Discotic columnar structure

Fig. 16 The molecular structure of two components, poly(ethylene oxide)-Wocfc-poly(i.-lysine) (32) and hexa-peri-hexabenzocoronene (33), used to form an ionic complex SLCP and a model of its a-helical-wif/im-discotic columnar structure. Each a-helical poly(L-lysine) chain is surrounded symmetrically by six columns of 33 (disks) [99]...

Figure 3.32 Schematic representation of the packing behavior of acceptor polymers having different spacer lengths in a discotic columnar structure with low molar mass donors (a) spacer too short, (b) spacer of correct length, (c) spacer too long (permission being sought from [73]).

X-ray measurements (5) make it probable, that here again meso-genic associates exist, but having certainly no discotic columnar structure. 

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. 

Figure 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

Liquid-crystalline phases are characterized to some degree by the shape of the molecules and by their packing arrangements and ordering in the mesomorphic state. Typically, molecules can have cither disc- or rod-like shapes and can form discotic or calamitic mesophases, respectively. Ferrocene liquid crystal systems that have so far been synthesized tend to have molecular structures that are lath- or rod-like in shape, and consequently the phases observed are calamitic. However, this does not preclude the possibility that a polysubstituted ferrocene could be prepared where the molecular shape is disc-like, thereby holding out the prospect of possibly producing discotic/columnar phases. 

The resulting mixtures revealed the existence of columnar mesophases, with a pronounced intracolumnar order, the stacking periodicity being in the range from 3.40 to 3.45 A. The same type of ordered columnar mesophase has been induced by mixing discotic nematic compounds with TNF. In this case, the interactions between the disks are too small in themselves to favour a columnar mesophase, but the incorporation of electron acceptor molecules such as TNF contributes to enhance the intermolecular interactions and to generate an ordered columnar mesophase. The stabilization of the columnar structure in these binary systems has been found for other types of molecules... 

Fig. 6.4.2. The discotic liquid crystal appearing in the isotropic phase when a sample is cooled very slowly. The growth pattern is diagnostic of the hexagonal symmetry of the columnar structure (a) Queguiner, Zann and Dubois , (b)...

Cellulose and oligocellulose derivatives have been studied and a variety of thermotropic LC phases established, i.e., cholesteric and colunmar structures for cellulose derivatives, discotic columnar and smectic-type for the oligomers, depending on the side group and the main-chain lengths. A list of compounds exhibiting thermotropic me-sophases is presented in Table 5. 

Discotic LC, based on the molecules having disc-like shapes [22, 23, 74], have attracted much attention in recent years owing to their inherent ability to form self-organized columnar structures, which can provide superior charge transport ability... 

C.V. Cerclier, M. Ndao, R. Busselez, R. Lefort, E. Grelet, P. Huber, A.V. Kityk, L. Noirez, A. Schonhals, D. Morineau, Structure and phase behavior of a discotic columnar liquid crystal confined in nanochannels. J. Phys. Chem. C 116, 18990-18998 (2012)... 

B. Brandi, J.H. Wendorff, Eutectic mixtures with plastic columnar discotics molecular structure, phase morphology and kinetics of phase separation. Liq. Cryst. 32, 553-563 (2005)... 

---