Water-Soluble Conjugated Polyelectrolytes with Branched Polyionic Side Chains

A new series of conjugated polyelectrolytes (CPE) consisting of an arylene-ethynylene backbone featuring phenyl (Ph), 2,1,3-benzothiadiazole (BTD), or 4,7-bis(2'-thienyl)-2,1,3-benzothiadiazole (TBT) units have been synthesized and characterized. On each polymer repeat unit the CPEs contain two branched ionic side groups each featuring a "triad" of carboxylate (R-CO(2)(-)Na(+)) or ammonium (R-NH(3)(+)Cl(-)) units, giving the polymers six ionic charges per repeat. The photophysical properties of the series of CPEs were investigated in CH(3)OH and H(2)O solution by absorption, steady-state fluorescence, and fluorescence lifetime spectroscopy. The different arylene units in the backbone lead to the variation of the HOMO-LUMO gap across the series. The branched, polyionic side chains suppress aggregation of the polymer chains, even in aqueous solution, leading to higher fluorescence quantum yields relative to similar CPEs with linear side chains. UV-vis absorption spectra show that CPEs with anionic branched side chains (R-(b)CO(2)(-)Na) aggregate at low pH, while retaining the photophysical properties of their organic-soluble precursors at high pH. CPEs having branched cationic side chains (R-(b)NH(3)(+)Cl(-)) exhibit the opposite response to pH change.