Fine tuning of excited states by trifluoromethylphenyl for blue-shifted color and enhanced EQEs in thermally activated delayed fluorescence emitters

3,6-di-tert-butyl-9H-carbazole and 3,6-diphenyl-9H-carbazole have been widely used to construct blue thermally activated delayed fluorescence (TADF) emitters. However, it is difficult for related emitters to achieve maximum external quantum efficiency (EQEmax) above 20 % with the Commission Internationale de l'Eclairage (CIE) y ≤ 0.12. A new type of donor was developed to construct blue TADF molecules, namely CF3P-BuCz-TRZ and CF3P-PhCz-TRZ. The weak electron-withdrawing trifluoromethylphenyl (CF3P) with moderate steric hindrance was introduced into the 1-site of carbazole donor for improving the color purity for blue TADF emitters. More importantly, the introduction of CF3P tightly compacts the energy levels for the significant excited states in RISC process. The energy gap between the lowest singlet excited (S1) state and the lowest triplet (T1) state (Δ E(S1−T1)), as well as between T1 and the higher triplet (Tn, n ≥ 2) states (Δ E(Tn−T1)) were reduced, so that Tn (n ≥ 2) could be effectively harvested to further promote reverse intersystem crossing (RISC). As a result, the OLEDs based on CF3P-BuCz-TRZ and CF3P-PhCz-TRZ displayed blue electroluminescence with CIE coordinates of (0.156, 0.107) and (0.155, 0.115). In particular, the maximum EQEs of the CF3P-PhCz-TRZ device reached 20.43 % without employing any light out-coupling enhancement or TADF-sensitized structure. This strategy should be valuable for designing more pure-blue and deep-blue TADF emitters. The weak electron-withdrawing trifluoromethylphenyl (CF3P) with moderate steric hindrance was introduced into the 1-site of carbazole donor. The kRISCs were remarkably facilitated with incorporation of CF3P group in CF3P-PhCz-TRZ. CF3P-PhCz-TRZ exhibited both blue-shifted EL in OLED with CIE (0.155, 0.115) and improved maximum EQE of 20.43 %, which are superior to the parent emitter PhCz-TRZ with CIE (0.154, 0.146) and EQE of 15.18 %. [Display omitted] •Trifluoromethylphenyl (CF3P) is introduced to 1-site of carbazole as new donors for blue TADF emitters.•The electron withdrawing CF3P group weakens donor strength, making hyprochromic shift by 10 nm.•CF3P group compacts the excited states energy levels and reduce energy differences ΔEST and ΔETT and facilitates RISC.•CF3P based TADF emitters exhibit higher EQE = 20 % (vs 15 %) and better CIEy = 0.115 (vs 0.146) in OLEDs.•Modifying carbazole donors with CF3P is a universal design strategy for better blue color and higher efficiencies.