Classification of discotic liquid crystalline phases

Discotic liquid crystal phases are classified into two main categories: discotic nematic mesophases (N_D) and discotic columnar mesophases (Figure 4). The discotic nematic phase is the least ordered mesophase formed by disc-like molecules and is analogous to the nematic phase formed by rod-like molecules. It is characterized by possessing 1-dimensional long-range orientational and no long-range positional order (Figure 3). Discotic columnar mesophases are obtained when the disc-like molecules stack on top of each other. These phases have both long-range orientational and 2-dimensional positional order with the exception of the columnar nematic phase that has no positional order. The different columnar mesophases with 2-dimensional positional order are gener­ally classified according to their packing symmetry. The symmetry is defined by the arrange­ment of the columns in space, the orientation of the molecular planes with respect to the colum­nar axis (Figure 3), and the degree of stacking order within the col­umns (Figure 6).²

Figure 3      Discotic liquid crystal phases: nematic discotic (N_D), nematic columnar (N_Col), columnar hexagonal (Col_h), and columnar rectangular (Col_r)³

Figure 4      Classification of Discotic Liquid Crystal mesophases

Figure 5      View down the stacking axis of columnar discotic mesophases

The degree of order and molecular dynamics within a columnar stack has been a matter of de­bate for more than a decade. It is commonly accepted to distinguish between at least four states of intra­columnar order (Figure 6). The disordered (Col_d) and ordered (Col_o) stacks of a columnar phase have been distinguished between based on X-ray diffraction data, but the transition be­tween the two is not strictly defined. A further increase in orientational and/or posi­tional order leads to the plastic columnar phase (Col_p,1D orientational and 3D positional order) and the helical co­lumnar phase (H, 3D orientational and 3D positional order), which have been identified more re­cently.²

Figure 6      Columnar phases can be sub-classified according to the degree of order and the dynamics of the molecules within the columnar stacks: disordered columnar phase (Col_d), ordered columnar phase (Col_o), plastic columnar phase (Col_p) and helical columnar phase (H).²


References:

1. S.Chandrasekhar; Sadashiva, B.; Suresh, K. Pramana 1977, 7, 471-480.
2.  Bushby, R. J.; Lozman, O. R. In Current Opinion in Colloid & Interface Science, 2002, 7, 343-354.

Discotic mesogens

Chandrasekhar discovered the first discotic liquid crystalline phase in 1977 that was formed by co­lumnar stacks of disc-like hexa-n-alkanoate substituted benzenes.¹ Since then thou­sands of dis­cotic liquid crystals have been prepared and they have attracted much attention over the last ten years because of their extraordinary electro-optical properties.

Figure 1      (A) Phthalocyanine and disc-shape; (B) 4-n-heptyl-N-(4-n-pentyloxy)benzylidene and rod-shape

Discotic mesogens are characterized by their disc-like molecular shape and usually con­sist of a con­jugated macrocyclic or polyaromatic core surrounded by flexible hydrocar­bon chains (e.g. the phthalocyanine derivative in Figure 1).

Many types of core structures have been converted into discotic mesogens such as an­thraquinones, ethynylbenzenes, naphthacenes, perylenes, porphyrins, phthalocyanines, triazines, tetraoxa[8]circulenes, trisoxadiazolylbenzenes, triphenylenes, and tristyrylpyridine derivatives.² The most extensively studied discotic LCs, however, are deriva­tives of triphenylene (Figure 2, B), porphyrin (Figure 2, C), and phthalocyanine (Figure 2, D).

Figure 2      Examples of discotic mesogens (R = alkyl or alkoxy chains)