observed for highly core fluorinated bent-core mesogens [420],... 

Polar and Apolar Columnar Phases Made of Bent-Core Mesogens... 

Keywords Liquid crystals Bent-core mesogens Polar propeties X-ray structure studies... 

Shreenivasa Murthy HN, Bodyagin M, Diele S, Baumeister U, Pelzl G, Weissflog W (2006) Reentrant SmCPA phases unusual polymorphism variant SmA-SmCsPA-Colob-SmCsPA observed in new bent-core mesogens. J Mater Chem 16 1634-1643... 

Ortega J, de la Fuente MR, Etxebarria J, Folcia CL, Diez S, Gallastegui JA, Gimeno N, Ros MB, Perez-Jubindo MA (2004) Electric-field-induced B1-B2 transition in bent-core mesogens. Phys Rev E 69 011703... 

Zhang Y, O Callaghan MJ, Baumeister U, Tschierske C (2008) Bent-core mesogens with branched carbosilane termini flipping suprastructural chirality without reversing polarity. Angew Chem Int Ed 47 6892-6896... 

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... 

Spontaneous resolved two chiral domains are formed in equal probability. In other words, enantiomeric excess (ee) is zero. We now ask whether ee can be controlled or not. The answer is yes. Several methods used in bent-shaped mesogenic phases will be introduced. The direct method is of course an addition of chiral dopant. Actually this has been shown to be a viable method [6, 61]. Use of chiral surface is also effective [62], By using polyimide with chiral side chains at both substrate surfaces, imbalance of two chiral domains (10% ee) has been achieved. Another method using macroscopic helical structure was demonstrated by Jakli et al. [63]. They used a nonchiral polymer network, which was formed in the N phase. After the polymerization, N compounds were washed out, then bent-core mesogens were introduced. Because of the polymer helical fibers, bent-core mesogen shows a chiral domain. 

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... 

Choi SW, Kang S, Takanishi Y, Ishikawa K, Watanabe J, Takezoe H (2006) Intrinsic chirality in a bent-core mesogen induced by extrinsic chiral structures. Angew Chem Int Ed 45 6503-6506... 

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