Seven unsymmetrical benzimidazdo-thiacarbocyanine dyes were synthesized by condensation of 2-(β-anioiovinyl)- 5-R-3-(γ-sylfopropyl) benzothiazolium betaine with 5,6- dichloro-1-ethyl-2-methyl-3-(γ-sulfopropyl) benzimidazolium betaine in Ac~2O-NEt~3 medium. A linear relationship between the Brooker deviation value of the unsymmetrical dyes and the Hammett σ~m-constant of the substituent R at the 5' position of benzene ring (benzothiazole heterocyclic nucleus) was observed. The sequence of the basicity of the nitrogen atom in the substituted benzothiazol heterocyclic ring was found to be 4,5-benzo＞5-CH~3＞5-H＞5-OCH~3＞5-F＞5-Cl＞ 5-CF~3 .

Key words: SYNTHESIS, SENSITIZER, DYES, SUBSTITUENT EFFECT