Abstract: The effect of counterions on the J-aggregai ion of an anionic cyanine dye, 4, 5, 4', 5'-dibenzo-9-ethyl-3, 3'-bis-(3-sulfopropyl)-thiacarbocyanine (1), in dilute aqueous solutions at room temperature was studied. Absorption spectra indicated that the counterions were not incorporated in the planar dye-ion stacks. Counterion activity measurements for dye I in solutions on adding simple metal ion salts resulted in that the dye-ion stacks with the surrounding counterions bound nearby behaved nearly as a salt-like "totality". The spectra also showed that, for the-added metal ion salts, the minimum Counterion concentrations necessary for initiating J-ag-gregation of dye I in dilute solutions of constant dye concentration depends mainly on the charge of the counterions, being much more efficient for the high velent ones. The effect of the nature-of the counterions was not significant.

Key words: cyanine dye, aggregation, counterion