| RESEARCH |
Nature put a hex on fives. Hexagon shapes, such as those found in beehive honeycombs, are common motifs in natural materials. But not pentagons. It's been known for ages that the geometry of regular five-sided objects prevents them from tiling a flat surface. Now, researchers in Europe have shown that custom-made molecules can assemble into liquid-crystal networks based on pentagonal honeycombs and other unusual patterns. The study presents new methods for controlling supramolecular architectures and eventually may lead to new types of materials used in liquid-crystal displays and related technologies. The shapes of molecules and incompatibility of molecular segments are two key factors that drive organization in liquid-crystal phases. Many liquid-crystal systems are based on molecules that feature a fairly rigid aromatic section surrounded by flexible molecular chains. That configuration often leads to an array of circular columns in a lattice with hexagonal symmetry. ORDERLY An optical micrograph (left) reveals the texture of liquid crystals with novel structures that form from tailor-made T-shaped molecules (bottom). In a related compound, alkyl chains assemble in triangular and square-shaped regions (yellow and green) and polar segments form pentagons (blue-purple), as seen in an electron-density map (right). To prepare the new types of honeycomb structures, the European researchers designed molecules that turn the common liquid-crystal structures inside out. In the latest work, rigid rodlike sections form the outer frame of polygons, and the interior is filled with flexible molecular chains of predetermined length. According to the team, the idea is to control the chain length--and hence the internal dimensions of the polygons--such that each polygon's perimeter works out to be an integral number of rod lengths. A perimeter equal to four rod lengths should result in square cylinders. A 5-rod-length perimeter should yield pentagonal columns, and so forth. And that's just what the group finds. Guided by their earlier work with liquid-crystals based on triblock molecules, the team made T-shaped molecules consisting of three incompatible segments: a rodlike aromatic core, a pair of terminal lipophilic and flexible alkyl chains (the top of the T), and a polar group (the body of the T). By varying the volume fractions of polar and nonpolar segments and controlling the temperature, the group observed several liquid-crystal phases, which they probed by using a combination of microscopy, X-ray diffraction, and computational procedures. In one case, the group determined that the liquid crystal was composed of pentagonal sections of polar chains separated from one another by triangular and square-shaped segments containing alkyl chains. A related compound formed square-shaped and triangular columns in a 2:1 ratio. In classical geometry, perfect pentagons cannot be used to tile a plane. But liquid crystals bend the rules--and the structures--slightly. the periodic organization of pentagonal cylinders observed in this case "is a result of the unique combination of order and mobility in the fluid state of liquid crystals. That fluidity allows the pentagons to be deformed slightly, which is required for this type of regular tiling." |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |