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decade that a liquid-crystalline nematic phase can be transformed into a cholesteric phase by addition of a small amount of an optically active molecule. In the nematic phase, the threadlike molecules of the liquid crystal are aligned along a common direction, like a school of fish traveling together. The cholesteric phase is actually a twisted nematic phase—that is, the molecules are arranged in nematic layers, but in each layer, the average orientation of the molecules is twisted relane of the holy grails sought by sci- length, a similarly small excess of right- tive to the adjacent layers so as to give the multilayered stack a helical configuraentists is a way to switch a liquid handed helices is produced. crystal between two states using a The Groningen chemists envision an tion—like a spiral staircase. Instead of using a single enantiomer single wavelength of light. Now, a team of optical data storage system based on chemists in the Netherlands has succeed- theirracemicalkene. The two mirror-im- as the dopant, the Dutch researchers ed, although in a technically marginal and age isomers represent the 0 and 1 states added 20% by weight of the racemic alnot very practical way. But it's a start. of a binary logic system. Writing would kene to a conventional liquid crystal. The Dutch group, led by chemistry be done by irradiating theracematewith When a thin film of this doped nematic professor Ben L. Feringa of the Universi- CPL. Written information would be phase was irradiated with left CPL for 90 ty of Groningen, learned how to make a erased using unpolarized or plane- minutes, the dopant became enriched in helical molecule twist from left to right polarized light at the same wavelength. the left-handed enantiomer, and a leftby changing the handedness of light And data would be read using plane- handed cholesteric phase formed. A shining on it. This is a well-known prin- polarized light at a different wavelength. right-handed cholesteric phase was obciple that has been used with other molWith this optical switching process in tained usingright-handedlight. When the cholesteric phases obtained ecules. The team then put these molecu- hand, the group moved to the next lar gyrations to good use: When the heli- stage: exploring its potential as a "chi- in this way were exposed to unpolarized cal molecules were added to a liquid roptical" trigger for switching between or plane-polarized light, the nematic phase returned. The researchers could switch crystalline film, they served as a light-ac- liquid-crystalline phases. tivated trigger to switch the film's molecScientists have known for more than a between nematic and cholesteric phases by switching between ular organization from racemic to chiral, " - 1 " - — ^ ~ ^ plane and circularly thus altering the film's appearance [Science, 273, 1686(1996)]. polarized light. Circularly polarized light changes This capability might one day be harLast year, Feringa molecule's chirality nessed for use in liquid-crystal displays, and coworkers reportoptical data storage systems, and other ed on a related alkene applications. However, Feringa stresses that could trigger chiLeft circularly to C&EN, the findings just reported are roptical switching bepolarized light fundamental and "still rather far from tween nematic and practical applications." cholesteric phases. But Right circularly polarized light that system required The chiral molecule central to Feringa's light of two different latest work consists of two related polycywavelengths [/. Am. clic units connected by a carbon-carbon Left-handed Right-handed Chem. Soc, 117, 9929 double bond. This alkene would like to lie helical isomer helical Isomer (1995)]. flat, but steric interactions between the Although Feringa's two bulky polycyclic units give it a slight latest system works twist. Two opposite helicities (mirrorwith a single wavelength, it has some big image isomers) exist, and when exposed problems. One is the rather laige amount to polarized light, they can interconvert Liquid-crystal phases of racemic dopant—compared with an through an excited state. optically active dopant—needed to obFeringa and graduate students Nina P. serve the cholesteric phase. M. Huck, Wolter F. Jager, and Ben de Another problem is the 0.07% ee. Lange discovered that when a solution of Chemistry professor Mark M. Green of the racemate (a 50/50 mixture of the Polytechnic University in Brooklyn, N.Y., two enantiomers) is irradiated with left notes this is very small and leads to only circularly polarized light (CPL) at 313 a tiny amount of twisting in the cholesnm, a tiny fraction of the right-handed teric phase. The twisting is almost imhelices isomerizes to the left-handed variNematic phase measurable, he points out, but it's "a beety. The enantiomeric excess (ee) of ginning toward something that may have 0.07% is unexpectedly small, they note. Cholesteric phase great practical importance.'' Likewise, when the solution is illuminatMuch better triggers have been obed with right CPL of the same wave-
MOLECULES FLIP WITH CHIRAL LIGHT Chemists use single-wavelength light to switch between liquid-crystal phases
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tained by chemistry professor Gary B. Schuster of Georgia Institute of Technology, Atlanta, who has been pursuing a similar approach for several years. Using chiral bicyclooctanones, Schuster and coworkers have achieved an ee of about 2%—"which is enormous" by comparison, Green says. It's probably the highest ee anyone will be able to attain in these systems, he adds. But Schuster's group hasn't yet observed any twisting to form the cholesteric state using these triggers. Even 2% ee, though, is small in absolute terms. So both Feringa and Schuster are interested in using a chiral helical polymer discovered by Green's group [Science, 268, I860 (1995)] to amplify the chirality of their trigger molecules. By attaching photoswitchable groups to such a polymer, they hope to produce a larger and more useful switching effect. Ron Dagani
Association may collect organic chemical data With the final government report on U.S. organic chemical production just issued, the National Petroleum Refiners Association is surveying its members to see whether NPRA should start collecting some of the abandoned statistics. Since 1917, the International Trade Commission (TTC) has published its quarterly Synthetic Organic Chemicals report and an annual reference listing U.S. producers, output, and sales. But last year, the House Ways & Means Committee ordered an end to the report by Oct. 1, 1996, as part of the Republican-led Congress' efforts to cut federal spending (C&EN, Dec. 4, 1995, page 7). ITC's final quarterly report covers 92 chemicals for April through June 1996. Annual figures will not be available for 1996, and 1995 data will not appear in a finalized form. Since 1971, NPRA has gathered production and supply-and-demand balance data on petrochemicals, based in part on ITC data. NPRA's petrochemical statistics subcommittee now is asking association members and others whether to continue and to increase data gathering to make up for ITC's phaseout. ITC data "was used in developing the benzene, ethylene, and propylene balances," says NPRA. "We are faced with the prospect of losing the information needed to continue to report these balances, or collecting much of this data ourselves." 18 SEPTEMBER 23, 1996 C&EN
Responses to NPRA's query are due produces the products we report." And Oct. 1, and its subcommittee will meet in data are collected by outside specialists and kept confidential. November to discuss the results. However, Petersen warns, as producNPRA says it will cost about $40,000 to add 20 chemicals and groups of chem- tion goes downstream from NPRA memicals to the 10 chemicals in its current bers, data will be harder to collect. And Lenz notes that ITC was able to require survey, which costs $100,000. "Aside from consultants, the industry responses, which no association or priis just starting to realize what's happen- vate analyst can. Elisabeth Kirschner ing," says Bruce L. Petersen, NPRA petrochemical director. ITC "was the only source of a number of production statistics for planners, business managers, and market researchers," notes Allen J. Lenz, director of economics at the Chemical Manufacturers Association (CMA). However, says Petersen, "if the data are important to the industry, ways will be found. We'll probably cover some BP Chemicals—whose current technolomore [products], consultants will cover gy is used by it and its licensees to proothers, and perhaps interest groups will duce more than 95% of the world's acryhire somebody to fill their needs." He lonitrile—has unwrapped a new process cites, for example, polymer production to make the important fiber and plastics data, long available from the Society of building block. The Cleveland-based company has worked out a much more the Plastics Industry. "We have good confidence in our economical route that starts with pronumbers," he adds. "We work very hard pane instead of propylene. Ammoxidation of propane—reaction to make sure we have everybody who of propane, ammonia, and oxygen to yield acrylonitrile—holds the promise of lowering production costs 20% comNPRA may add 20 chemical pared with ammoxidation of propylene, according to BP Chemicals. The compagroups to current survey ny's economic analyses are based on conservative U.S. Gulf Coast conditions, Current survey: where propane has enjoyed a long-term Acrylonitrile Benzene average cost advantage over propylene Butadiene of 7 to 9 cents per lb. Increased producCumene tion of valuable coproducts and lower Ethylene waste add to the economic advantages of Ethylene oxide Propylene the process. Styrene In timing that was mostly coincideno-Xylene tal, BP Chemicals announced its breakp-Xylene through on Sept. 13 at its Warrensville Proposed additions: Heights, Ohio, laboratory at ceremonies Acrylic acid designating the company's propyleneAniline Chlorobenzene based process a National Historic ChemiCyclohexane cal Landmark. It is the 11th such landEthylene-propylene rubber mark to be named under the American Ethylbenzene Chemical Society program. Ethyl chloride The earlier technology dates back to Ethylene dichloride (exports) Halogenated hydrocarbons 1957, when the company was Standard Isopropyl alcohol Oil Co. of Ohio, or Sohio, before its acquiLinear olefins and alcohols sition in 1987 by British Petroleum. ExperOligomers iments at the Sohio lab led to the fluidizedOxo chemicals (includes n-butyl alcohol and isobutyl alcohol) bed process based on a bismuth phospho2-Ethylhexanol molybdate catalyst that has now garnered Low-density polyethylene some 35 licensees in 20 countries. High-density polyethylene As in the propylene-based approach, Polypropylene the key to the new process is the catalyst Propylene oxide Vinyl acetate system. Frank Kocjancic, project managVinyl chloride er for business development at BP Chemicals, explains that the catalyst for the
Propane route to acrylonitrile holds promise of savings
