Liquid crystals of disc-like molecules

The first liquid crystals of disc-shaped molecules, now generally referred to as discotic liquid crystals, were prepared and identified in 1977. Since then a large number of discotic compounds have been synthesized and a variety of mesophases discovered. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked one on top of the other aperiodically to form liquid-like columns, the different columns constituting a two-dimensional lattice (fig. 1.1.8 (a)). The structure is somewhat similar to that of the hexagonal phase of soap-water and other lyotropic 

Polymers that exhibit liquid crystalline phases. The basic monomer units are low molar mass rod-like or disc-like mesogens which may form part of the main chain (a) or attached as side groups (b). 

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Chandrasekhar S, Sadashiva B Kand Suresh KA 1977 Liquid crystals of disc-like molecules Pramana 9 471-80... 

Chandrasekhar S 1983 Liquid crystals of disc-like molecules Phil. Trans. R. Soc. A 309 93-103... 

Chandrasekhar, S., "Liquid Crystals of Disc-Like Molecules, "Advances in Liquid Crystals, 47-78 (1982). 

Chandrasekhar, S. Sadashiva, B. K. Suresh, K. A. Liquid crystals of disc-like molecules. Pramana 1977, 9, 471-480. 

The Bj textures show characteristic helical filamentary growth in cooling from isotropic melt. i- 2 However they denote at least two distinct phases. X-ray measurements clarified that the original By materials, which form freestanding strands, - just like columnar liquid crystals of disc-shape molecules (see Chapter 2), indeed have a columnar phase (hereafter we will label them as Bj j). Other By-type materials (we call them Byu) were foimd to have modulated layer structures. We note that the By and B2 type banana-smectics also form strands of fibers. ... 

Brown GH, Shaw WG (1957) The mesomorphic state— liquid crystals. Chem Rev 57 1049-1157 Cardinaels T, Ramaekers J, Guillon D, Donnio B, Binnemans KA (2005) Propeller-like uranyl metallomesogen. J Am Chem Soc 127 17602-17603 Chai CP, Zhu XQ, Wang P, Ren MQ, Chen XF, Xu YD, Fan XH, Ye C, Chen EQ, Zhou QF (2007) Synthesis and phase structures of mesogen-jacketed liquid crystalline polymers containing 1,3,4-oxadiazole based side chains. Macromolecules 40 9361-9370 Chandrasekhar S, Sadashiva BK, Suresh KA (1977) Liquid crystals of disc-Uke molecules. Pramana J Phys 9 471 80... 

In the effort to make free-standing films of the first B1 material it was found that instead of films they form free-standing strands [25] just like columnar liquid crystals of disc-shape molecules [95], Such observation was confirmed with other B7 materials [98], and later it was found that the B7 and B2 type banana-smectics also form strands of fibers [99]. Although the most stable fibers are the B7 materials with slenderness ratio as large as 5000, the B7 and B2 fibers also have aspect ratios over 1000 and 100, respectively. The values are orders of magnitudes larger than of the Rayleigh-Plateau limit [100] of Newtonian isotropic fluids and of nematic and smectic liquid crystals of rod-shape molecules [96]. 

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... 

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... 

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... 

If this section of this chapter hardly reads as a chronological history, this is because the last 15 years have seen so many developments in different directions that a simple pattern of evolution does not exist. Instead, developments have occurred in an explosive way, emanating outward from the core of fundamental knowledge acquired up to the end of the 1970s. We have already looked briefly at display applications, liquid crystals from disc-shaped molecules, liquid crystal polymers, and metallomeso-gens, but to follow all the developments of recent years radiating out from the central core is hardly possible in a short chapter like this. 

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. 

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-... 

In the last few years disc-like molecules have been shown to form liquid crystals (Chandrasekhar, 1994). Typical of them are hexasubstituted esters of benzene (I) and certain porphyrin esters (II) (see below). In the liquid crystalline state, the disc-like molecules are stacked aperiodically in columns (liquid-like), the different columns packing in a two-dimensional array (crystal-like). The phases have translational periodicity in two dimensions but liquid-like disorder in the third. In addition to the columnar phase(D), the disc-like molecules also exhibit a nematic phase (Nj,). A transition between D and phases has been reported. 

FIG. 6.13 Schematic representation of some characteristic liquid crystal phase structures. Nematic, smectic and cholesteric phases formed from rod-like molecules columnar phases formed from disc-like molecules (from Jansen, 1996). 

Disc-like molecules may also exhibit the liquid crystal phase, de Gennes predicted this possibility in his famous book The Physics of Liquid Crystals ... 

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... 

It can be easily extended to disc-like molecules (de Gennes, 1973) that form a liquid crystal phase because of their anisotropic shape. The Onsager theory has also been extended to treat the poly-disperse polymer system (Parsons, 1979) and semi-rigid chain systems (Lekkerkeker et al, 1984). 

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... 

Of all liquid crystalline phases, the nematic phase is the phase with the highest symmetry, i.e. Dooh, and the least order. As shown in Fig. 3.3a, b, the mesogens solely possess orientational order. Positional order of the mass centers does not occur in this phase. Nematic phases are usually built up by either rod-like or disc-like mesogens. For thermotropic liquid crystals these mesogens are therefore calamitic or discotic molecules, respectively. In both cases the phase is simply denoted with the abbreviation N. For lyotropics, the notation typically distinguishes between nematic phases Nc, which are formed by rod-like micelles, and nematic phases Np, which are composed of disc-like micelles. 

The nematic phase was described in Chapters 1, 3 and 4 and is exhibited by certain rod-like molecules and certain disc-like molecules. The nematic phase is the least ordered liquid crystal phase and it is usually very easy to identify a nematic phase by optical polarising microscopy. The high degree of disorder of the phase stmcture means that the nematic phase is very fluid and dust particles within the sample are seen to undergo intense Brownian motion. When the coverslip is displaced (a vital operation when identifying liquid crystal phases by optical polarising microscopy), it is easy to detect how fluid the phase actually is and the sample shirmners quite intensely at the time of impact. 

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. 

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