International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2010). Vol. B, ch. 4.4, p. 561   | 1 | 2 |

Section 4.4.5. Discotic phases

P. S. Pershana*

aDivision of Engineering and Applied Science and The Physics Department, Harvard University, Cambridge, MA 02138, USA
Correspondence e-mail: pershan@deas.harvard.edu

4.4.5. Discotic phases

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In contrast to the long thin rod-like molecules that formed most of the other phases discussed in this chapter, the discotic phases are formed by molecules that are more disc-like [see Fig. 4.4.1.3(f)[link], for example]. There was evidence that mesomorphic phases were formed from disc-like molecules as far back as 1960 (Brooks & Taylor, 1968[link]); however, the first identification of a discotic phase was by Chandrasekhar et al. (1977[link]) with benzene­hexyl hexa-n-alkanoate compounds. Disc-like molecules can form either a fluid nematic phase in which the disc normals are aligned, without any particular long-range order at the molecular centre of mass, or more-ordered `columnar' (Helfrich, 1979[link]) or `discotic' (Billard et al., 1981[link]) phases in which the molecular positions are correlated such that the discs stack on top of one another to form columns. Some of the literature designates this nematic phase as [N_{\rm D}] to distinguish it from the phase formed by `rod-like' molecules (Destrade et al., 1983[link]). In the same way that the appearance of layers characterizes order in smectic phases, the order for the discotic phases is characterized by the appearance of columns. Chandrasekhar (1982[link], 1983[link]) and Destrade et al. (1983[link]) have reviewed this area and have summarized the several notations for various phases that appear in the literature. Levelut (1983[link]) has also written a review and presented a table listing the space groups for columnar phases formed by 18 different molecules. Unfortunately, it is not absolutely clear which of these are mesomorphic phases and which are crystals with true long-range positional order.

Fig. 4.4.5.1[link] illustrates the molecular packing in two of the well identified discotic phases that are designated as [D_{1}] and [D_{2}] (Chandrasekhar, 1982[link]). The phase [D_{2}] consists of a hexagonal array of columns for which there is no intracolumnar order. The system is uniaxial and, as originally proposed, the molecular normals were supposed to be along the column axis. However, recent X-ray scattering studies on oriented free-standing fibres of the [D_{2}] phase of triphenylene hexa-n-dodecanoate indicate that the molecules are tilted with respect to the layer normal (Safinya et al., 1985[link], 1984[link]). The [D_{1}] phase is definitely a tilted phase, and consequently the columns are packed in a rectangular cell. According to Safinya et al., the [D_{1}] to [D_{2}] transition corresponds to an order–disorder transition in which the molecular tilt orientation is ordered about the column axis in the [D_{1}] phase and disordered in the [D_{2}] phase. The reciprocal-space structure of the [D_{1}] phase is similar to that of the crystalline-E phase shown in Fig. 4.4.4.7(b)[link].

[Figure 4.4.5.1]

Figure 4.4.5.1 | top | pdf |

Schematic illustration of the molecular stacking for the discotic (a) [D_{2}] and (b) [D_{1}] phases. In neither of these two phases is there any indication of long-range positional order along the columns. The hexagonal symmetry of the [D_{1}] phase is broken by `herringbone-like' correlations in the molecular tilt from column to column.

Other discotic phases that have been proposed would have the molecules arranged periodically along the column, but disordered between columns. This does not seem physically realistic since it is well known that thermal fluctuations rule out the possibility of a one-dimensional periodic structure even more strongly than for the two-dimensional lattice that was discussed above (Landau, 1965[link]; Peierls, 1934[link]). On the other hand, in the absence of either more high-resolution studies on oriented fibres or further theoretical studies, we prefer not to speculate on the variety of possible true discotic or discotic-like crystalline phases that might exist. This is a subject for future research.

References

Billard, J., Dubois, J. C., Vaucher, C. & Levelut, A. M. (1981). Structures of the two discophases of rufigallol hexa-n-octanoate. Mol. Cryst. Liq. Cryst. 66, 115–122.
Brooks, J. D. & Taylor, G. H. (1968). In Chemistry and Physics of Carbon, Vol. 3, edited by P. L. Walker Jr, pp. 243–286. New York: Marcel Dekker.
Chandrasekhar, S. (1982). In Advances in Liquid Crystals, Vol. 5, edited by G. H. Brown, pp. 47–78. London, New York: Academic Press.
Chandrasekhar, S. (1983). Liquid crystals of disk-like molecules. Philos. Trans. R. Soc. London Ser. A, 309, 93–103.
Chandrasekhar, S., Sadashiva, B. K. & Suresh, K. A. (1977). Liquid crystals of disk like molecules. Pramana, 9, 471–480.
Destrade, C., Gasparoux, H., Foucher, P., Tinh, N. H., Malthête, J. & Jacques, J. (1983). Molecules discoides et polymorphisme mesomorphe. J. Chim. Phys. 80, 137–148.
Helfrich, W. (1979). Structure of liquid crystals especially order in two dimensions. J. Phys. (Paris) Colloq. 40, C3–105–C3–114.
Landau, L. D. (1965). In Collected Papers of L. D. Landau, edited by D. ter Haar, pp. 193–216. New York: Gordon and Breach.
Levelut, A. M. (1983). Structures des phases mésomorphes formée de molécules discoides. J. Chim. Phys. 80, 149–161.
Peierls, R. E. (1934). Transformation temperatures. Helv. Phys. Acta Suppl. 2, 81–83.
Safinya, C. R., Clark, N. A., Liang, K. S., Varady, W. A. & Chiang, L. Y. (1985). Synchrotron X-ray scattering study of freely suspended discotic strands. Mol. Cryst. Liq. Cryst. 123, 205–216.
Safinya, C. R., Liang, K. S., Varady, W. A., Clark, N. A. & Andersson, G. (1984). Synchrotron X-ray study of the orientational ordering D2–D1 structural phase transition of freely suspended discotic strands in triphenylene hexa-n-dodecanoate. Phys. Rev. Lett. 53, 1172–1175.








































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