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The recent introduction of thermally activated delayed fluorescence (TADF) emitters is regarded as an important breakthrough for the development of high efficiency organic light-emitting devices (OLEDs). The planar D and A groups are generally used to construct TADF emitters for their rigid structure and large steric hindrance. In this work, it is shown that many frequently used nonaromatic (noncontinuous conjugation or without satisfying Hückel's rule) planar segments, such as 9,9-dimethyl-9,10-dihydroacridine, are actually pseudoplanar segments and have two possible conformations-a planar form and a crooked form. Molecules constructed from pseudoplanar segments can thus have two corresponding conformations. Their existence can have significant impact on the performance of many TADF emitters. Two design strategies are presented for addressing the problem by either (1) increasing the rigidity of these groups to suppress its crooked form or (2) increasing the steric hindrance of the linked group to minimize energy of the emitters with the highly twisted form. Following these strategies, two new emitters are synthesized accordingly and successfully applied in OLEDs demonstrating high external quantum efficiencies (20.2% and 18.3%).

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A novel thermally activated delayed fluorescence (TADF) emitter 12,15‐di(10H‐phenoxazin‐10‐yl)dibenzo[a,c]dipyrido[3,2‐h:2′,3′‐j]phenazine (DPXZ‐BPPZ) is developed for a highly efficient red organic light‐emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron‐donor (D) and electron‐acceptor (A) segments, DPXZ‐BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet–triplet energy split to DPXZ‐BPPZ. Therefore, DPXZ‐BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen‐free conditions. The OLED based on DPXZ‐BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward‐viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A−1 (current efficiency), and 30.9 ± 1.3 lm W−1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ‐BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.

Highly twisted electron donor (D)–electron acceptor (A)-type thermally activated delayed fluorescence (TADF) emitters can achieve high efficiency while suffering from serious structural relaxations and broad emissions. Multiple resonance (MR)-type TADF emitters can realize narrow emission. However, until now, only a few efficient MR-emitting cores are reported and custom tunning of their emission color remains a major challenge in their wider applications. In this work, by combining the conventional TADF and MR-TADF designs, we demonstrate that color tuning and narrowing the spectral width of conventional TADF emission can be easily achieved simultaneously. We select a prototypical carbonyl (CO)/N-based MR core as a backbone and attach it with D segments of different electron-donating abilities and numbers to obtain three different TADF emitters with emissions from sky blue to green and orange-red while maintaining the narrow emission of the original MR core. The corresponding sky blue, green, and orange-red organic light-emitting diodes achieve maximum external quantum efficiencies of 20.3, 27.3, and 26.3%, respectively, and narrow full widths at half-maximum all below 0.28 eV. These results provide a new molecular design strategy for developing narrowband TADF emitters with easily tunable emissions covering the full visible range.

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be...

A novel molecular model of connecting electron-donating (D) and electron-withdrawing (A) moieties via a space-enough and conjugation-forbidden linkage (D-Spacer-A) is proposed to develop efficient non-doped thermally activated delayed fluorescence (TADF) emitters. 10-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-o-TRZ) was designed and synthesized accordingly. As expected, it exhibits local excited properties in single-molecule state as D-Spacer-A molecular backbone strongly suppress the intramolecular charge-transfer (CT) transition. And intermolecular CT transition acted as the vital radiation channel for neat DMAC-o-TRZ film. As in return, the non-doped device exhibits a remarkable maximum external quantum efficiency (EQE) of 14.7 %. These results prove the feasibility of D-Spacer-A molecules to develop intermolecular CT transition TADF emitters for efficient non-doped OLEDs.