Water-soluble cationic poly(p-phenyleneethynylene)s (PPEs): Effects of acidity and ionic strength on optical behavior

Three cationic PPEs P1′-P3′ were obtained through quaternization of their neutral polymers, of which all side chains were respectively featured with a tertiary amino group (P1), an alternative tertiary amino and tri(ethylene glycol)methyl ether group (P2), and an alternative tertiary amino and dodecyloxy group (P3). PI′ showed intrinsic water-solubility although its quaternization degree was only 45%. Slightly blue-shifted UV-vis absorption maxima in an acidic environment (maximal Δλ_max ≈ 3 nm) and obvious red-shifted absorption (maximal Δλ_max ≈ 30 nm) and emission maxima (maximal Δλ_max ≈ 20 nm) in an alkaline environment at pH < 13 are explained in terms of pH-induced planarization of the conjugated backbone. After the pH was increased to 14, a new absorption (λ _max ≈ 478 nm) and emission (λ_max ≈ 495 nm) peak appeared in the redder region and the fluorescence intensity further dramatically decreased, implying the occurrence of the conformational change from the planarization of conjugated backbone to the interchain aggregation. The fluorescence enhancement of P1′ in an acidic environment and the fluorescence decrease of P1′ in an alkaline environment are attributed to the pH-influenced quenching ability of the nonbonding electron pair on the nitrogen atom, which belongs to the unquaternized tertiary amino group on the side chain, and the interchain aggregation. Such pH-influenced fluorescence intensity can be efficiently buffered by adding salts at pH < 10. Although adding salts in a neutral environment only creates the planarization of PPE conjugated backbone, adding salts in an alkaline environment significantly promotes the formation of interchain aggregation. Amplified quenching of P1′ by anionic quencher Fe(CN)₆^4- was observed and its K_sv increased significantly after adding salts or increasing pH to 14, for example from K_sv = 9.6 × 10⁵ M ^-1 at pH = 7 to 7.7 × 10⁶ M^-1 at pH = 10 when [NaCl] = 0.1 M, due to the formation of elongated π-conjugation.