Metallic liquid crystals

A new class of materials formed by combining liquid crystals and metal clusters glow intensely red in the infra-red region of the electromagnetic spectrum when irradiated over a broad range of wavelengths. The materials, dubbed clustomesogens, could be used in analytical instrumentation and potentially in display technologies.

Liquid crystals are well known in display technologies from digital watches to flat panel televisions. As their name suggests, they are at once liquid and can flow, but their molecules can also be oriented into something akin to a crystal state, usually under the influence of an electric field.

A second class of materials of interest to the optoelectronics field is metal clusters. Clusters are aggregates of just a few atoms, and so their properties are not those of individual atoms nor of the bulk metal, but somewhere in between. Indeed, metal clusters show some rather unusual electronic, magnetic, and optical properties because of the presence of the particular types of bonds that form between metals when just a few are present.

Now, Yann Molard, of the University of Rennes, in France, and colleagues there and at the University of Bucharest have united the two classes in clustomesogens to create metal clusters that exist in a liquid-crystalline phase.

Liquid crystals containing bonds between metal atoms are rare and usually limited to compounds in which just two metal atoms are connected in each unit. Molard and colleagues have produced liquid crystals that contains octahedral clusters made of six molybdenum atoms. Eight bromide ions sit on the eight surfaces of the octahedron, six fluorides and an aromatic organic group, or ligand, is at each vertex of the octahedron. These aromatic ligands each have three long hydrocarbon chains also ending in a pair of aromatic rings.

Yann Molard
Yann Molard

Simple warming these materials initiates a process of self-organization in which the clusters stretch out to form long, narrow units arranged in what is known as a lamellar, plate-like, structure. The flat rings at the ends of the ligands of neighbouring layers are interleaved and the structure has liquid-crystalline properties.

“The association of mesomorphism with the peculiar properties of metallic clusters should lead to clustomesogens that offer great potential in the design of new electricity-to-light energy conversion systems, optically based sensors, and displays,” the team says.

Links

Angew Chem Int Edn, 2010, 49, 3351-3355
Yann Molard homepage

Bringing a sense of order to plastics

US chemists are using liquid crystals as templates to help them synthesise novel plastics that conduct electricity. The technique could be used in the leap from laboratory to mass production of polymer-based components for displays, foil-batteries, and microelectronics devices.

Conducting polymers were first discovered in the 1970s by Alan Heeger, Alan MacDiarmid and Hideki Shirakawa, which won the three a share in the 2000 Nobel Prize for Chemistry. They and technologists around the world were quick to realise that polymers would have many advantages over the conventional semiconductor materials used for electronics components. For instance, they would be operable at lower, more efficient, voltages, they would be easily processable, into devices of almost any shape, and would ultimately be much cheaper to manufacture in bulk because of their ease of processibility.

Liquid crystals

Liquid crystals

Plastic electronics was to be the revolution of the turn of the century and once it was found that many conducting polymers also glowed the possibility of a flat-screen TV that could be rolled up in your pocket based on flexible conducting polymers was the classic image of chemistry in action. But, and there is always a but, the electrical conductivity of polymers has not yet reached the levels needed for the wide range of potential applications. This is due in part to structural disorder – a problem that prevails in plastics.

Now, Samuel Stupp and doctoral candidate James Hulvat at Northwestern University in Evanston (USA) have developed a new technique that forces conducting polymers into an ordered structure, which could overcome this obstacle.

Sam Stupp

Sam Stupp

There are many research teams around the world looking for ways to bring order to polymers. Stupp and Hulvat have concentrated their research on one particular group of conducting polymers, the polythiophenes, which the say are the most industrially important class of conducting polymers. They have used the self-organising power of liquid crystals to provide a template in which these polymers can be synthesised with an underlying order not possible in a free reaction.

Liquid crystal structures

Liquid crystal structures

Liquid crystals are fluid phases in which the individual particles have some degree of order as if they were in a solid crystal, which means they are structured. The US team produced a liquid crystalline gel composed of tiny, mutually parallel, hydrophobic (water repellent) cylindrical units, which are suspended in a hydrophilic (water loving) environment. They dissolve the building blocks for the polymer in this gel.

Liquid crystals showing how order generated

Liquid crystals showing how order generated

The building blocks, monomers, are themselves hydrophobic so they remain exclusively within the hydrophobic cylinders. The polymerisation reaction is kick-started with an electric current and the monomers start to link up to form long polymer chains. In a free solution, such polymer chains would kink and coil forming a conglomeration of random chains, reminiscent of a tangled bowl of spaghetti. However, the template cylinders keep the growing chains on the straight and narrow preventing them from becoming entangled. Once the gel is removed the polymeric material retains its order.

Our very simple new method could help in the production of conducting plastics with improved electronic properties, says Stupp. After the liquid crystals template is washed away, the polymers films remain stuck to the surface, an electrode, on which they are formed. Interestingly, these polymeric films essentially ‘copy’ the liquid crystal texture, revealing birefringent domains that match those of the liquid crystal medium, say the researchers. They suggest that the polymerization takes place within the confined nanoscale environment of the liquid crystals’ hydrophobic cores and produces polymer chains oriented parallel to the direction of the liquid crystal order.

The science and technology of low cost organic electronics could be advanced significantly by utilizing self-assembly processes to pattern and control the nanostructure of conducting polymers, adds Hulvat, this could simplify fabrication and improve efficiency of organic light-emitting diodes (OLEDs), organic field effect transistors and other devices.

Further reading

Angew. Chem. Int. Ed. 2003, 42(7), 778-781
http://www3.interscience.wiley.com/cgi-bin/abstract/103019628/START

DOI: 10.1002/anie.200390206

2000 Nobel Prize for Chemistry
http://nobelprize.org/nobel_prizes/chemistry/laureates/2000/

Samuel Stupp
http://www.matsci.northwestern.edu/faculty/sis.html

Suggested searches

Conducting polymers
Liquid crystals
Polymerisation

Batting around molecular shuttlecocks

A molecule shaped like a badminton shuttlecock with a buckyball molecule at the base and five rod-like compounds forming the feathers has been made by Japanese chemists. This spectacular molecule stacks with others of its kind, just like a pile of real-life shuttlecocks. However, on the molecular scale these shuttlecocks display liquid crystal behaviour and could find use in new organic-based electronics devices.

Liquid crystals (LCs) flow like fluids, but have regularity and the direction-dependent properties more commonly seen in solid crystals. These characteristics have been brought to bear in a number of devices, such as flat-panel displays (LCDs). An applied electric field is used to change the organization of the liquid crystals and display an image.

The shuttlecock

The shuttlecock

Materials scientists continue to look for novel liquid crystals with potential as non-linear optical devices for fibre-optic communication and tuning lasers and other technologies. Now, Eiichi Nakamura of the University of Tokyo and colleagues have synthesised a whole new class of liquid crystals.

Their new LCs are based on the soccer-ball shaped all-carbon molecule known colloquially as the buckyball. Buckyballs, or fullerenes to give them their proper name, represent the third major form, or allotrope, of carbon after graphite and diamond.

3D shuttlecock structure

3D shuttlecock structure

Nakamura and his team have used the [60]fullerene molecule as the base of a shuttlecock-shaped molecule that display liquid crystal behaviour.

Stack ’em high 3D structure

Stack ’em high 3D structure

The molecular shuttlecocks, with their idealistically conical shape, stack together to form columns. The molecules stack together because of attraction between the spherical fullerene base and the cone-shaped part of the next molecule in the stack. By attaching feather-like units with different chemical properties the researchers can make different versions of their liquid crystals.

According to Carsten Tschierske of the Martin-Luther-University Halle-Wittenberg, Halle Saale, Germany, writing in the journal Nature, 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 have undoubtedly provided a new design principle for research teams to play with. He adds that, The polar order within the columns is especially interesting and could possibly lead to macroscopically polar structures which could be of interest for nonlinear optics and other applications.

Nakamura adds, In combination with our recent work on the synthesis of bucky ferrocene, we expected that the present technology would be provide a fundamental tool for making electronic, magnetic and optoelectronic devices.

Further reading

Nature, 419, 702 (2002)
http://dx.doi.org/10.1038/nature01110

Eiichi Nakamura
http://www.adm.u-tokyo.ac.jp/IRS/IntroPage_E/intro60968724_e.html

Carsten Tschierske
http://www2.chemie.uni-halle.de/org/ak_tschierske/

Suggested searches

Liquid crystals
Fullerenes