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 spectrum. The insensitivity of the spectral effect to the oxidation potential of the borate indicates that this interaction is not a charge-transfer 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 water, the hydrophobic borate anion-dye interaction reduces the dye solubility with the concomitant formation of dimers of the cyanine dye-borate complexes that facilitate electronic interactions of the dye