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 mi crocrystal. 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 molecu les. Here we describe a new concept for increasing the efficiency of photog raphic systems, two-electron sensitization, which makes use of the chemical potential stored in the oxidized dyes. In conventional photography, subseq uent reactions of the oxidized dyes are not controlled and may in fact incl ude counterproductive return electron transfer reactions (recombination). I n the two-electron sensitization scheme, an appropriately designed electron donor molecule, X-Y, that is added to the photographic dispersion transfer s 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 sta ble cation, Y+. The radical X-. is chosen to be sufficiently reducing so th at it can inject an electron into the silver halide conduction band. In thi s way, the oxidized dye, which is a strong oxidant, is replaced by the radi cal, 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 crit eria for the X-Y molecules. Several electron-rich carboxylate molecules tha t meet these criteria have been identified. Solution-phase experiments to d etermine the fragmentation (decarboxylation) kinetics and the reducing powe r of the resultant radicals are described. Photographic data demonstrate th at increases in sensitivity by factors approaching 2 can be obtained, confi rming the viability of the two-electron sensitization concept.