1 of 5) 1600675 emitter combined with sophisticated device engineering, including an appropriate choice of neighboring materials, is absolutely imperative to achieve high η p OLEDs realizing both high η ext and low drive voltages at high brightness over 1000 cd m −2 . While we developed novel pyrimidine conjugated emitters and the devices, Yang's group reported a pyrimidinebased green TADF emitters realizing a high η ext,max close to 25% and an η p of 74 lm W −1 . [16] However, we note that this device showed a high drive voltage of 3.4 V at 1 cd m −2 and a large efficiency roll-off with η p,1000 of 37 lm W −1 at a high brightness of 1000 cd m −2 .Herein, we investigated the structure-property relationships among pyrimidine conjugate emitters revealing an effective molecular design for high-performance OLEDs. Consequently, we successfully developed a highly luminescent pyrimidine conjugate emitter named PXZ-PPM exhibiting a high η ext of over 25%, with low driving voltages at Commission Internationale de l'Eclairage chromaticity coordinates (CIE) of (0.36, 0.58), and exceptionally low efficiency roll-off realizing η ext of over 22% at a high brightness of 1000 cd m −2 . These OLEDs also exhibited an η p of over 110 lm W −1 (84.9 cd A −1 , η ext = 25.1%) while maintaining extremely low voltages of 2.2 V at 1 cd m −2 and 3.0 V at 1000 cd m −2 , and an η p,1000 of 78.3 lm W -1 (75.5 cd A -1 , η ext = 22.3%). These performances clearly exceed those of previous TADF devices and are comparable to those of their stateof-the-art phosphorescent counterparts. [17][18][19][20][21][22][23] So far, our TADF molecules, denoted as Ac-RPM (R = H, CH 3 , and phenyl), were synthesized introducing two 9,10-dihydro-9,9-dimethyl-10-phenylacridine moieties at the 4-and 6-positions with regard to the pyrimidine unit and exhibited high photoluminescent quantum yields (η PL s) of approximately 80% and strong TADF properties. However, all Ac-RPM derivatives showed similar emission peaks (λ em ) around 490 nm and ΔE ST of 0.19 eV. Thus, the structure-property relationships should be clearly identified for effective molecular design and superior OLED performances based on pyrimidine conjugate emitters. With this aim, we designed two novel pyrimidinebased emitters named PXZ-PPM and Ac-NPM (Figure 1). Ac-NPM possessed a piperidine moiety at the 2-position of pyrimidine. In this sense, stronger electron-donating properties are expected to increase the lowest unoccupied molecular orbital (LUMO) levels, thereby leading to wider energy gaps (E g ) as compared to the phenyl-based Ac-PPM emitter. On the other hand, PXZ-PPM possessed phenoxazine (PXZ) moieties

By introducing an acceptor which contains double sulfonyl groups and a twisted biphenyl core, an efficient blue thermally activated delayed fluorescence emitter is successfully developed. Comparative study and analysis are carried out on it with an analogous molecule, which also imports two sulfonyl groups as acceptors and exactly same donor units but shows stretched shape and displays green emission. Results of quantum chemistry computation estimate that the introduction of such a twisted backbone is beneficial to increase energies of both singlet and triplet states. As a contrast to its counterpart, the newly developed blue emission compound with contorted structure enables a desirable emission with a peak wavelength of 460 nm and superior photoluminescence quantum yield up to 82%. Moreover, remarkable external quantum efficiency over 24% and Commission International de l'Eclairage coordinates with total (x + y) value <0.4 are achieved from the electroluminescent device based on the blue emitter, while this value is almost twice to that of the green emission device based on its analog.