Dye desensitization studies: AgBr core—shell versus AgBr core—AgIBr shell emulsions

Abstract

Four 0.73 μm core-shell octahedral emulsions, sulphur-plus-gold sensitized either internally or on the surface, were used to study dye desensitization by anionic J-aggregating dyes. Two of the emulsions were pure AgBr and two contained AgBr cores with AgI0.01Br0.99 shells. Four dyes having a systematically varying electrochemical oxidation potential, Eox, were studied at 20, 40, 60, 80 and 100 per cent monolayer coverage. None of the dyes desensitized the internal image of either emulsion. However, desensitization of the surface-sensitized emulsions was observed, particularly for the dyes with Eox less than about 1.1 V (versus Ag/AgCl in acetonitrile), and it was greater for the iodobromide surface. Photobleaching studies showed that the dye bleaching rate was greater on the iodobromide surface. Spectral speeds for internal image formation as a function of dye coverage agreed with predicted speeds obtained from integrated absorption spectra for three of the dyes. However, the dye whose lowest unoccupied molecular orbital (LUMO) was the lowest in energy of all four dyes only agreed with the predicted spectral speed at lower dye coverages. Deviation from predicted spectral speed at higher dye coverage for this dye was attributed to the mean of its LUMO energy-level distribution being near the bottom of the conduction band. At low dye coverages formation of J-aggregates near ‘special sites’ populated the upper levels of this distribution, but additional aggregates formed at higher dye coverages populated the lower levels of the distribution. The higher desensitization for dyes adsorbed to the iodobromide surface suggested that the dye energy levels lie higher in energy on this surface than on the AgBr surface.