Absorption Spectra of Sensitizing Dyes Adsorbed to Silver Bromide and to Silver Iodobromide

Abstract

The reflectance spectra have been obtained for three thiacarbocyanine dyes adsorbed to silver bromide emulsion crystals having either 111 or 100 faces. The effects of digestion, crystal face, incorporated iodide, and the adsorbed organic stabilizer 2-methylthio-4-hydroxy-6-methyl-I,3,3a,7-tetraazaindene, on dye aggregation have been measured.On 111 faces the dyes form conventional M-, H- and J-bands, but the results are complex as the bands are extremely sensitive to changes in conditions: they are dependent on iodide, stabilizer and the time of addition of dye in relation to digestion. Digestion or incorporated iodide enhances J-aggregation, and iodide gives the J-aggregate a bathochromic shift of 5-10 nm. Both these effects can be explained by postulating that the 111 face is relatively rough, and has nucleation centres which initiate dye aggregation. Iodide provides particularly good nucleation centres.By contrast, on 100 faces the dye aggregation is simple in that the dyes form only two main peaks (517 nm and 595 nm), and these are unaffected by any of the variables mentioned above. The relative heights of the two peaks are also unaffected by changes in the dye concentration. The assignment of the peaks on this surface is more difficult but they are consistent with a herringbone structure for the dye aggregate with two molecules per unit cell.