Alkanediyl bridged benzimidazolo monomethine cyanine dyes, processes for their preparation, and photographic emulsions and elements containing such dyes

摘要

Alkanediyl bridged benzimidazolo monomethine cyanine dyes wherein at least one benzimidazolo nucleus is provided with an electron-withdrawing group are photographically useful e.g., useful as blue sensitizers in silver halide emulsions and photographic elements. A disclosed preparation of the dyes includes producing an electron-withdrawing group substituted bis(2-nitrophenyl)-2-R'-propanediamide, where R' is hydrogen or alkyl: converting the nitro groups to primary amine substituents; removing the elements of water to effect ring closure; and thereafter providing the -CR'- linked benzimidazole nuclei so formed with an N,N'-alkanediyl bridging group and quaternizing substituents. Dye synthesis is completed by reducing the -CR¹-linking group to the corresponding methine.