Formation of poly(9,9-dioctylfluorene) β-phase by incorporating aromatic moiety in side chain

• Two blue light-emitting polymers with aromatic unit in side chain are synthesized.
• The incorporated aromatic units can induce the formation of PFO β-phase.
• Both luminous efficiency and color purity can be simultaneously improved.
• A peak luminous efficiency of 2.25 cd A−1 was obtained for PFO-SO.

Two novel polyfluorene-based blue-light emitting copolymers consisting of aromatic carbazole or dibenzothiophene-S,S-dioxide (SO) moiety in side chain are designed and synthesized based on a palladium-catalyzed Suzuki polymerization. The UV–vis absorption, photoluminescence characteristics and wide-angle X-ray diffraction measurements of the resulting copolymers demonstrate that the incorporation of such aromatic units can induce the formation of the β-phase. The resulting copolymers consisting of aromatic units exhibit improved electroluminescence properties relative to the pristine PFO film. A maximum luminous efficiency of 2.25 cd A−1 with Commission Internationale de l’Eclairage coordinates of (0.17, 0.09) is obtained from the light-emitting device based on the copolymer consisting of SO unit (PFO-SO). Device based on copolymer PFO-SO also exhibits stable electroluminescent spectra and relatively low roll-off of luminous efficiency at high current densities. These observations indicate that the incorporation of aromatic moiety in side chain is a promising strategy to obtain stable blue-light-emitting polyfluorenes with enhanced efficiency.

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