A STUDY OF BENZOTHIOLINIUM CYANINE DYES AS A NOVEL CLASS OF SENSITIZING DYES

The properties and photographic behavior of benzothiolinium cyanine dyes have been studied and compared with those of thiacyanine and benzimidacyanine dyes to study the role of heteroatoms in cyanine dye molecules. The transition energy of a benzothiolinium cyanine dye, as determined by its absorption spectrum, was smaller than those of the thiacyanine and benzimidacyanine dyes. The height of the lowest unoccupied molecular orbital of a benzothiolinium cyanine dye, as estimated by its polarographic reduction potential and electronic energy levels calculated by the Hueckel method, was lower than those of the thiacyanine and benzimidacyanine dyes. In accord with this result, a benzothiolinium cyanine dye decreased the lifetime of photoelectrons in cubic AgBr microcrystals and depressed latent image formation on the surface of the microcrystals, acting as a trap for photoelectrons. Calculation has indicated that large heteroatoms in heteronuclei in a cyanine dye tend to localize an electron around the heteroatoms in its lowest unoccupied molecular orbital, lowering the height of its lowest unoccupied molecular orbital and thus decreasing its transition energy. Adsorption of a benzothiolinium cyanine dye to the microcrystals increased the concentration of interstitial silver ions in the microcrystals It is considered that sulfur atoms in a cyanine dye were positively charged and electrostatically repelled a silver ion at a kink site on the surface of the microcrystal into its interstitial position.