The Spectroscopic Studies on the Aggregation Behavior of Cyanine Dyes

Aggregations of 1,1′-diethyl-2,2′-carbocyanine iodide (DCI) and 1,1′-diethyl-2,2′-dicarbocyanine iodide (DDI) in aqueous solution have been investigated by the steady-state absorption spectroscopy. The equilibrium constants for dimerization of DCI and DDI are found to be (9.8±0.5) × 10⁵ and (1.6±0.5)× 10⁵ M^-1, respectively, at 293 K. The enthalpy changes for the dimerization of DCI and DDI are -6.7±0.7 and - 7.7±0.8 kcal/mol, respectively. The results show that the dispersion force plays an important role in the aggregation of DCI and DDI in aqueous solution. The absorption bandwidth of DCI/ethanol system has been measured as a function of temperature, providing the evidence for no strong interaction between DCI and solvent molecules. The participation of hydrophobia force in driving the aggregation is suggested. For the first time, DCI in aqueous solution is found to form a new aggregate which has both J- and H- bands.