Synthesis, characterization, and structure-property relationship of novel fluorene-thiophene-based conjugated copolymers

A novel series of soluble alternating conjugated copolymers comprised of 9,9-dihexylfluorene and substituted bithiophene or thiophene moieties were synthesized by palladium-catalyzed Suzuki coupling reaction. In the backbone structure of the polymers, the thiophene rings in the bithiophene moieties are β-substituted with decyl chains, and the bithiophene moieties are in two different coupling configurations of head-to-head and tail-to-tail regioregularities. For the thiophene moieties, they are either mono- or disubstituted with decyl chains. By means of the controllable structure modifications, the electronic properties of the polymers were tuned. In addition, the structure-property relationship of the backbone structure was studied. All of the polymers demonstrate efficient blue-to-green light emission, good thermal stability, and relatively high glass transition temperatures (T_g to approximately 78 °C). For the green-light-emitting polymers comprised of bithiophene moieties, their optical and electrochemical properties are independent of the coupling configurations of the substituted bithiophene moieties, whereas the head-to-head regularity demonstrates higher glass transition temperature and better environmental stability. As for the polymers composed of thiophene moieties, the monosubstitution of a decyl chain on the β-position of thiophene ring produces bluish green emission with high fluorescence quantum efficiency (0.65 in solution). The second attachment of a decyl chain on another β-position of thiophene ring pushes the emission to the blue region with reduced fluorescence quantum efficiency (0.39 in solution). The effort of synthesizing the polymers with the bithiophene moieties through FeCl₃ oxidative polymerization is also reported.