Blue Light-Emitting Polymers Containing Twisted Carbazole-Based Benzothiophene-S,S-Dioxide Derivative

A twisted five-ring fused compound, N-(2-decyltetradecyl)-7H-benzo[4,5]thieno[2,3-b]carbazole-S,S-dioxide (i-BTOCz), was synthesized and characterized. By introducing i-BTOCz into the main chains of polyfluorene (PF) and polycarbazole (PCz), respectively, blue light-emitting polymers (PF-i-BTOCzs/PCz-i-BTOCzs) were synthesized. Excellent thermal stability was achieved, along with a strong intramolecular charge transfer (ICT) effect of i-BTOCz and the resulting polymers. For the polymers, as the content of i-BTOCz increased, the lowest unoccupied molecular orbital (LUMO) energy levels decreased slightly and the highest occupied molecular orbital (HOMO) energy levels were almost unchanged. The photoluminescence (PL) spectra showed slight broadening in solution and basically no change in film, which due to the twisted molecular structure suppressed the decrease in the energy levels and redshift of spectra. With the current density in the range of 100–500 mA cm⁻², the electroluminescence (EL) spectra of the polymers were almost unchanged, indicating outstanding EL stability. As the content of i-BTOCz increased, a slight redshift of the EL spectra was observed for the PF-i-BTOCzs and PCz-i-BTOCzs, with basically unchanged Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.14) and (0.17, 0.16), respectively. The maximum luminous efficiency (LE_max) of PF-i-BTOCz20 and PCz-i-BTOCz20 was 1.11 cd A⁻¹ and 1.41 cd A⁻¹, while the LE_max of PF and PCz was 0.58 cd A⁻¹ and 0.35 cd A⁻¹, respectively, indicating that introducing the twisted unit into the main chains of PF or PCz could be a viable strategy for restraining the redshift of spectra and obtaining efficient blue light-emitting polymers.