Electrostatic interactions of cationic dyes with negatively charged polyelectrolytes in aqueous solution

The electrostatic binding of the cationic dyes rhodamine 6G (MG), acridine orange (AO), bisindolenylpentamethine (Cy5) and 1,1 ' -diethyl-2,2 ' -cyanine (PIC) to the anionic polyelectrolyte polystyrene sulfonate (PSS) was investigated by absorption and fluorescence spectroscopy. R6G exhibits little changes in the absorption and fluorescence spectra upon binding to PSS. Adsorption of Cy5 to PSS shifts the absorption 10 nm to the red, if the dye/polymer ratio (D/P) is below 0.1. This is caused by polarization of the extended pi -electron system through the PSS charges. Larger D/P ratio results in the formation of H-aggregates. Transition dipole interactions between the dye molecules shifts the absorption band to higher energy and quenches the fluorescence. This effect is even more pronounced in case of AO bound to PSS. Adsorption of the non-fluorescent dye PIC on PSS lead to a narrow fluorescence emission caused by formation of PIC J-aggregates, The J-aggregates requires a sterical fit of the aggregate structure to PSS binding sites, which is optimal fulfilled at a D/P ratio of 0.55. Adsorption of more dye destroys the J-aggregates again, which is explained by a model of the J-aggregate-PSS complex. The study of competitive binding between AO, Cy5 and poly(allylamine) hydrochloride (PAH) onto PSS showed comparable binding constants for the dyes but a remarkably higher one for PAR (C) 2001 Elsevier Science BN. All rights reserved.