Molecules that form liquid crystals are usually rod-like, but bend them and a new liquid-crystal phase — a biaxial nematic — should form. Strong evidence for the existence of this phase has only now emerged.
Liquid crystals form the basis of several flat-screen display technologies. The alignment of their rod-like molecules, in what is known as the ‘nematic’ phase, can be achieved rapidly and with low voltage. According to theory, these materials should also exist in another nematic phase with potential for use in display applications. In Physical Review Letters, Madsen et al.1 and Acharya et al.2 report firm evidence for this so-called biaxial nematic phase. The announcement has created considerable excitement, for it opens up new areas of both fundamental and applied research. It seems that a Holy Grail of liquid-crystal science has at last been found.
The hunt for this new liquid-crystal phase began more than 30 years ago, when it was recognized that the molecules forming liquid crystals deviate from their presumed cylindrical shape3. In fact, the molecules are more lozenge-like, and it is because of this lowering of the molecular symmetry that two nematic phases should be possible. In both phases, the molecular axes tend to align over large distances, although their centres of mass have no long-range order. In the common nematic phase, the long axes of the molecules tend to be aligned, in a direction known as the ‘director’, but the short axes are not ordered (Fig. 1a). This is known as the uniaxial nematic, because its properties are cylindrically symmetric about the director. The other nematic phase is predicted to occur at lower temperatures than the uniaxial nematic. In this instance, the molecular short axes tend to be aligned over large distances as well, giving a phase with three directors, about which three molecular axes tend to align. The phase has two optic axes (directions along which the optical properties appear to be cylindrically symmetric), hence its description as ‘biaxial’.
The design of molecules with sufficient biaxiality is a difficult task. There are many forms that the molecules could take, in addition to the lozenge, that deviate from a cylindrical shape. For example, a series of molecular shapes could be formed by linking rod-like, disc-like and semicircular moieties together4 (Fig. 1b). Such molecules would be long enough to exhibit a nematic phase and to deviate sufficiently from cylindrical symmetry to give a biaxial nematic. Indeed, the first claimed discovery of a biaxial nematic5 was for a compound formed of spoon-like molecules, and similar claims soon followed for cross-shaped6 and bone-shaped7 molecules.
Although such molecules should certainly exhibit biaxial nematics, the problem lies in identifying the symmetry of the nematic phase unambiguously8. It is now recognized that optical techniques commonly used to investigate other liquid-crystal phases may incorrectly identify a uniaxial nematic as biaxial. NMR spectroscopy seems to provide one of the most definitive tests. When applied to materials purportedly forming biaxial nematics, it has always shown the phase to be uniaxial9. This disappointing result indicates just how exacting the design criterion must be if the transition to the biaxial nematic is not to be blocked by the material freezing first. It is also frustrating, given that biaxial nematic phases are already known to form in classes of liquid crystal other than that used for display technologies (such as lyotropic10 and polymeric11 systems).
One design not previously explored was that of a V-shaped molecule, formed simply by bending a rod-like molecule. Such boomerang-like molecules should form a biaxial nematic9,12 (Fig. 1c) and, more importantly, theory has indicated how the stability of the elusive phase should depend on the angle between the two arms. Researchers have been synthesizing materials with bent molecules, and some had begun to suspect that the nematic phases formed were biaxial13. However, the definitive NMR experiments proved especially challenging and have only now been performed1. Although some of Madsen and colleagues' results1 are puzzling, others do indicate a small biaxiality for the nematic phase, in agreement with optical results. This identification is consistent with Acharya and colleagues' X-ray diffraction studies2, in which the minor director was aligned by applying an electric field.
The discovery has important implications. For example, it raises the question of the mechanism underlying the formation of the biaxial nematic. So far, this has been attributed to molecular shape. But for these boomerang molecules this seems unlikely, because their apex angle of around 140° is far from the predicted optimum value9,12 of 109°. Indeed, Madsen et al.1 recognize this and have suggested that strong electrostatic forces are crucial, at least for these materials, in stabilizing the biaxial nematic. This would be especially intriguing, for it could result in a nematic with ferroelectric properties.
What of the future? Clearly, theoreticians will want to think again about the molecular design of biaxial nematics. No doubt, given the controversial nature of previous claims, others will seek to test this latest subtle identification of the biaxial nematic phase. More importantly, the discovery will stimulate the search for other examples of biaxial nematics, especially those formed by V-shaped molecules. In fact, many such materials are already available, although produced for other reasons, and would certainly merit re-examination. The fundamental behaviour of this phase, both static and dynamic, must be explored, creating a new area of research in the field of liquid-crystal science and technology.
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