Synthesis and optical properties of pH-responsive conjugated polyampholytes

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Conjugated polyampholytes (CPAs) containing both positive and negative ionic groups in their side-chains were designed and synthesized. Two types of random copolymers were prepared by the incorporation of fluorene, phenylene and 2,1,3-benzothiadiazole (BT) moieties in the main chain. Both quaternary ammonium bromide and carboxylic acid functionalities were introduced successfully in the side-chain through sequential protection, quaternization and deprotection reactions. The resulting ionic polymers were soluble in water and their optical characteristics were examined by changing pH. The fluorescence resonance energy transfer (FRET)-sensitized BT emission of the polymers increased with increasing solution pH. Under basic conditions, deprotonation of the carboxylic acid groups induced intra- and/or interchain aggregation via electrostatic complexation between the cationic ammonium and anionic carboxylate groups. The FRET ratio between the green and blue emissions showed a linear relationship with solution pH. This new type of water-soluble fluorescent bioassays and bioimaging applications through bioconjugation with a targeting moiety.