Blue-light-emitting fluorene-based polymers with tunable electronic properties

A series of soluble alternating polyfluorene copolymers with different main chain structures and those of the same main chain structure polyfluorene-co-alt-phenylene with different functional groups attached at the 2- and/or 5-positions of the phenylene ring were synthesized by a palladium-catalyzed Suzuki coupling reaction. All 10 polymers had the band gaps ranging from 2.81 to 3.35 eV, corresponding to blue-light emission. Through controllable modification for both the main chain structures and the side chains, not only the optical and electronic properties of the blue emissive polymers had been tuned, but also the structure-property relationships, especially the HOMO and LUMO energy level engineering, had been studied. Relatively high PL efficiency in both solution and film states, good thermal stability, and relatively high glass transition temperatures were demonstrated on these polymers. In general, polymers with the main chain structure of polyfluorene-co-alt-phenylene were found to have higher Φ_fl both in solution and in solid states than those copolymers with other main chain structures. For the polymers with the same main chain structure of polyfluorene-co-alt-phenylene, attachment of electron-donating alkoxy groups on phenylene ring (P4) had caused a spectral red shift, corresponding to slightly decreased HOMO and increased LUMO energy levels, while attachment of electron-withdrawing ester groups (P6) had led to an obvious blue shift in the absorption spectrum with a decrement in both the HOMO and LUMO energy levels as compared to that of the unsubstituted polymer (P2). As for the polymers of different main chain structures, in comparison with the homopolymer P1, carbazole comonomer had caused an obvious spectral blue shift with increased HOMO and decreased LUMO energy levels. A decrement in both the HOMO and LUMO energy levels had been observed for P9 in which naphthalene was chosen as the comonomer. However, for P10, although there was no obvious difference between the absorption and emission spectra of P10 as compared to those of P1, both the HOMO and LUMO energy levels were reduced greatly when they were compared with those of P1.