Investigation of the Effect of Concentration on Molecular Aggregation of Cyanine Dyes in Aqueous Solution

The aggregation of the cyanine dye, 3,3’-disulfobutyl-5,5’-dichloro thiacarbocyanine triethylamine, (NK-3796) in aqueous solution was investigated using absorption and emission spectroscopy. The equilibrium, n(monomer) ⇆ n(dimer) ⇆ (H-aggregate)n , was observed over a series of dye concentrations ranging from 10-4 mM to 0.1mM. At concentrations <10-3 mM, the dye exists in solution mostly in the monomeric form. However, dimers become more significant when the concentration of the dye exceeds 10-2 mM. Unlike the substituted dye in the 9th position, the NK-3796 dye tends to form H-aggregates at higher concentration (>10-1 mM). Monomers and dimers exhibit strong emission in the visible region. Also notable, is that the emission from the H-aggregates was very weak due to self quenching.