Two-faced Liquid Crystals

John Goodby

A new class of programmable liquid crystals could be used to make variable optical filters for laboratory instrumentation and digital cameras; they might even be used to treat dyslexia.

Chemists John Goodby and Isabel Saez at the University of Hull in the UK, have for several years been developing novel approaches to liquid crystals with the aim of making them stable over a wider range of temperatures than current molecules. Compounds such as 5CB (pentyl cyano-biphenyl), which was the first commercially viable liquid crystal, led to such innovations as the liquid crystal display (LCD). The next generation of liquid crystals could contribute to other technological developments.

"Originally, we were researching in the area of liquid crystals with large discrete molecular structures," Goodby told Reactive Reports. "We worked mostly with dendrimers [branching polymeric molecules], with the objective of creating optical filters, and selective reflectors." The resulting chiral nematic giant liquid crystals had wide temperature ranges for their liquid crystal phases, from room temperature to about 80° Celsius. In their chiral nematic states, the materials have a helical macrostructure which acts like a diffraction grating and selectively reflect light just like a compact disc (CD) does.

	Two-faced liquid crystal structure

Goodby and Saez had also incorporated earlier octasilsesquioxane units into the middle of these materials making hybrid-inorganic-organic systems with the view of moving towards the development of photonic band gap materials. The next developments included building giant liquid crystals using buckyballs, C₆₀ fullerene moieties, which demonstrated that liquid crystals could be created with non-liquid crystal groups built in. "We realized then that we could do this with any structure," Goodby explains. "We could put a chemical unit that had a functional property into a giant liquid crystal and have it self-organise. This is effectively what proteins do, but we were doing it in an advanced materials sense!"

The next logical step was to use two different functional pieces and join them together to make what Goodby has called "Janus" materials, after the two-faced Roman god. Possible couplings might be fluorinated groups with hydrocarbons, hydrophilic and hydrophobic moieties, siloxanes, and hydrocarbons, etc, and in the present case, chiral and achiral halves.

The texture of a liquid crystal chiral nematic phase

The team has used two different dendritic halves - one carrying cyanobiphenyl-type mesogens which forms smectic (layered) liquid crystal phases and the other one with chiral phenyl-benzoate ester that normally forms chiral nematic phases. Both remain stable liquid crystals at high temperatures. Combined into a single giant material with a very low melting point, they have a much wider liquid crystal temperature range than the archetypal digital watch liquid crystal 5CB.

The new materials selectively reflect light and so, suggests Goodby, might be used in optical filters and coatings for laboratory instruments, cameras, and spectacles. "Such a system would also be of interest to pilots using up-head displays, above the cloud limit these are washed out by the sun, but with a dye included you would see the image much better," explains Goodby.

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There is perhaps greater potential still with these materials, if their self-organizational skills are taken into account. Proteins, although complex structures, also have functionality, they catalyse certain reactions for instance, but to do this they need flexibility. This may involve the self-organization of the natural liquid crystal environment. Membranes for instance are quasi-liquid crystals, and in fact most of the molecules found in biological cell membranes exhibit liquid crystal properties. The Hull researchers have created what might be the synthetic analog of this organizational ability. "Our materials open the door to the development of materials with protein-like properties, in that they self-organize and can have in-built functional properties - they are on the way towards becoming molecular machines."

Chem. Commun., 2003, 1726-1727; http://dx.doi.org/10.1039/b305152h