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Two novel bipolar isomers, namely 3,5-bis(3-(9 H -carbazol-9-yl)phenyl)isonicotinonitrile ( m -DCzPyCN), and 3,5-bis(9-phenyl-9 H -carbazol-3-yl)isonicotinonitrile (3-DCzPyCN), are designed and synthesized by selecting cyanopyridine as n-type unit and N -phenylcarbazole as p-type unit. Two peripheral carbazoles are conjugated through their C-3 atom with the central cyanopyridine in 3-DCzPyCN but are indirectly linked through the N-9 atom via meta -phenylene bridges in m -DCzPyCN. The linking strategy had significant effect on the physical properties of the isomers. The single-carrier device study reveals that 3-DCzPyCN has much better positive and negative charge balancing ability than m -DCzPyCN. The 3-DCzPyCN hosted green phosphorescent organic light-emitting diode exhibits superior performance over m -DCzPyCN, with the maximum current efficiency of 84.1 cd A −1 and external quantum efficiencies of 24.2% and low efficiency roll-off. At an extremely high brightness of 10000 cd m −2 , the efficiency still remained at 78.4 cd A −1 , which corresponds to a roll-off of 6.7% from the maximum value. Even at an ultra-high brightness of 100000 cd m −2 , the efficiency is reduced by 47.0%. The superior performance of 3-DCzPyCN is ascribed to its remarkable charge balancing capability, which is the combined result of the cyanopyridine as n-type unit and the conjugated linking strategy between p- and n-type moieties. [ABSTRACT FROM AUTHOR]

The non-doped organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) materials are attracting increasing attentions due to the excellent performance and simple fabrication. We report an effective design strategy of TADF emitters to improve the non-doped performance by adjusting the valence state of the S atom in TADF molecules and utilizing the steric hindrance effect. Two novel TADF materials PT-BZ-DMAC and PS-BZ-DMAC were designed and synthesized with benzophenone acceptor and acridine donor. In particular, the thioether or sulfone group was attached to benzophenone. Remarkably, the bluish-green TADF-OLED with non-doped PT-BZ-DMAC turned on at 2.5 V and realized a maximum external quantum efficiency (EQE) of 17.34%, which was comparable with its doped OLED (17.71%) but showed much slower roll-off. At 100 and 1000 cd m−2, the non-doped PT-BZ-DMAC device still maintained acceptable efficiencies at 14.65% and 12.59%. In contrast, sulfone-modified PS-BZ-DMAC exhibited good efficiency of 20.55% only in doped OLED. The single crystal analysis and DFT calculation revealed that the phenyl thioether unit in PT-BZ-DMAC hardly contributes to the LUMO or the electronic transitions, instead determines the highly twisted geometry and acts as the barrier group, finally improving the performance of PT-BZ-DMAC in neat film device. Two TADF emitters are developed by attaching thioether or sulfone groups to the benzophenone acceptor. Unlike the strong electron withdrawing sulfone group that contributes to the electronic transition and TADF, the phenyl thioether blocked the conjugation of molecular backbone. The steric hindrance effect of phenyl thioether moiety along with the intermolecular hydrogen bondings can effectively prevents exciton quenching, thus further raising the EQE of non-doped TADF-OLED to 17.34% with low efficiency roll-off. [Display omitted] • Thioether and sulfone were incorporated as substituents to design TADF molecules PT-BZ-DMAC and PS-BZ-DMAC. • Thioether was found to strongly suppress intermolecular interactions and prevent exciton quenching via steric hindrance effect. • Non-doped OLED of thioether based PT-BZ-DMAC exhibited comparably high efficiency (17.34%) to its doped device (17.71%), but with much slower roll-off. • Sulfone based PS-BZ-DMAC exhibited high efficiency of 20.55% only in doped OLED, while non-doped device did not work. [ABSTRACT FROM AUTHOR]

Saturated red iridium phosphors are strongly desired for application in organic light-emitting diodes (OLEDs). Most of the reported iridium complexes with 2-arylquinoline or its derivatives as cyclometalating ligands are orange-red phosphorescent with peak wavelength below 620 nm. Herein trifluoromethyl (CF 3) or Fluorine (F) is introduced into the 6-site of quinoline ring to design and synthesize the cyclometalated ligands 2-phenyl-6-CF 3 /F-quinoline (CF 3 -phq or F-phq) and heteroleptic iridium complexes Ir(CF 3 -phq) 2 (acac) (Ir-1), Ir(CF 3 -phq) 2 (tmd) (Ir-2), Ir(F-phq) 2 (acac) (Ir-3) and Ir(F-phq) 2 (tmd) (Ir-4). Owing to the electron-deficient feature of CF 3 and F, the electron affinity of these complexes are increased, the corresponding LUMOs are pulled downwards, and the phosphorescence of CF 3 -containing iridium complexes are saturated red with peaks at 627–632 nm in photoluminescence. At the same time, by varying the ancillary ligand from acetylacetonate (acac) to 2,2,6,6-tetrametylheptane-3,5-dionate (tmd), the emitting performance are dramatically improved. As a result, the Ir(CF 3 -phq) 2 (tmd) based red OLED realized pure red electroluminescence with Commision Internationale de L'Eclairage (CIE) coordinates of (0.67, 0.33) with a high brightness of 12695 cd m−2 and a maximum external quantum efficiency of 14.96%, respectively, which maintained as high as 9.62% at a practical luminance of 1000 cd m−2. This is the first example of saturated red iridium phosphors with structure modification at the 6-site of quinoline ring of 2-phenylquinoline ligand. By incorporating electron-withdrawing fluorine (-F) or trifluoromethyl (-CF 3) at the 6-site of quinoline ring of the 2-phenyl-quiniline cyclometalating ligands and changing the auxiliary ligand from acac to bulky tmd, the heteroleptic iridium complexes realize pure red phosphorescence with CIE coordinates of (0.67, 0.33) and high efficiency of 14.96% in doped organic light-emitting diodes, which still remains at 9.62% at a practical brightness of 1000 cd m−2. Image 1 [ABSTRACT FROM AUTHOR]