Water-Soluble Poly(phenylene Ethynylene)s That Contain Phosphonium Pendant Groups

Expanding the functionality of conjugated polyelectrolytes (CPEs) is important for their use in biosensing and biocidal applications. Here we report the design and synthesis of a novel class of water-soluble poly(phenylene ethynylene) (PPE)based CPEs, featuring cationic trimethyl phosphonium (-PMe3+) moieties as pendant groups. A series of four CPEs were prepared which vary in the number of -PMe3+ groups and the length of the side-chains linking the phosphonium groups to the PPE backbone (3- or 6-carbon linkers). The polymers' molecular weights (MW) were estimated by diffusion-ordered NMR spectroscopy (DOSY) in methanol relative to poly(ethylene glycol) standards, and MW values ranging from 10 to 30 kDa were recovered. The photophysical properties of the new CPEs were assessed by steady state and time-resolved fluorescence spectroscopy analysis, using polymer solutions in water and methanol. Interesting relationships were observed between the optical properties and the structural variations of the new CPEs. Namely, the fluorescence reveals that the CPEs aggregate in water, with the degree of aggregation increasing with fewer -PMe3+ groups, and longer side-chains. The fluorescence quantum yields of the CPEs in methanol are high, with one CPE exhibiting >90% yield. Transient absorption spectroscopy on time scales from picoseconds to microseconds probes the excited state structure and dynamics of the CPEs in water and methanol. The singlet exciton state decays more rapidly in water due to nonradiative decay promoted by interchain aggregation. Excitation of the polymers produces triplet excitons and studies using singlet oxygen (1O2) phosphorescence and an anthracene-based probe confirm that 1O2 is produced by sensitization. Addition of pyrophosphate (PPi) to the phosphonium CPEs in methanol leads to fluorescence quenching; the mechanism of quenching is attributed to aggregation induced quenching.