We have designed and synthesized a series of deep-blue light-emitting polyfluorenes, PF-27SOs and PF-36SOs, by introducing electron-deficient 9,9-dimethyl-9H-thioxanthene 10,10-dioxide isomers (27SO and 36SO) into the poly(9,9-dioctylfluorene) (PFO) backbone. Compared with PFO, the resulting polymers exhibit an equivalent thermal decomposition temperature (>415 C), an enhanced glass transition temperature (>99 C), a decreased lowest unoccupied molecular orbital energy level (E LUMO ) below −2.32 eV, a blue-shifted photoluminescence spectra in solid film with a maximum emission at 422 nm, and a shoulder peak at~445 nm. The resulting polymers also show blue-shifted and narrowed electroluminescence spectra with deep-blue Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.07) for PF-27SO20 and (0.16, 0.06) for PF-36SO30, compared with (0.17, 0.13) for PFO.Moreover, simple device based on PF-36SO30 achieves a superior device performance with a maximum external quantum efficiency (EQE max = 3.62%) compared with PFO (EQE max = 0.47%). The results show that nonconjugated 9,9-dimethyl-9H-thioxanthene 10,10-dioxide isomers can effectively perturb the conjugation length of polymers, significantly weaken the charge-transfer effect in donor-acceptor systems, substantially improve electroluminescence device efficiency, and achieve deep-blue light emission.