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DOUBLING FILM SPEED, RADICALLY Cation-based strategy exploits chemical potential stored in oxidized dyes
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one silver atom per absorbed photon. But in conventional film, three to 10 photons are required to generate one silver atom because of recombination of the photogenerated electrons and holes. In the Kodak approach, an additional electron donor molecule reacts with the oxidized dye, forming a radical cation," Gould tells C&EN. "This radical cation undergoes irreversiblefragmentationto produce a radical that donates a second electron to the silver halide."
novel photosensitization concept developed by researchers at East{man Kodak in Rochester, N.Y., and Arizona State University could double the speed of photographic materials. And that has "a number of practical advantages," says Joseph P. Dinnocenzo, a chemistry professor at the University of Rochester. "For example, it enables pictures to be taken under low-light conditions or without flash." This two-electron sensitization concept "provides a means to prevent elec-
Electron donor fragments irreversibly CH3 >-C02
©-< -co Electron-rich biscarboxylate
CH3
CH 3 -CO,
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Radical cation
tron-hole recombination and, because two electrons are injected per absorbed photon, has the potential for doubling the photosensitivity of silver halide dispersions," claim Kodak researchers Jerome R. Lenhard, Samir Farid, and their colleagues and Arizona State assistant professor of chemistry Ian R. Gould \J.Am. Chem. Soc, 122,11934 (2000)]. During the conventional photographic process, photons falling on photographicfilmscause halide ions to generate electrons paired with 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," which is then developed into a visible image. "The photoactive materials infilmsare silver halide microcrystals, which are made sensitive to visible light by dye molecules adsorbed to the crystal surface," Gould explains. "The primary image-capture process is donation of an electron from the excited dye to the silver halide." The theoretical limit of the process is
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