Efficient deep‐blue light‐emitting polyfluorenes based on 9,9‐dimethyl‐9H‐thioxanthene 10,10‐dioxide isomers

Abstract

AbstractWe 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 (ELUMO) 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 (EQEmax = 3.62%) compared with PFO (EQEmax = 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.