Materials chemistry

Liquid crystals stack up

Take a spherical carbon 'buckyball', feather it with rod-like molecules, and the result is a distinctive shuttlecock shape that can easily be stacked into columns. Liquid-crystal phases thus formed should have unusual properties.

Liquid crystals (LCs) are materials that can flow like fluids, but also have some of the regularity and direction-dependent properties of crystals. This combination of order and mobility enables these systems to respond to external stimuli, such as an applied electric field, and change their organization. For this reason, they are ideal components for flat-panel devices in computers, cellular phones and televisions, but numerous future applications in nonlinear optics, molecular electronics and molecular photonics are conceivable1. On page 702 of this issue, Sawamura et al.2 describe a new type of liquid-crystalline material. They have synthesized molecules that are shaped like a badminton shuttlecock and stack together in a directed manner.

Conventional LC materials are mostly formed by rod-like or disc-like molecules that have flexible chains attached to rigid cores (Fig. 1a, b). In most cases, rod-like molecules organize themselves into layers (known as smectic LC phases), whereas disc-shaped molecules form columns (columnar LC phases)1. The chains are fluid and provide the necessary mobility which, in particular temperature ranges, inhibits the formation of solid crystals.

Figure 1: Liquid-crystal phases.
figure1

a, Rod-like molecules organize themselves into layers, whereas, b, disc-like molecules form columns that can be arranged parallel to each other in a two-dimensional lattice. c, A bend introduced in the rigid core leads to 'banana-shaped' molecules. The rotation of these molecules around their long axis is restricted and they adopt a directed order within the layers. Depending on the bending direction in adjacent layers, either antiferroelectric or ferroelectric smectic phases may result. d, Molecules with a conical shape can lead to a polar order within columns. The polar direction of neighbouring columns may be parallel or antiparallel. e, Sawamura et al.2 have made a 'shuttlecock-shaped' molecule based on the C60 molecule, whose distinctive shape leads to directed organization in columns.

If the symmetry of such molecules is reduced, different LC phases can arise. For example, the introduction of a bend of about 110–140° at the centres of rod-like molecules leads to 'banana-shaped' molecules (Fig. 1c) which can organize to provide LC phases not found in linear rod-like molecules3. When such bent-core molecules organize in layers, their specific shape gives rise to a directionality of alignment, causing a polar order in each of the layers. This polar order and the reduced phase symmetry yield extremely interesting phenomena, such as the spontaneous formation of chiral and helical superstructures, and ferroelectric or antiferroelectric properties3,4,5,6. In ferroelectric phases, the polar directions of neighbouring layers point in the same direction, leading to macroscopic polar order, whereas in antiferroelectric phases the polar direction in adjacent layers is opposite, so that the polar moments are cancelled (Fig. 1c). Such ferroelectric and antiferroelectric phases are potentially useful, because they can be switched by electric fields between distinct states.

Several attempts have been made to realize polar order in columnar LC phases7,8 — which could be expected if the flat shapes of disc-like molecules were bent into bowl-like or hollow cone shapes (Fig. 1d). Sawamura et al.2 provide a new concept for the design of hollow cone molecules, using the buckminsterfullerene molecule (C60)9 as the central unit. C60 has special redox and photophysical properties and so there is great interest in incorporating it into LC materials10. Sawamura et al. attached five rod-like aromatic units, each bearing two flexible non-aromatic chains, to one side of the C60 molecule to form a cone-shaped molecule resembling a shuttlecock (Fig. 1e).

The C60 apex of each of these molecules fits perfectly into the cavity of a neighbouring molecule. As a result, the molecules prefer to organize head-to-tail in columns, and the columns themselves organize parallel to each other in a regular manner, forming a hexagonal two-dimensional lattice. The flexible chains at the periphery fill the space between the columns and provide the necessary mobility for LC formation. But, in contrast to polar layers of banana-shaped molecules which can easily adopt a macroscopically non-polar antiferroelectric arrangement by alternation of the polar direction of adjacent layers (Fig. 1c), the organization of polar columns in a hexagonal two-dimensional lattice is more complex. There are many ways for the polar columns to be arranged (examples are shown in Fig. 2), but it is not possible to arrange them in such a manner that each column only has neighbours of opposite polar direction. Hence, for these systems many new polar LC phases could be expected.

Figure 2: Examples of hexagonal lattices with polar columns (coloured green and blue for the two orientations).
figure2

Note that it is not possible to arrange the columns in such a manner that each polar column only has neighbours of the opposite polar direction.

For such polar LC phases to be exploited in switching processes, the molecules must be mobile enough to respond quickly to external stimuli. High viscosity is a general problem of columnar phases, but it can be much reduced by adding solvents that dissolve around and between the flexible chains11,12. As well as reducing viscosity, this can lead to a change of the organization in the LC phase: in the case reported by Sawamura et al.2, the hexagonal columnar LC phase was replaced by a columnar 'nematic' phase at higher solvent concentrations and temperatures. In this phase, the long-range hexagonal lattice is lost and the columns are only on average aligned parallel to each other. Nevertheless, these nematic phases may also have special properties resulting from the polar order within the columns13.

Many of the advances in LC research have been stimulated by fresh designs of molecules that form new LC phases. In their 'shuttlecock' molecules, Sawamura et al.2 have undoubtedly provided a new design principle for research teams to play with.

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Correspondence to Carsten Tschierske.

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Tschierske, C. Liquid crystals stack up. Nature 419, 681–683 (2002). https://doi.org/10.1038/419681a

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