Two-electron sensitization: A new concept for silver halide photography

The primary process in conventional photography involves electron transfer from an excited dye molecule into the conduction band of a silver halide microcrystal. Repeated events of this type ultimately lead to formation of a small, stable cluster of silver atoms in the silver halide that acts as the latent image, along with the one-electron oxidized forms of the dye molecules. Here we describe a new concept for increasing the efficiency of photographic systems, two-electron sensitization, which makes use of the chemical potential stored in the oxidized dyes. In conventional photography, subsequent reactions of the oxidized dyes are not controlled and may in fact include counterproductive return electron transfer reactions (recombination). In the two-electron sensitization scheme, an appropriately designed electron donor molecule, X - Y, that is added to the photographic dispersion transfers an electron to the oxidized dye to give a radical cation, X - Y·⁺. The X - Y·⁺ then undergoes a fragmentation reaction to give a radical, X·, and a stable cation, Y⁺. The radical X· is chosen to be sufficiently reducing so that it can inject an electron into the silver halide conduction band. In this way, the oxidized dye, which is a strong oxidant, is replaced by the radical, X·, which is a strong reductant. The two-electron transfer scheme has the potential of doubling the photographic speed because two electrons are injected per absorbed photon. Here we describe the mechanistic details of the two-electron sensitization scheme and the structural and energetic criteria for the X - Y molecules. Several electron-rich carboxylate molecules that meet these criteria have been identified. Solution-phase experiments to determine the fragmentation (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, confirming the viability of the two-electron sensitization concept.