Banana molecules

Fig. 16. Structure of a banana molecule 4,6-dichloro-l,3-phenylene bis[4 -(9-deceny-loxy)biphenyl] carboxylate and 13C NMR spectra in the nematic (top) and isotropic (bottom) phase.

Flexoelectricity is usually explained as being due to asyirmietric molecular shapes. If a molecule is thicker on one side (drop shape) then in an unbent media the up and down orientations of the molecules are equally occupied. But if the material is bent, one of the two positions is more favourable. Molecules still rotate around the short axes however, they spend more time in one position than in the other (Fig. 5.3a,b). If a molecule has a longitudinal component of polarization, the bent material is polarized as well. Similarly we can consider molecules, which are bent like banana molecules. Then, the two orientations (Fig. 5.3c) are not equivalent and rotation along the long molecular axis is hindered when the material is bent. The molecules spend more time in one of the orientations. If the polarization has a component in the bent plane perpendicular... 

It has been shown [34] that molecules containing a bent rigid core, so-caUed banana molecules, exhibit spontaneous polarization an chirality in their smectic LC phases in spite of the fact that the molecule itself is achiral. The banana molecules in a 1.3-benzene bis [4- (4- -alkoxyphenyliminomethyl) benzoate] (P -0-PIMB) homologous series, as illustrated in Fig. 1C, have a various characteristic physical properties in the LC phases, which are classified and designated as B-phases [35]. It has been reported by... 

On the other hand, detailed physical properties of the B4 phase remain uncertain. One of the most interesting features in this smectic phase is the presence of transparency and visible blue color [59], from which the formation of the hehcal structure is deduced. So far, two models have been proposed to describe such hehcal structures the twisted grain boimdary (TGB)-hke model [59] and the helical nanofilament model [60]. Although arguments for the packing systems continue, the presence of a twisted conformation in banana molecules is expected to be a driving force for the formation of the hehcal structure in both models [59-61],... 

An analysis of the CS tensors for the doublet peaks of alkoxy carbon sites (a/a ) is expected to be important for deducing torsion angles between Planes II and III and between Planes iP and III. It can be seen from Table 1 that the and S22 components as well as the isotropic shifts are almost similar to each other, but a clear difference is found for the S32 components. To understand such a difference, as shown in Fig. 32A, we constructed a simple model of the banana molecule in which the torsion angle between Planes II and III (0) was artificially changed from 0° to 360° in steps of 30°, and quantum chemical calculations for the CS tensors were performed at each step. In this model, the conformation in which the two planes are coplanar and carbon sites (a/a ) are located on the same side of the C=N bond... 

However, it should be noticed here that the introduction of chirality is one of the simplest ways to reduce the packing symmetry. The research of banana-shaped liquid crystal is actually bom from this fact. In other words, the attachment of a bent shape into a molecule could give rise to a smectic (Sm) stmcture which has the polar twofold axis along the bent direction and thus to ferroelectric and antiferro-electric Sm LCs without a chiral center. This was firstly suggested in the polymer molecules, successfully demonstrated in bent dimers and then in banana molecules. 

Fig. 9.24 The smectic layer structure formed by banana molecules. The banana molecules are packed into a layer with the same directionality, showing the spontaneous polarization along the bent direction...

Fig. 9.25 (a) Phase transition behavior of banana molecules, P-n-O-PIMB, and (b) antiferro-electric switching behavior observed in B2 phase on the triangular-wave voltage application... 

The banana molecules are too interesting to say now since the ferroelectric phase has been demonstrated, let s move on. Their bizarre behavior sometimes leads to ferroelectric and sometimes to antiferroelectric phases. Their optical texture is also not uniform, but they exhibit a polydomain liquid crystal phase, as if they would contain a large amount of impurities (of course the compounds were as pure as possible). The reason for this erratic behavior was proposed at a Gordon conference in 1997 the complexity of the behavior stems from the tilting of the molecules with respect to the smectic layer. Initially, we considered the molecular tilting in the B2 phase because the measured thickness of the smectic layers was far less than the calculated molecular length [131]. This fact had always been in the back of my head, but I could not find a reason why the molecular axis must tilt. 

The tilting of molecules in the B2 phase is clearly confirmed from the observation that the spherulites emerging from the isotropic phase show an electric field dependence of the position of the optical extinction lines (Fig. 9.26). Because of the tilting of banana molecules to the layer, chirality is spontaneously generated in addition to the polarity this fact sounds shocking but is so simple to be understood [132, 133]. If the molecule is rotated around their polar axis (the orientation of the bent in the molecules), which is akin to tilting the molecules in the layer, the rotation operation cannot be achieved by a simple translation (see Fig. 9.27). That is, these two states are in a mirror relation with left-handed and right-handed chirality. This is called the layer chirality. When the chirality couples with the polarity of the molecules, one would consider various smectic liquid crystal structures. There are two homochiral phases in which either (—) or (-I-) chiral molecules stack in the layers and a racemic phase in which layers are alternately stacked with layers of (—) and (-I-) chiral molecules. Each of those phases can be either ferroelectric or antiferroelectric, so that in total six different phases are present... 

Fig. 9.27 Illustration of the chiral symmetry breaking event by tilting of banana molecule to the layer normal...

The Nematic Phase Including Banana Molecules Is Also Chiral ... 

Fig. 9.32 (a) Phase transition behaviors of N (1,7)-Sn including hexagonal columnar phase (Colh) and optical isotropic cubic (Cub) as a function of carbon number of alkyl tail, (b-d) Schematic illustration of the column and cubic formation by the winding of the layer, (b) The asymmetric molecules are packed with the same directionality. Then, two rigid side arms of the banana molecule take a different orientation to the layer normal, producing a significant difference in the density if the layer is flat. Hence a dilatirai occurs in one half-layer (upper part divided by the dotted line in (a)) while a compression occurs in the other half (lower part) as in (c). This frustration can cause cylindrically and spherically enclosed deformations of the layer as in (d)... 

Recent optical experiments have probed the effect of flexible polymeric solutes in both lyotropic [36] and thermotropic liquid-crystal [37, 38] environments. Long-chain solutes feel an average effective anisotropy, and are found to exhibit much stronger ordering than smaller solutes. The synthesis of the bent-core mesogens ( banana molecules) has resulted in a novel biaxial smectic-A (both pure phase [39] and solute-induced [40]) as well as nematic phases [41,42]. 

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