Optical studies of the solution phase reduction and stabilization of indigo tetrasulfonate in polyelectrolyte complexes

Ultraviolet-visible (UV-vis) and fluorescence spectroscopy have been used to characterize the polyelectrolyte complexes (PECs) formed when potassium indigo tetrasulfonate (ITS) interacts with poly diallydimethylammonium chloride (PDADMAC) through columbic attraction in the presence of the reducing agent sodium bisulfite, NaHSO3. The PDADMAC facilitates both the reduction of the ITS and the stabilization of the reduced state of the ITS in an atmospheric oxygen environment. Dilutions of the dye solution show that the PEC is stable to dilutions of at least 1 to 1000. UV-vis studies indicate that the reduced ITS (ITSred) forms what is likely a J-aggregate in the presence of PDADMAC with an absorbance band red shifted from the normal absorbance band of reduced ITS by roughly 130 nm, 390 nm to 520 nm. Excitation of the PEC solution at either 390 nm or 520 nm produces an emission spectrum of the aggregated complex with an emission maximum near 534 nm. Monomer emission at 480 nm of ITSred represents only 3.0 ± 0.5% of the emission signal of the aggregated complex. Kinetic studies using fluorescence spectroscopy over a temperature range of 30 to 70 °C and dilutions of dye solutions ranging from 1:10 to 1:1000 yield data for the oxidation of ITSred that is best fit by a first order rate constant. Kinetic data displays two distinctive regimes, a short time rate and a long time rate. These two distinct kinetic regimes are likely due to the reduced ITS interacting with an outer PEC environment and an inner PEC environment. First order rate constants could be used to estimate Δ‡H and Δ‡S of the oxidation reaction. Fluorescence data was used to calculate the partitioning of reduced ITS molecules between the outer and inner PEC environments. Partitioning from the inner to outer PEC environment was found to be entropically driven. Addition of NaCl to the diluted dye solutions could alter the kinetics of the oxidation but the significance of the effect depended on the initial dye solution preparation.