Reversal of interfacial dipole orientation in polyelectrolyte superlattices due to polycationic layers

Repeated bilayers of a polycation and a polyanion were assembled by adsorption from their aqueous solutions. The polycations used were PEI (poly(ethylenimine)), PAH (poly(allylamine hydrochloride)), and generation 3.0 dendrimer (poly(propylenimine)). The polyanion was PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)-benzenesulfonamido]-1,2-ethanediyl, sodium salt]). Second harmonic generation (SHG) and UV-vis spectroscopy were used to study the structure and formation of the superlattices. The linear increase in absorbance versus the number of bilayers suggested that a uniform and consistent amount of polymer adsorbed during each deposition cycle. The observation of a second harmonic (SH) signal from the film showed that the conjugated chromophores in the PAZO polymer were aligned with a net dipole. However, the SHG signal did not continue to increase as the number of bilayers was increased. On the basis of thorough studies of the UV and SH properties for each bilayer for 10 total bilayers, we propose that the asymptotic nonlinear optical response is due to loss of charge density on the polycation. The SH signal from multilayers terminated with the polycation is smaller than when PAZO is the terminal layer. We attribute this to a change in orientation of the dipole of the outermost PAZO chromophores at the interface of the PAZO and the terminal polycation layer. We also propose multilayer assemblies that will lead to self-assembled polyelectrolyte superlattices with a high nonlinear optical susceptibility.