DIMERIZATION OF CYANINE DYES IN WATER DRIVEN BY ASSOCIATION WITH HYDROPHOBIC BORATE ANIONS

The titrametric addition of hydrophobic borate anions to water-soluble cationic cyanine dyes readily perturbs the dye visible absorption spe ctrum. The insensitivity of the spectral effect to the oxidation poten tial of the borate indicates that this interaction is not a charge-tra nsfer complex. Both concentration and solvent dependencies suggest the effect is due to the formation of cyanine dye-borate complexes, which are driven primarily by the hydrophobicity of the borate. Thus, in wa ter, the hydrophobic borate anion-dye interaction reduces the dye solu bility with the concomitant formation of dimers of the cyanine dye-bor ate complexes that facilitate electronic interactions of the dyes. Fur thermore, the addition of hydrophobic borate anions to a mixture of tw o water-soluble cyanine dyes results in the formation of mixed dimers. The effect of the borate in these phenomena is opposite to the more f amiliar effect of surfactant solubilization of a hydrophobic dye. Inde ed, the hydrophobic borate anion is acting as an asurfactant, in the s ense that it interacts with a water-soluble species to form a defined aggregate structure.