Discotic systems

The number of examples of Uquid crystalline systems is limited. A simple discotic system, hexapentyloxytriphenylene (17) (Fig. 4), has been studied for its hole mobUity (24). These molecules show a crystalline to mesophase transition at 69°C and a mesophase to isotropic phase transition at 122°C (25). 

There have been several simulations of discotic liquid crystals based on hard ellipsoids [41], infinitely thin platelets [59, 60] and cut-spheres [40]. The Gay-Berne potential model was then used to simulate the behaviour of discotic systems by Emerson et al. [16] in order to introduce anisotropic attractive forces. In this model the scaled and shifted separation R (see Eq. 5) was given by... 

Berardi et al. [66] have also investigated the influence of central dipoles in discotic molecules. This system was studied using canonical Monte Carlo simulations at constant density over a range of temperatures for a system of 1000 molecules. Just as in discotic systems with no dipolar interaction, isotropic, nematic and columnar phases are observed, although at the low density studied the columnar phase has cavities within the structure. This effect was discovered in an earlier constant density investigation of the phase behaviour of discotic Gay-Berne molecules and is due to the signiflcant difference between the natural densities of the columnar and nematic phases... 

In calamitic systems it is the long axis of the molecules that is correlated in the mesophases but in discotic systems it is the short axis and different types of organization are seen, although disk-like molecules also form a nematic phase in which a unique axis is orientationally correlated (Fig. 25). However, below the nematic phase is a series of columnar phases (Col) in which the disks are stacked up into columns, which are themselves arranged according to some symmetric pattern. Typical... 

A number of supramolecular discotic systems have been investigated in which hydrogen bonding has been used to form or stabilize the disc rather... 

Star-shaped compounds having a 1,3,5-triazine core and stilbenoid arms have been prepared. These compounds form nematic discotic systems <04T6881>. Discotic liquid crystalline... 

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... 

Figure 12. Orientational dynamics of the discotic system GBDII (N = 500) at several temperatures across the isotropic-nematic transition along the isobar at pressure P — 25. (a) Time evolution of the single-particle second-rank orientational time correlation function in a log-log plot. Temperature decreases from left to right, (b) Time dependence the OKE signal at short-to-intermediate times in a log-log plot. Temperature decreases from top to bottom on the left side of the plot T = 2.991,2.693,2.646, and 2.594. The dashed lines are the simulation data and the continuous lines are the linear fits to the data, showing the power law decay regimes at temperatures. (Reproduced from Ref. 115.)...

When a different isobar was chosen at a lower pressure for the discotic system, a direct transition from the isotropic phase to the columnar phase was observed [119]. In this case, decay of neither the single-particle second-rank OTCF nor the OKE signal showed any power law regime [119]. As compared... 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The a-diketone precursors used to prepare the dithiolene complexes discussed above can be used for the preparation of other potential ligands such as dioximes. Such ligands are directly obtained by the reaction of the corresponding benzil species with hydroxylamine chloride. Dioximato complexes with cf metals crystallize in columnar stacks with the formation of a chain of metals orthogonal to the molecular planes, and as such are potentially interesting candidates for the design of discotic systems with low-dimensional semiconducting properties. Ohta et al. reported a... 

This section will concentrate on rod-like complexes which typically form nematic and smectic phases. While the approach does not concentrate on design aspects, it may become apparent that in rod-like systems, design is not a simple matter and there are many factors to be taken into account to do with anisotropy, disposition of ligands and functional groups associated with the metal and introduced on complexation. Thus, while it has been said (albeit with tongue slightly in cheek) that for discotic systems it is possible to realize liquid crystal systems by taking a... 

These examples of study on binary mixtures with discotic systems show an importance of solubility and molecular orders in mesophase for the resultant structure of mesogenic binary systems. 

Discotic systems (see Chapter 3) can be made chiral by incorporating a chiral unit into one or more of the peripheral units that surround the discotic core. 

Relatively few examples of chiral discotic nematic compounds exist, probably because of the perceived lack of applications for discotic systems and the relative expense of chiral units. The chiral discotic nematic (N ) phase has an analogous structure to the calamitic... 

Ringsdorf et al. could form discotic systems from amorphous polymers 26-28 containing disc-shaped r-rich... 

However, the majority of the work in this area has been carried out with discotic systems and it is this area that will be discussed here as indicative of what is possible. 

Dynamic processes that can be investigated by NMR include both the motions of individual molecules, e.g., conformational dynamics and molecular rotations, and collective motions, e.g., director fluctuations in nematic systems, layer undulations in smectic systems, or density modulations in columnar phases of discotic systems. Self-diffusion can be measured by NMR relaxation or field gradient methods, as discussed in Sec. 13 of Chap. VII of this Volume. Table 1 gives an overview of the time scales accessible by the most common experimental techniques and examples of the type of motion that can be studied. 

Experiments on lyotropic SmA systems determined the layer compressibility modulus, B (intermembrane interactions) and the bending modulus, K (interpreted as membrane flexibility) [102], supporting the view of membranes with flexibility of the order of k T. The special case of Rayleigh scattering from columnar undulations in a columnar discotic system [103] showed a 3-D solid like elasticity attributed to column entanglement. 

In discotic systems, the roles of ATu and K22 are reversed, because in such phases the bend deformations require the lowest energy [181]. Measurements of splay and bend constants in a homologuous series of discotic n-hexa(alkanoylox)truxenes [76] revealed that K22 is always smaller than A, . The splay/bend ratio approached unity at the high temperature transition to the columnar phase. Qualitatively different results have been obtained, however, by Raghunathan et al. [75], who found K22>Kn in a disco-tic nematic phase enclosed between two columnar phases. The authors interpreted this unexpected result as being a consequence of short-range columnar order. 

Chain-branching leads, in some cases [190-195], to more complicated phase behaviour and polymesomorphism. The octa(2-ethylhexyloxy)phthalocyanine (as a mixture of diastereomers) provides a good example [190, 192]. The crystalline solid melts to form a discotic tetragonal (Dtet) mesophase at 170 °C. Heating further yields a nematic mesophase (still relatively rare for discotic systems) between 233 °C and 270 °C. Even more complex phase sequences are observed when large central metal ions are introduced into the central cavity of this material [194]. 

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