Water-Soluble Cationic Poly(p-phenyleneethynylene)s (PPEs):  Effects of Acidity and Ionic Strength on Optical Behavior

Abstract

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). P1‘ showed intrinsic water-solubility although its quaternization degree was only 45%. Slightly blue-shifted UV−vis absorption maxima in an acidic environment (maximal Δλmax ≈ 3 nm) and obvious red-shifted absorption (maximal Δλmax ≈ 30 nm) and emission maxima (maximal Δλmax ≈ 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 (λmax ≈ 478 nm) and emission (λmax ≈ 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.

Recent Applications of Conjugated Polyelectrolytes in Optoelectronic Devices

“…However, it is noteworthy that higher concentrations of CPEs can be dissolved into water by the action of surfactants. [51,52] Electrostatic interactions between CPE chains containing acidic [53,54] and basic [55][56][57] functionalities can be modified by adjustments of the solution pH. Protonation of anionic units at low pH and deprotonation of cationic units at high pH ultimately decreases electrostatic repulsion as determined by light scattering experiments and a variety of optical characterization techniques.…”

“…[6,7,58] Changes in ionic strength typically do not lead to changes in properties that are as pronounced as those attained by modulating backbone charge with pH, but can influence the threshold pH required to modify intra-and inter-chain interactions. [53,54,59] Replacement of typical alkali or halide counterions in CPEs with organic ions is also known to modify chain packing. [39,56,[59][60][61] Such modifications may lead to interesting nanostructures in the solid state.…”

Sensitive fluorescent sensing for DNA assay

“…Some CPs, including polythiophene [51], poly-fluorene [52], poly (p-phenylenevinylene) [53], poly (p-phenylene ethynylene) [54], and oligonucleotidefunctionalized polymer, were synthesized and their sensing properties were investigated. Compared with small-molecule dyes, their advantages stem from the light-harvesting and optical-signal-amplification properties of conjugated backbones with efficient intrachain and interchain energy-transfer mechanisms.…”

Polyfluorenes

“…Mo (1) precursors in organic solvents, water-soluble conjugated polymers (WSCPs) usually exhibit much lower PL efficiencies in aqueous solutions [189][190][191][192]. The water solubility of PF can be achieved by functionalizing the substituted side chains with ammonium (NR 3 + ) to form cationic PF or with terminal carboxylate (CO 2 − ), sulfonate (SO 3 − ), and phosphonate (PO 3 2− ) groups to form anionic PF.…”