Liquid crystals have transformed our daily lives, with the LCD industry currently being worth £300 bn per year worldwide. The nematic phase, the most common LC phase used in displays and also the simplest, is formed by rod-like aromatic molecules bearing a flexible chain at one or both ends. Such molecules also form layered, or smectic, phases (Fig. 1, left). Thirty years ago a third type of LC, the columnar phase, was discovered in disc-like molecules, with columns of stacked discs placed on a 2D ordered lattice, usually hexagonal (Fig. 1, middle). Overlap of p-orbitals of stacked aromatic discs has led to interesting materials for molecular electronics and photovoltaics.1,2 In recent years a new type of LC molecules, pioneered by C. Tschierske, gained prominence, having flexible chains attached to the side rather than the ends of the rods. With sticky hydrogen-bonding groups at the ends of the rod, these T-shaped “bolaamphiphiles” (Fig. 1, right) were found to assemble in a range of new structures, many of which can be described as honeycombs.3 The rods form the walls and the side chains fill the channels. In a recent study using grazing-incidence small-angle X-ray scattering (GISAXS) on beamline I07 at Diamond Light Source, researchers from Sheffield University and Martin Luther University in Halle have discovered a new type of liquid crystals.
By increasing the volume of the side-chain of the T-shaped molecules, the shape of the polygonal honeycomb cells can be changed from triangular, via rhombic, square, pentagonal all the way to stretched hexagonal, as more and more rods are required to encircle the cell (Fig. 2 a,b). A further increase in side chain volume, e.g. by attaching two instead of one side chain, honeycombs give way to layered structures (Fig. 2c). Unlike in the classic smectics (Fig. 1 left), here the rods lie in plane of the layers (Fig. 2c). The work highlighted here has gone a step further still, by replacing the linear side-chains with branched “swallow-tail” ones (Figs. 2d and 3a). Interestingly, such compounds exhibit the hexagonal columnar phase again. However, this time the structure is an inverted honeycomb, in fact it is closer to the classical columnar discotic, with the columns having a hard aromatic core and a soft aliphatic sheath. However, unlike the discotic, the new columnar phase is made up of bundles of around 10 rod-like molecules in cross-section, with the rods aligned parallel to the column. The flexible side-chains provide the surrounding continuum (Fig. 2d). The axial alignment of the rods is confirmed by the distribution of colours in polarized optical micrographs using an optical compensator (Fig. 3d).
Figure 1: Some types of liquid crystal phases and the schematic representation of molecules that form them.
At high temperatures the rod-like molecules are distributed evenly along the column length. However, at lower temperatures they gradually group together forming periodic modulation of the columns, as clearly seen in the electron density map in Figure 3e (pink), reconstructed from the X-ray diffraction pattern. In fact the structure changes from 2D periodic to 3D periodic, with rhombohedral symmetry. This means that column B is shifted by 1/3 of a molecule longitudinally relative to column A, and column C by 2/3 of a molecule. This information is obtained from GISAXS patterns from very thin aligned LC films on silicon wafer (Figs. 3b,c).
Figure 2: Some representative LC structures in bolaamphiphiles with increasing volume of the side chains. Blue: hydrogen-bonding glycerol groups; grey: aromatic rod-like cores; green: laterally attached chains.
In the compounds used in this work the two branches of the swallow tails are not the same – one is an aliphatic hydrocarbon, while the other contains a fluorinated Teflon-like segment (RF in Fig. 3a). These two branches tend to separate. For the high-temperature phase, Figs. 3e and f show schematically the distribution of the fluorinated regions (green) for a compound with a shorter and a longer RF segment, respectively. As the temperature is lowered into the rhombohedral phase, the fluorinated regions condense into right- and left-handed helices (Fig. 3h), with the blue-purple blobs showing the regions of maximum fluorine content (see also Fig. 3e).
Figure 3: (a) The compounds used in this work. (b) GISAXS pattern of a thin LC film at 155°C (hexagonal columnar), and (c) at 120°C (rhombohedral phase). (d) Optical micrograph of the columnar phase between crossed polarizers with a l-wave plate. (e) 3D electron density map of the rhombohedral phase: blue-purple are regions of highest density (fluoroalkyl), pink are regions of low density, i.e. the aromatic bundles in the centres of the columns. (f),(g) Schematic representation, based on electron density maps, of the hexagonal columnar phase in a compound with a shorter and a longer fluorinated segment RF, respectively (green: RF regions, grey: aromatic). (h) As in (f) but after cooling to the rhombohedral phase, showing RF helices.
This work illustrates the fact that there is much more yet to be discovered in liquid crystals. There has been a flurry of discoveries of new and ever more complex structures with great promise of new nanomaterials for optics and molecular electronics, photovoltaic, LED, ceramics templating and other applications.
Prehm, M., Liu, F., Zeng, XB., Ungar, G. & Tschierske. C. Axial-bundle phases – New modes of 2d and 3d helical columnar self-assembly in liquid crystals of bolaamphiphiles with swallow tail lateral chains. J. Am. Chem. Soc. 133, 4906-4916 (2011)
- Bushby, R. J. & Lozman, O. R. Discotic liquid crystals – 25 years on. Curr. Opin. Colloid Interf. Sci., 7, 343-354 (2002).
- Sergeyev, S., Pisula W. & Geerts, Y. H. Discotic liquid crystals – A new generation of organic semiconductors. Chem. Soc. Rev. 36, 1902–1929 (2007).
- For a recent review see Ungar, G. et al. Self-Assembly at Different Length Scales: Polyphilic Star-Branched Liquid Crystals and Mictoarm Star Copolymers. Adv. Funct. Mater., 21, 1296-1323 (2011).
This work was supported by the ESF Eurocores SONS2 program, project SCALES, funded by EPSRC and DFG.