Molecule rod-like

Metallomesogens. It is also possible to synthesize compounds based on metal atoms which possess Hquid crystal phases. The series based on dithiolene complexes (1), where M = Ni, Pd, or Pt, contains a number of compounds which show the Hquid crystal phases typical of rod-like molecules (13,14). 

Figure 14.1 Each polypeptide chain in the collagen molecule folds into an extended polyproline type II helix with a rise per turn along the helix of 9.6 A comprising 3.3 residues. In the collagen molecule three such chains are supercoiled about a common axis to form a 3000-A-long rod-like molecule. The amino acid sequence contains repeats of -Gly-X-Y- where X is often proline and Y is often hydroxyproline. (a) Ball and stick model of two turns of one polypeptide chain.

The liquid crystal polymers consist of rod-like molecules which, during shear, tend to orient in the direction of shear. Because of the molecular order the molecules flow past each other with comparative ease and the melts have a low viscosity. When the melt is cooled the molecules retain their orientation, giving self-reinforcing materials that are extremely strong in the direction of orientation. 

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers... 

The way forward was proposed by Berne and Pechukas [11] many years later. Their important idea was to consider the overlap between two prolate ellipsoidal gaussian distributions. From the expression for this overlap they evaluated a range parameter which was taken to be the contact distance g and a strength parameter which was set equal to the well depth, e. If the orientations of the two rod-like molecules in the laboratory frame are represented by the unit vectors Ui and Uj and the orientation of the intermolecular vector by the unit vector f then the expression for the angular dependence of the contact distance is... 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

We now consider the alternative strategy with which to create chiral molecules using Gay-Berne molecules as a basis. In this two rod-like molecules are linked together resulting in a wide range of chiral conformations. One of... 

Here, the tilde indicates the parameter for the combination of a rod-like molecule and an atom so denotes the contact distance when the atom is at the end of the molecule and dj that when it is at the side. The analogous expression used for the well depth is... 

It is not possible to predict from the related crystal structure alone whether the compound will melt to a liquid crystalline phase or not, because the anisotropic molecules (calamitic and discotic ones) form in favourable anisotropic packing. As a rule long shaped rod-like molecules quite often possess a layered arrangement in the solid state regardless of whether the compound is mesogenic or not. 

An orthogonal layered structure in the solid state of rod-like molecules is the exception rather than the rule. Therefore, there is no conclusive evidence that a tilted layer structure in the solid state melts to a tilted smectic phase. In other words, if we consider the solid state as precursor for the type of the liquid crystalline state, no real precursor for an orthogonal fluid smectic phase would exist. As demonstrated in Fig. 19, the compound B-A for example exhibiting a smectic A phase has a tilted layer structure in the solid state. 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

Introduction of lateral chains in rod-like molecules helps, as discussed above, to lower transition temperatures and avoid the thermal decomposition of this type of... 

In the phrase "liquid-crystalline", the "crystalline" adjective refers to the fact that these materials are sufficiently ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials. On the other hand, the "liquid" part specifies that there is frequently sufficient disorder for the material to flow like a liquid [145]. The disorder is typically in one dimension as is the case, for example, with rod-like molecules having their axes all parallel but out of register with regard to their lengths. 

The concept of a molecular wire is a remarkable case in this a conjugated rod-like molecule bridges the gap between nanoelectrodes with molecular dips as crudal components.161... 

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

For solutes comprising thin rod-like molecules the analysis of Holtzer127 for the asymptotic reciprocal scattering function yields ... 

Rod-hke polymers such as polyglutamate [21], polysiloxane [22], alkylated cellulose [23], and discotic crystals [24], have been reported to form LB films in which rod-like molecules aligned in one direction during the compres-... 

Mesogen composed of rod-like molecules with large lateral branehes such as alkyl, alkoxy or ring-containing moieties. 

Note 3 A cubic mesophase may be formed by rod-like molecules with strong, specific intermolecular interactions, such as hydrogen bonding, between them. However, they are also found in polycatenary compounds where there are no specific, strong interactions. 

Note 2 The symmetry and structure of a nematic mesophase formed from disc-like molecules is identical to that formed from rod-like molecules. It is recommended therefore, that the subscript D is removed from the symbol No , often used to denote a nematic formed from disc-like molecules. 

Note 2 For rod-like molecules, the order parameter of the effective molecular symmetry axis at the nematic-isotropic transition is about 0.3 and can increase to about 0.7 in the... 

Note 2 fri low molar mass nematics composed of rod-like molecules, the bend constant is the largest while the twist constant Ki is the smallest. Typical values of K are 10 " - lO Nm l... 

Grazing incidence x-ray diffraction (GID) measurements have indicated that both precollapse and collapsed state monolayers at the air-water interface can be crystalline (Birdi, 1989). A general procedure was delineated that could provide near-atomic resolution of two-dimensional crystal structures of -triacontanoic acid (C29H59COOH). A monolayer composed of rod-like molecules would generally pack in such a way that each molecule has six nearest neighbors, that is, hexagonal cell. 

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic LC have been of much interest in such fields as biomimetic chemistry. In fact, biological membranes and cell membranes are a form of LC. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface yet, the membrane is fluid and elastic. The constituent molecules can flow in plane quite easily but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These LC membrane phases can also host important proteins such as receptors freely floating inside, or partly outside, the membrane. 

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