Improved electroluminescence efficiency of polyfluorenes by simultaneously incorporating dibenzothiophene-S,S-dioxide unit in main chain and oxadiazole moiety in side chain

A series of efficient and spectrally stable blue light-emitting polyfluorene derivatives containing 3,7-dibenzothiophene-S,S-dioxide (SO) unit in main chain and oxadiazole (OXD) moiety in the side chain were synthesized via Suzuki copolymerization. It was realized that the glass transition temperatures of the resulted copolymers PFSO-OXD increased gradually with the content of OXD, while the UV-vis absorption, photoluminescence spectra, as well as electrochemical properties were not significantly influenced by the molar ratio of OXD unit. Apparent solvatochromism of copolymers PFSO-OXD can be realized by varying polarity of solvents from toluene to dichloromethane. Light-emitting devices based on PFSO-OXD exhibited superior performances to those of PFSO and PF-OXD20 due to the more balanced charge carrier mobility of the devices. The electroluminescence spectra of all copolymers are independent with the current densities and thermal annealing. The best device performance was achieved based on PFSO-OXD20 with a maximal luminous efficiency of 4.9 cd A−1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.12). The results indicated that the strategy of concurrently incorporating SO and OXD unit into the main chain and side chain of polyfluorenes, respectively has great potential to achieve efficient blue light-emitting polymers.