Exciton peak redshifting and broadening in polyaniline chains during ion-induced hydrophobic collapse and aggregation

The ion-induced reprecipitation of the emeraldine base form of polyaniline from an aqueous-organic binary solution is a facile method for obtaining polymer nanoparticles and microscale clusters. The hydrophobic collapse and aggregation that accompanies the addition of various cosolutes induces changes in the peak wavelength and linewidth of the main exciton absorption. In particular, we find that the addition of ionic cosoutes leads to a redshifting and broadening of this spectroscopic feature, with divalent coions exhibiting an additional hypsochromic reversal at high concentrations. The denaturant urea, in contrast, causes a blueshift and line-narrowing at all concentrations. Using a quantitative Lorentz fit, we show how the redshifting and broadening of this spectral feature can be attributed, at least in part, to the forced planarization of the polymer chains in the collapsed state.

► Adding monovalent cosolutes to polyaniline suspensions causes the chains to collapse.
► The forced planarization of the polymer leads to a redshifting and broadening of the exciton absorption peak.
► Urea decreases the hydrophobic interaction and therefore has the opposite effect.
► Divalent coions cause redshifting at low concentrations, then a hypsochromic reversal.
► We preformed a quantitative spectral analysis to study the magnitude of these changes.