Here, we developed a series of pyrimidine-based TADF emitters, 2-functionalized-4,6-bis[4-(9,9-dimethyl-9,10- dihydroacridine)phenyl]pyrimidine called Ac-RPM. We introduced a phenylacridine moiety into the 4,6-position of pyrimidine core to induce a twisted structure leading to a high PLQY of ~80%, and a small singlet and triplet excited energy difference of <0.20 eV. The optimized device realized a power efficiency of 62 lm W-1, a high EQEmax of 25%, light-blue emissions with the Commission Internationale de l'Eclairage chromaticity (CIE) coordinates of (0.19, 0.37) and a low turn-on voltage of <3.0 V. Further, we investigated the strucutre-property relationship to unlock the potential of pyrimidine-based emitter. Consequently, we developed a green emitter realizing a power efficiency of over 110 lm W-1 while maintaining extremely low voltages of 2.2 V at 1 cd m-2 and 3.0 V at 1000 cd m-2 at CIE of (0.36, 0.58). Apparently, these performances exceed those of previous TADF devices and are comparable to those of their state-of-the-art phosphorescent devices.