Photophysical properties and morphology of fluorene-alt-benzene based conjugated polymers

A series of fluorene-alt-benzene based conjugated main chain polymers attached chemically with different lengths of alkyl side chains on phenylene rings were designed and synthesized by a palladium catalyzed Suzuki coupling reaction. The UV-vis absorption and fluorescence spectra, thermal stability of spectral property, phase transition behavior and morphology of the synthesized polymers were investigated. With increasing the length of the alkyl side chain, the UV and fluorescence spectra exhibit obvious blue shift compared with those of the unsubstituted polymer. The alkyl substitution improves the thermal stability of the spectra due to the steric hindrance of the alkyl side chains and thus the effective separation of the main chain backbones. The phase transition behavior is closely related to the length of alkyl side chains attached on the phenylene rings. The annealed polymer films display characteristic nematic liquid crystalline texture. Transmission electron microscopy (TEM) observations indicate that solvent-cast thin deposits of all the polymers show typical fibrillar morphology.