Liquid crystal bent-core

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

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

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... 

Benidipine, 5 130, molecular formula and structure, 5 125t Bentazon, 13 323 Bent-core liquid crystals, 15 98 Bentonite, 6 664, 686, 696 9 11, 17. 

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

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. 

In the following sections we shall focus on the structure and properties of the two-dimensional phases formed by the bent-core liquid crystals. In Sect. 2 we describe the structure studies by the X-ray diffraction (XRD) method, optical studies, and the response of different structures to the external electric field. In Sect. 3 we give theoretical models of the director and layer structure in 2D modulated phases and discuss how to reconstruct electron density maps from XRD data. 

So far we have discussed 2D density modulated phases that are formed by deformation or breaking of the layers. However, there are also 2D phases with more subtle electron density modulations. In some cases additional peaks observed in the XRD pattern (Fig. 10) are related to a double layer periodicity in the structure. As double layer periodicity was observed in the bent-core liquid crystals formed by the asymmetric as well as symmetric molecules [22-25] it should be assumed that the mechanism leading to bilayers must be different from that of the pairing of longitudinal dipole moments of molecules from the neighboring layers, which is valid for smectic antiphases made by asymmetric rod-like molecules. 

The experimental observations could be consistently explained if the general tilt structure (SmCo) inside the layers is assumed. For most bent-core smectics the polar vector is perpendicular to the tilt plane, defined by the layer normal and averaged long axis direction, just as polarization in the ferroelectric rod-like liquid crystalline systems. However, since in the bent-core liquid crystals the polar order is decoupled from the tilt order, the polar director can in general have any direction in space thus it can also have a non-zero component along the layer normal. This can be achieved by a combination of tilting (rotation around the polar director) and leaning (rotation around the direction perpendicular to the polar director) of... 

The layer and director structure in the bent-core liquid crystal phases can be studied by the Landau-de Gennes type model [32, 34, 35], The layer and director structure is such that the free energy (F = J/dV) has the minimum value. The free energy density (f) is written in terms of ... 

The free energy density terms introduced so far are all used in the description of the smectic phases made by rod-like molecules, the electrostatic term (6) being characteristic for the ferroelectric liquid crystals made of chiral rod-like molecules. To describe phases made by bent-core molecules one has to add symmetry allowed terms which include the divergence of the polar director (polarization splay) and coupling of the polar director to the nematic director and the smectic layer normal ... 

Bent-core liquid crystals are especially interesting materials for basic research as in these systems the polar and tilt order are decoupled and polarization splay seems to be an inherent property of the system. Both effects lead to a variety of structures with unusual properties, e.g., formation of the 2D density modulated phases built of the smectic layers fragments. We have presented the current knowledge... 

Redy RA, Tschierske C (2006) Bent-core liquid crystals polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems. J Mater Chem 16 907-961... 

Szydlowska J, Mieczkowski J, Matraszek J, Bmce DW, Gorecka E, Pociecha D, Guillon D (2003) Bent-core liquid crystals forming two- and three-dimensional modulated structures. Phys Rev E 67 031702-1-031702-5... 

Shimbo Y, Gorecka E, Pociecha D, Araoka F, Goto M, Takanishi Y, Ishikawa K, Mieczkowski J, Gomola K, Takezoe H (2006) Electric-field-induced polar biaxial order in a nontilted smectic phase of an asymmetric bent-core liquid crystal. Phys Rev Lett 97 113901... 

Gimeno N, Barbera J, Serrano JL, Ros MB, de la Fuente MR, Aloso I, Folcia CL (2009) Terminal chains as a tool to modulate the properties of bent-core liquid crystals. Chem Mater 21 4620-4630... 

Pociecha D, Vaupotic N, Gorecka E, Mieczkowski J, Gomola K (2008) 2-D density-modulated structures in asymmetric bent-core liquid crystals. J Mater Chem 18 881-885... 

Takezoe H, Takanishi Y (2006) Bent-core liquid crystals their mysterious and attractive world. Jpn J Appl Phys 45 597-625... 

Bailey C, Jakli A (2007) Role of molecular shape on bent-core liquid crystal structures. Phys Rev Lett 99 207801... 

Vaupotic N, Copic M, Gorecka E, Pociecha D (2007) Modulated structures in bent-core liquid crystals two faces of one phase. Phys Rev Lett 98 247802-1-247802-4... 

Earl DJ, Osipov MA, Takezoe H, Takanishi Y, Wilson MR (2005) Induced and spontaneous deracemization in bent-core liquid crystal phases and in other phases doped with bent-core molecules. Phys Rev E 71 021706-1-11... 

Sathyanarayana P, Mathew M, Sastry VSS, Kundu B, Le KV, Takezoe H, Dhara S (2010) Splay bend elasticity of a bent-core nematic liquid crystal. Phys Rev E (Rapid) 81 010702(R)-... 

Taushanoff S, Le KV, Williams J, Twieg RJ, Sadashiva BK, Takezoe H, Jakli A (2010) Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material. J Mater Chem 20 5893-5898... 

Keith C, Dantlgraber G, Reddy RA, Baumeister U, Prehm M, Hahn H, Lang H, Tschierske C (2007) The influence of shape and size of silyl units on the properties of bent-core liquid crystals - from dimmers via oligomers and dendrimers to polymers. J Mater Chem 17 3796-3805... 

Takanishi Y, Shin GJ, Jung JC, Choi S-W, Ishikawa K, Watanabe J, Takezoe H, Toledano P (2005) Observation of very large chiral domains in a liquid crystal phase formed by mixtures of achiral bent-core and rod molecules. J Mater Chem 15 4020-4024... 

Araoka F, Sugiyama G, Ishikawa K, Takezoe H (2011) Electric-field controllable optical activity in the nano-segregated system composed of rod- and bent-core liquid crystals. Opt Mater Exp 1 27-35... 

Prasang C, Whitwood AC, Bruce DW (2008) Spontaneous symmetry-breaking in halogen-bonded, bent-core liquid crystals observation of a chemically driven Iso-N-N phase sequence. Chem Commun 2008 2137-2139... 

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

Finally, our group reported on gold nanoparticles decorated with bent-core liquid crystals showing pattern formation on TEM grids after slow solvent evaporation (18 in Fig. 22). These particles showed interesting self-assembly effects in different bent-core liquid crystal hosts (SmCPA and Colr) and slightly improved electro-optic effects such as shorter response time, x, and unaltered spontaneous polarization in the SmCPA host, but no mesophase formation [547]. 

Supramolecular chemistry can be used to create the bent cores that give rise to the symmetry breaking in this family of liquid crystals [139]. The formation of a complex between a calamitic benzoic acid derivative and a bent core terminated with a pyridyl group—neither of which display mesomorphic behaviour—gave rise to a material which displayed SmCP mesophases. The achiral bent cores can also give rise to symmetry breaking when they are attached to flexible polymeric chains, such as poly(siloxane) [140]. 

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

Liquid crystals are interesting supramolecular systems which can show second harmonic generation when they are aligned appropriately. Ferroelectric LCs [250] as well as bent-core molecules have been used to this purpose, and show reasonable second harmonic generation [251]. These materials combine non-linear optical effects with simple processing procedures on account of their liquid crystalline flow characteristics and the possibility of organising them with electric and magnetic fields. 

While such a general model is quite appropriate to describe many known liquid-crystal species, it is increasingly inadequate, not because it is inaccurate, but rather because developments in the synthetic chemistry of liquid crystals have been such that, in reality, the usefulness of such general models might be called into question. For example, a new and significant area of research concerns on so-called bent-core (or banana) liquid crystals where the structural anisotropy rules given above simply do not hold and a 120° bend in the molecule is effectively a requirement. Some other developments may be found elsewhere. 

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