REDOX LIMITATIONS FOR THE SPECTRAL SENSITIZATION OF SILVER-HALIDE IN THE INFRARED

The efficiency of spectral sensitization of silver halide by cyanine d yes in the infrared region was found to be dependent on both the one-e lectron reduction and oxidation potentials of the dye. Correlation of sensitizing efficiency with reduction potential is expected, since thi s measurement reflects the electron-injecting ability of the dye's exc ited state. The dependence on oxidation potential results from the cor relation of this measurement with a dye's propensity toward oxidation in the liquid state of the silver halide microcrystalline dispersions used to prepare photographic coatings. Oxidative decomposition of thes e dyes in such dispersions may lead to destruction of dye and creation of species that reduce spectral sensitizing efficiency and lower over all photographic sensitivity. Infrared diffuse-reflectance spectrophot ometry was used to confirm that cationic tricarbocyanine sensitizing d yes undergo a one-electron oxidation in AgCl liquid dispersions to yie ld the corresponding dye radical dications. Correlations from spectra obtained for dye radical dications formed by electrochemical generatio n in acetonitrile solution were used to aid in identification of the d ye-related species present in silver halide systems. The rate of sensi tizing dye oxidation in a silver halide microcrystalline dispersion at 40-degrees-C was found to depend on the dye oxidation potential and o n the aggregation state of the unadsorbed dye.