Bent-core molecules packing

As you can imagine, when bent-core molecules pack together, a variety of different phases may form distinct from the more conventional calamitic phases formed from rod-like molecules. These different phases result from the additional complexity added to packing bent molecules instead... 

Fig. 17 The stereochemical possibilities for packing in bent-core molecule phases...

Assuming the head-to-tail symmetry of a bent-shape molecule, the highest symmetry of a uniaxial non-tilted smectic A layer is Dooh- Then, according to molecular packing presented in Fig. 13.29a, the highest symmetry of the biaxial polar layer is 2, there is a rotation axis C2 parallel to x, and two symmetry planes xz and xy. The layer polarization is possible along the C2 axis. Had the layer consisted of the rod-like molecules tilted within the xz plane the symmetry would be C2h as in SmC. However, when the bent-core molecules are tilted in the y-direction (forward... 

The smectic C phase formed by chiral molecules (SmC phase) has also a helical superstructure having a pitch incommensurate with the smectic layer thickness. Theoretically chiral phases can also be formed by achiral molecules due to very specific packing [16]. For instance, three achiral rod-Uke molecules of dijfer-ent length may form a chiral trimer or a tripod due to Van der Waals interactions between their fragments, see Fig. 4.25a, and such trimers, in their turn, may form a kind of helical structure. Another example is bent-core or banana like-molecules [17]... 

N. Gimeno, M.J. Clemente, P. Forcen,J.L. Serrano, M.B. Ros, On the snpramolecular packing of bent-shaped molecules the influence of the central hent-core on the hquid crystalline behaviour, NewJ. Chem. 33 (2009) 2007. 

In this chapter we will only review the first ten years of experimental physical results on bent core materials. We consider the molecules as bent rods and their packing and macroscopic phase behavior will be described based on the cartoons we present in Figure 2-2 to Figure 2-6. 

Chirality can also be introduced when one or more chiral carbons are incorporated in the molecules, for example in the hydrocarbon terminal chains [67, 68], within the bent-core [69], or by addition of chiral dopants [6, 70], It was noted during the early research that the handedness of the homochiral structures is very sensitive to chiral dopants [6], or even on chiral surfaces [71]. On the other hand, it was observed that banana-smectics made of enantiomeric chiral molecules form synclinic - antiferroelectric [44] and anticlinic ferroelectric [67] domains. This combination of tilt and polar order implies that the phase is racemic, with a rigid alternation of right- and left-handed chiral layers. This shows that the molecular chirality has no or minor effect on deciding about anticlinic or synclinic packing (which is mainly determined by entropic reasons), but it can bias the otherwise degenerate tilt directions. 

Experiments by Binet et al. [72], carried out on chiral bent core materials containing biphenyl (BP) cores with S or R hydrocarbon chains, and on achiral biphenyl core molecules with chiral dopants, reveal the effect of the molecular chirality on the polarization stmcture. In addition the polarization Pb due to the closed packing of the bent-shape molecules, another polarization, P is introduced due to the chiral and tilted molecular structure. It was found [72] that in the antiferroelectric racemic domains at low fields, of the synclinic - racemic... 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

Another difference between the ternary and the binary complex is a bent conformation of the L2 finger in the complex with monastrol. Furthermore, the pointed tip of the core structure (jS6—L10-/17) has a variable conformation in the structure with monastrol. In the two molecules of the asymmetric unit, the tip bends and moves by about 7 A. The conformation observed without monastrol is intermediate between the two conformations of the ternary complex. The double conformation of L10 is clearly a crystal packing effect. However, it shows that the tip of the core domain is rather flexible. 

Cholesterol, which is interspersed between the phospholipids, maintains membrane fluidity. In the phosphoacylglycerols, unsaturated fatty acid chains bent into the cis conformation form a pocket for cholesterol, which binds with its hydroxyl group in the external hydrophilic region of the membrane and its hydrophobic steroid nucleus in the hydrophobic membrane core (Fig. 10.5). The presence of cholesterol and the cis unsaturated fatty acids in the membrane prevent the hydrophobic chains from packing too closely together. As a consequence, lipid and protein molecules that are not bound to external or internal structural proteins can rotate and move laterally in the plane of the leaflet. This movement enables the plasma membrane to partition between daughter cells during cell division, to... 

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