Liquid Crystals Align on Two Axes

  In work that has the potential to revolutionize liquid-crystal displays (LCDs), two teams of researchers have provided unequivocal evidence of the long-sought biaxial nematic liquid-crystal phase in boomerang-shaped molecules.

One team, led by chemistry professor Edward T. Samulski at the University of North Carolina, Chapel Hill, observed the phase in oxadiazole derivatives using polarized light microscopy and provided conclusive evidence of the biaxial structure by NMR spectroscopy [Phys. Rev. Lett., 92, 145505 (2004)]. Samulski carried out the work with postdoc Louis A. Madsen, Ph.D. student Theo J. Dingemans, and Michi Nakata, a visiting student from Japan.

“These are the first examples of conventional biaxial nematic LCs,” Samulski says. “All prior
nematic liquid crystals—thousands of them—have uniaxial symmetry.”

In collaborative work, a team headed by physics professor Satyendra Kumar at Kent State University, in Ohio, examined the low-angle X-ray diffraction patterns of Samulski’s oxadiazole derivatives and, in the same issue, confirm the biaxial nature of the LC phase [Phys. Rev. Lett., 92, 145506 (2004)].

Kumar’s group had provided preliminary evidence of the biaxial nature of this nematic phase in 2000, he notes.

The use of biaxial nematics could result in LCDs with faster refresh rates and dramatically lower power consumption, according to Madsen. “After a voltage is applied across an LC cell (the pixel element), it takes some time for the cell to respond and appear dark or bright,” he explains. “Biaxial nematics show promise to reorient more quickly in response to such a voltage.”

The faster response time should be possible because reorienting the molecules’ secondary alignment axis with an external electric field should require less energy than rotating the primary axis, which is what is currently done in LCDs.

“The lower symmetry of the biaxial nematic, in comparison with its well-known uniaxial counterpart commonly used in display devices, means that a greater number of variables is needed to describe its behavior, both static and dynamic,” comments liquid-crystals expert Geoffrey R. Luckhurst, a professor of chemistry at the University of Southampton, in England.

“This additional level of complexity is especially exciting because it is expected to open up new areas of fundamental physics and reveal novel applications for the biaxial nematic phase,” he adds.

At present, the biaxial nematic phases of the oxadiazole derivatives are observed only at temperatures near 200 °C.

Many thousands of known nematic LCs could theoretically exhibit a biaxial nematic phase, Samulski points out.

“The oxadiazole heterocycle in our mesogen [LC molecule] has a very large electric dipole moment and hence dipolar associations, normal to the primary [axis], that may reinforce the shape-driven interactions,” Samulski notes.

“According to theory, molecular shape alone should be sufficient for having a biaxial nematic. But the electrostatic ‘push’ might, in fact, be essential to see this amazing supramolecular arrangement in a fluid.”

Samulski is continuing to investigate whether the biaxial nematic phase can be stabilized in a useful temperature range—preferably room temperature.