Disc-like molecules, discotics

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]. 

Since the discovery of discotic liquid crystals [121], the mesophases formed by rod-like and disc-like molecules have been considered as belonging to different liquid crystalline classes. Indeed, the conventional rod-like and disc-... 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

In discotic phases the orientation of the molecules is perpendicular to the molecular plane. Here, the columns can be arranged in a nematic or columnar manner. In the nematic phase the molecules possess a centre of gravity randomly ordered, but with the short molecular axis of each molecule more or less parallel. In the columnar phase, beside the preferable orientation of the short molecular axes, the disc-like molecules are ordered forming columns. Depending on the correlation strength between he columns these phases can be subdivided into ordered or disordered. A third possibility is to have a thermodynamically preferable position of the columns in the mesophase, like in a hexagonal cell. Additionally, a tilt of the columns is also possible. 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

In the simplest liquid-crystalline phase, namely the uniaxial nematic, there is at rest a special direction designated by a unit vector n called the director (see Fig. 10-2). In the plane transverse to the director, the fluid is isotropic. The most common nematics are composed of oblong molecules that tend to point in a common direction, which defines the director orientation. Oblate, or disc-like, molecules can also form uniaxial nematics for these discotic nematics, the director is defined by the average orientation of the short axis of the molecule. Lath-like molecules or micelles (shaped like rectangular slabs), in which all three dimensions of the molecule are significantly different from each other, can form biaxial nematics (Praefcke et al. 1991 Chandrasekhar 1992 Fialtkowski 1997). A biaxial... 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

The liquid crystals discussed so far are composed of rod-like molecules. As discussed in the last section, disc-like molecules may show liquid crystal phases and are called discotic liquid crystals. The first discotic liquid crystal was synthesized by Chandrasekhar (1977) and many more discotic liquid... 

In the quest for a universal feature in the short-to-intermediate time orientational dynamics of thermotropic liquid crystals across the I-N transition, Chakrabarti et al. [115] investigated a model discotic system as well as a lattice system. As a representative discotic system, a system of oblate ellipsoids of revolution was chosen. These ellipsoids interact with each other via a modified form of the GB pair potential, GBDII, which was suggested for disc-like molecules by Bates and Luckhurst [116]. The parameterization, which was employed for the model discotic system, was k = 0.345, Kf = 0.2, /jl= 1, and v = 2. For the lattice system, the well-known Lebwohl-Lasher (LL) model was chosen [117]. In this model, the particles are assumed to have uniaxial symmetry and represented by three-dimensional spins, located at the sites of a simple cubic lattice, interacting through a pair potential of the form... 

The existence of the thermotropic biaxial nematic phase was theoretically predicted almost thirty years ago [119] but such a phase has yet to be unambiguously identified. Indeed, as we saw in the previous section, a biaxial nematic phase has been claimed for a class of discotic dimers [111], Theory predicts that mixtures of rod-like and disc-like molecules should exhibit the biaxial nematic phase [120] but experimentally such systems phase separate... 

Just as chiral induction can be realized in discotic liquid crystals, so it can in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical fm-amides (Fig. 6), which exhibit discotic liquid-crystalline phases, also form chiral columnar stacks through n-n interactions when dissolved in apolar solvents,which are depicted schematically in... 

DLCs generally exhibit two types of phases nematic phase and columnar phase (Fig. 7.1). Discotic nematic phases are rarely observed while columnar phases are commonly found. In the nematic phase, the molecules possess only orientational ordering however in the columnar phase, the disc-like molecules spontaneously self-assemble into infinite one dimensional (ID) stacks, which in turn self-organize to various two dimensional (2D) liquid crystalline lattices. The hexagonal columnar phase is the most commonly encountered phase in DLCs. Some discotic compounds are also known to exhibit lamellar and cubic phases. 

It should be evident, that a perfect peripheric space filling of the lateral alkylchains, which is regarded as an essential criterion for the ability of conventional disc-like molecules to form disophases (6), determinates also the behaviour of this novel discotic associates. 

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

Thermotropic liquid crystal phases are formed by anisotropic molecules with long-range orientational order, and in many types of structure some degree of translational order. The main types of mesogens are those that are rod-like or calamitic and those that are disc-like or discotic. 

Nematic discotic liquid crystals consist of disc-like molecules with a preferred parallel orientation of their short molecular (symmetry) axes. There is no translational order. The symmetry of the liquid crystal agrees with that of usual calamitic nematic liquid crystals consisting of rod-like molecules. Accordingly, the stress tensor for calamitic and discotic nematic liquid crystals are identical. Nevertheless, there are some differences in the flow phenomena due to differences in flow alignment. 

A more systematic investigation of such structures was performed by Thomson et al. [65] who found that complexes, 25, with the rod-like structure A showed monotropic nematic mesophase of discotic type while those with the more disc-like molecule B show enantiotropic phases of calamitic type. 

In the discotic nematic (No) phase (Fig. 16.3a), molecules have orientationally ordered arrangement of discs with no long-range translational order. This is the least order (usually high temperature) in the disc-like molecules (Kumar 2009). The nematic mesophase can be assimilated to a lamellar nematic liquid crystal, in which the director vector (an optical axis) is perpendicular to the average direction along which the flat molecules are aligned, as illustrated in Fig. 16.3a. 

Comparable to the nematic (N) phase of rod-like compounds the least orda ed (usually highest temperature) mesophase exhibited by disc-like molecules is also the nematic (discotic. No) phase the index D simply refers to their molecular shape. Both nematic phases are of the same symmetry and identical types of defects are seen in both cases [5] they exhibit similar fluid Schlieren textures [3,6]. However, the nematic phases of these two low-molar-mass liquid crystals are not miscible [3,6] and phase separation occurs due to fundamental differences in their molecular structures. 

Prior to discussing the structural features of discotic mesogens in Section C, some other distinguishing features of these fascinating, discotic - for many still exotic - materials will be discussed here induction of nematic phases and the possibility of thermotropic phase biaxiality envisaged for them. Along with thermotropic Nd phases of pure compounds, further fundamental modes of formation of nematic phases of disc-like molecules have become known. 

FIGURE I Schematic rqjresentations [7] of the nematic phase (N, left) of rod-like molecules cwnpared with die discotic nematic (Nq, coitre) and die non-tihed columnar nematic (Ngdi, right) phases of disc-like molecules. D = discotic. Col = columnar, n = director. 

In contrast to the single kind of fluid nematic phase known to be exhibited by calamitic mesogens, the disc-like mesogens described in this Datareview can fmm two nematic phases, each showing a Schlieren texture a discotic nematic or a columnar nematic type of phase both differ in their supra-molecular array of the disc-like molecules and viscosity. 

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