Two-Electron Sensitization: A New Concept for Silver Halide Photography

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• 作者：Ian R. Gould ; Jerome R. Lenhard ; Annabel A. Muenter ; Stephen A. Godleski ; and Samir Farid
• 刊名：Journal of the American Chemical Society
• 出版年：2000
• 出版时间：December 6, 2000
• 年：2000
• 卷：122
• 期：48
• 页码：11934 - 11943
• 全文大小：128K
• 年卷期：v.122,no.48(December 6, 2000)
• ISSN：1520-5126

文摘

The primary process in conventional photography involves electron transfer from an excited dyemolecule into the conduction band of a silver halide microcrystal. Repeated events of this type ultimately leadto formation of a small, stable cluster of silver atoms in the silver halide that acts as the latent image, alongwith the one-electron oxidized forms of the dye molecules. Here we describe a new concept for increasing theefficiency of photographic systems, two-electron sensitization, which makes use of the chemical potentialstored in the oxidized dyes. In conventional photography, subsequent reactions of the oxidized dyes are notcontrolled and may in fact include counterproductive return electron transfer reactions (recombination). In thetwo-electron sensitization scheme, an appropriately designed electron donor molecule, X-Y, that is added tothe photographic dispersion transfers an electron to the oxidized dye to give a radical cation, X-Y^{ages/entities/bull.gif">+}. TheX-Y^{ages/entities/bull.gif">+} then undergoes a fragmentation reaction to give a radical, X^{ages/entities/bull.gif">}, and a stable cation, Y⁺. The radical X^{ages/entities/bull.gif">}is chosen to be sufficiently reducing so that it can inject an electron into the silver halide conduction band. Inthis way, the oxidized dye, which is a strong oxidant, is replaced by the radical, X^{ages/entities/bull.gif">}, which is a strong reductant.The two-electron transfer scheme has the potential of doubling the photographic speed because two electronsare injected per absorbed photon. Here we describe the mechanistic details of the two-electron sensitizationscheme and the structural and energetic criteria for the X-Y molecules. Several electron-rich carboxylatemolecules that meet these criteria have been identified. Solution-phase experiments to determine thefragmentation (decarboxylation) kinetics and the reducing power of the resultant radicals are described.Photographic data demonstrate that increases in sensitivity by factors approaching 2 can be obtained, confirmingthe viability of the two-electron sensitization concept.