NEXAFS spectroscopic study of organic photographic dyes and their adsorbed states on AgCl and AgBr

The near-edge X-ray absorption fine structure (NEXAFS) spectra were studied at the S K-edge using, synchrotron radiation for various sulfur-cantaining organic photographic materials in the solid stale and in the adsorbed states on AgCl and AgBr grains. The features in the spectra of neat solids were assigned by mutual comparison and polarization dependence. The spectra of merocyanine dyes containing thiocarbonyl group were drastically affected by the adsorption on AgCl and AgBr, with significant suppression of the peak corresponding to the excitation of the S 1s electron into the pi* orbital at the S=C bond. This change is interpreted as indicative that these compounds are adsorbed at the S atom in the thiocarbonyl group, such rearrangement of the chemical bonds leading to a contribution from the zwitterionic form with S--C=N+ < structure. By taking account of the area occupied by a molecule estimated from the measurements of the adsorption isotherm, a rather detailed picture emerged about the adsorbed state of this kind of dye. In contrast, the spectra of merocyanine and cyanine dyes containing sulfur in the five-membered aromatic ring showed little change by the adsorption on AgCl, indicating that the S atom is not the adsorption site or that the interaction between the molecule and AgCl is weak.