Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide unit in alkyl side chain

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide (SO) unit in alkyl side chain (PF-FSOs and PF-CzSOs) were synthesized. All the polymers show high thermal stability with the decomposition temperatures over 400 °C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the copolymer slightly decrease with the increase of SO content in side chain. PL spectra of the polymers show slightly red shift and broadening with the increase of solvent polarities, indicating unremarkable intramolecular charge transfer (ICT) effect in the polymers containing SO unit in alkyl side chain. EL spectra of the polymers are almost unchanged in the current densities from 100 to 400 mA cm−2, indicating the superb EL stability of the resulted polymers. The EL spectra of the copolymers exhibit obvious blue-shift and narrowing with the CIE of (0.18, 0.11) for PF-FSO10 and (0.17, 0.11) for PF-CzSO10, respectively, compared with PF-SO10 containing SO unit in main chain with the CIE of (0.16, 0.17) and PFO with the CIE of (0.18, 0.18). The superior device performances were obtained with the luminous efficiency (LEmax) of 1.17 and 0.68 cd A−1 for PF-FSO15 and PF-CzSO20, respectively, compared with the LEmax of 0.37 cd A−1 for PFO. The results indicate that linking SO unit to alkyl side chain of the polyfluorene is a promising strategy for efficient blue light-emitting polymers.