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

Catalytic troublemaker

Porous solid catalysts are a mainstay of the modern chemical industry, allowing reactions that would otherwise take an age to progress to be run much, much faster. One group of such catalysts are the zeolites and particularly important among them is one known as ZSM-5, an aluminosilicate material with an MFI structure. However, despite its attractions, ZSM-5 can behave badly because its chemical building blocks do not join together perfectly. This leads to chemical starting materials on which the catalyst is to act often becoming stuck before they can get into the reactive pores and be converted into product. Now, Dutch scientist Marianne Kox has discovered the nature of the miniscule deviations that can make ZSM-5 such a troublemaker.

Catalytic ZSM-5 isn't always on its best behaviour (Credit: Nature Materials/Weckhuysen et al)

Catalysts are essential to the production of a vast array of pharmaceutical drugs, agrochemicals, fuels and countless other chemical products that are made from simple starting materials. Kox and colleague Lukasz Karwacki, together with researchers at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, Germany, ExxonMobil Chemical Europe Inc, Machelen, Belgium, the Centre for Nanoporous Materials, at the University of Manchester, UK, UOP LLC, a Honeywell Company, in Des Plaines, Illinois, USA, and Nicholas Copernicus University, Torun, Poland, have used a raft of spectroscopic techniques, on the micro scale to analyse the structure of zeolite ZSM-5 and have obtained spatial and time-resolved data on the three-dimensional interior of these porous materials. The data reveal the deviations from one porous unit to the next that can lead to reduced efficiency, catalytic poisoning, and unwanted chemical by-products.

Catalytic ZSM-5 (Credit: Nature Materials/Weckhuysen et al)

Kox is working as part of the Vici project run by Bert Weckhuysen, Professor of Inorganic Chemistry and Catalysis at Utrecht University in The Netherlands. Details of the research were published in Nature Materials. The team developed a new approach that correlates confocal fluorescence microscopy with focused ion beam–electron back-scatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped zeolite crystals of differing shapes, sizes, structures, and chemical compositions.

The powerful combination of techniques demonstrates “a unified view on the morphology-dependent MFI-type [zeolite] intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion,” the team say. “It has been found that internal-surface barriers originate not only from a 90° mismatch in structure and pore alignment but also from small angle differences of 0.5 to 2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nanometres.”

LINKS

Nature Mater, 2009, 8, 959-965
http://dx.doi.org/10.1038/nmat2530

Bert Weckhuysen

Water, water

Scientists in Canada have identified a toxic chlorination by-product of chlorination in tap water, dichloroquinone, using a newly developed procedure based on liquid chromatography (LC), electrospray ionization (ESI), and tandem mass spectrometry (tandem-MS). The levels of the compound may be present at a few nanograms per litre, which the researchers suggest may represent a bladder cancer risk as well as being associated with adverse reproductive effects.

Untreated water can carry several potentially lethal diseases, including typhoid, dysentery, cholera, and diarrhoea. Treatment and disinfection through chlorination usually renders water safe to drink and helps keep these illnesses at bay, at least in those parts of the world where chlorination facilities exist.

However, some scientists are concerned that the chlorination of water itself may be a risk factor for more insidious diseases. For instance, some studies have suggested that there may be a link between drinking chlorinated tap water and an increased risk of bladder cancer. Now, researchers at the University of Alberta, in Canada, have identified a chlorination by-product that may represent a risk factor, dichloroquinone.

Xing-Fang Li group
Xing-Fang Li group

Xing-Fang Li and colleagues explain that common reactions between chlorinating agents and natural organic molecules in water are known to produce tiny quantities of chloroform and chlorinated acetic acid derivatives. The presence of these compounds in drinking water are tightly regulated but the team suspected that there are other compounds formed in disinfected tap water that are not, among them chlorinated quinones. The researchers point out that concentrations of such compounds were below detection thresholds previously but are now under suspicion.

Quinones are organic molecules containing a six-membered carbon ring to which are attached two oxygen atoms bound by double bonds on opposite sides of the ring; microbial activity can lead to the presence of these compounds in water. Earlier, independent work suggests that halogenated quinones, those containing chlorine or bromine, can react with DNA and proteins even at very low concentrations and cause damage to these critical biomolecules. The Canadian team has now successfully identified a representative of this class of compounds, 2,6-dichloro-1,4-benzoquinone, in chlorinated drinking water.

Chlorination graph
The chemistry of clean water

“The scientific understanding of drinking-water quality has advanced substantially since trihalomethanes [e.g. chloroform] were first discovered [in water] in 1974,” the team says. “Effective management of disinfection by-products health risks requires better knowledge of disinfection chemistry combined with toxicology,” the researchers add.

LINKS

Angew Chem Int Edn, 2010, in press
http://dx.doi.org/10.1002/anie.200904934

Fang group