Optimizing Optoelectronic Properties of Pyrimidine-Based TADF Emitters by Changing the Substituent for Organic Light-Emitting Diodes with External Quantum Efficiency Close to 25 % and Slow Efficiency Roll-Off

A series of green butterfly‐shaped thermally activated delayed fluorescence (TADF) emitters, namely PXZPM, PXZMePM, and PXZPhPM, are developed by integrating an electron‐donor (D) phenoxazine unit and electron‐acceptor (A) 2‐substituted pyrimidine moiety into one molecule via a phenyl‐bridge π linkage to form a D–π–A–π–D configuration. Changing the substituent at pyrimidine unit in these emitters can finely tune their emissive characteristics, thermal properties, and energy gaps between the singlet and triplet states while maintaining frontier molecular orbital levels, and thereby optimizing their optoelectronic properties. Employing these TADF emitters results in a green fluorescent organic light‐emitting diode (OLED) that exhibits a peak forward‐viewing external quantum efficiency (EQE) close to 25 % and a slow efficiency roll‐off characteristic at high luminance. Butterfly‐shaped TADF emitters: Optoelectronic properties of green butterfly‐shaped thermally activated delayed fluorescence (TADF) emitters on the basis of pyrimidine/phenoxazine hybrids are finely optimized by simply changing the substituent at the pyrimidne unit. Devices employing these emitters exhibit a peak forward‐viewing external quantum efficiency close to 25 % and a slow efficiency roll‐off characteristic at high luminance (see figure).