Carboxylate group side-chain density modulates the pH-dependent optical properties of PPEs

The synthesis of two new water-soluble, carboxylate-substituted poly(p-phenyleneethynylene)s (PPE) is reported. These PPEs differ in their concentration of free carboxylate groups per repeat (two aryleneethynylenes) unit. The photophysical properties of these two polymers are compared to each other and to that of the known dicarboxylate-PPE. An increasing concentration of carboxylate groups (on a per repeat basis) makes the polymers less fluorescent but also more sensitive toward quenching by mercury ions. The least carboxylate functionalized PPE shows the highest quantum yield (0.33). All three PPEs are pH-sensitive and show dramatic changes in absorption and emission at around pH 3.5, which is the pKa of methoxyacetic acid, the common structural subunits in all of these PPEs.