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"Gin's method is a signif nois researchers correctly icant advance that address anticipated they could devel One-pot method makes 2-acetamidosugars es simultaneously two of the op a method for nitrogen ο most difficult problems in transfer as well. "One of the II s oligosaccharide synthesis: key features is that an N-si(CF 3 S0 2 ) 2 0 installation of the 2-acetamilylated amide must be used RO do group that is found in to carry out the nitrogenR O ^ Y CH3CNHSi(CH3)3, (C2H5)2NC6H5 many carbohydrate struc transfer process," Gin says. tures of biological impor 'The use of unprotected pri OR tance, and construction of mary amides as nitrogen nu r >Nu glycosidic linkages at such cleophiles does not appear RO Acid 1 CH, 2-acetamidosugars," com to be effective." Nu-H RO^Y^N-jjCHs RO^Y""N ments Carolyn R. Bertozzi, Gin's group next plans to associate professor of chem OR OR extend the scope of nitrogen istry at the University of Cali substituents they can trans R = protecting groups; Nu = nucleophiles fornia, Berkeley. "Personally, fer to glycals. They also will I can say that my laboratory apply the reaction to other spends much time and effort navigating research efforts, which are oriented more electron-rich alkenes with the hope of de the synthesis of 2-acetamidosugars, and toward the biological side." veloping a method for asymmetric nitro we are ready to try this new method im Based on their previous work on oxy gen transfer, Gin tells C&EN. mediately; it will certainly accelerate our gen transfer to glycal substrates, the Illi Pamela Zurer
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Dopant boosts AgBr emulsion photosensitivity The use of a dopant may significantly im prove the performance of photosensitive silver halide emulsions used to make photographic films, according to scien tists in Europe. A research team has shown that the sensitivity of silver bromide emulsions increases by a factor of 10 when doped with formate ions (HC02"). The work was carried out by Jacqueline Belloni, a director of research, and co workers MonaTreguer and Hynd Remita at the National Center for Scientific Re search (CNRS), University of ParisSouth, and René De Keyzer, an R&D manager for the Agfa-Gevaert group in Mortsel, Belgium [Nature, 4 0 2 , 865 (1999)]. During the conventional photographic process, photons falling on silver halide microcrystals cause the halide ions to generate pairs of electrons and positive holes. The silver ions capture the electrons and the resulting neutral silver atoms group into clusters of metallic silver that form the "latent image." This image is then developed into a visible image. "One of the major inefficiencies in image recording with the silver halides used in conventional film is the recombination between photogenerated electrons and holes," explains Richard Hailstone, professor of imaging science at Rochester Institute of Technology, Rochester, N.Y. He points out in a commentary in Nature that to improve sensitivity modern films and print materials use silver halides that are chemically sensitized with reagents containing labile sulfur and gold atoms. Even so, recombination oc8
JANUARY 3.2000 C&EN
curs and, as a result, three to 10 photons are required to generate one silver atom. TTie theoretical limit is one silver atom per absorbed photon. To date, model systems that achieve this limit give rise to fogging caused by side reactions that lead to latent-image formation in the absence of light. "The novelty and importance of our work is that, for the first time, the doped crystal behaves as an ideal system," Belloni tells C&EN. "All the light energy is used to produce the latent image without electron recombination and without fogging." The dopant formate ions act as hole scavengers and produce C02*~ radicals in the process. These radicals transfer electrons to silver cations. The overall process is known as photoinduced bielectronic transfer because two electrons are generated, one by the photon and, subsequently, one by the radical. The theoretical yield for this process is two silver atoms per photon absorbed. According to De Keyzer, nanostructured silver halide materials represent one of the major classes of industrially produced microcrystalline ionic semiconductors. "Our discovery proves the concept of active hole trapping in ionic semiconducting materials and the total annihilation of recombination," he says. "The bielectronic nano-silver halide system leads to the highest light efficiencies ever measured." Hailstone notes that AgBr absorbs only blue light. Dyes are therefore added to silver halide emulsions to absorb red and green light. Belloni, De Keyzer,
and coworkers show that formate doping increases the sensitivity of not only AgBr but also green-dyed AgBr. "Improving the overall efficiency of color films might be possible if similar increases in sensitivity can be shown for red-dyed emulsions," Hailstone says. Michael Freemantle
One plasticizer okay, another still suspect, says federal panel At current exposure levels, a plasticizer widely used in polyvinyl chloride to make toys—diisononyl phthalate (DINP)— poses no risk to human reproduction or development, a federal advisory panel concluded during a mid-December meeting in Research Triangle Park, N.C. However, the panel did not give di(2-ethylhexyl) phthalate (DEHP) an equally clean bill of health. The panel will reconsider the fate of DEHP, which has been shown to leach from certain PVC medical devices, at a meeting in early 2000.
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DEHP, R : : 2-ethylhexyl DINP, R = isononyl
