Improving the Efficiency of Red Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diode by Rational Isomer Engineering

The development of efficient red thermally activated delayed fluorescence (TADF) emitters with an emission wavelength beyond 600 nm remains a great challenge for organic light‐emitting diodes (OLEDs). Herein, two pairs of isomers are designed and synthesized by attaching electron‐donor 9,9‐diphenyl‐9,10‐dihydroacridine (DPAC) moiety to the different positions of two kinds of highly rigid planar acceptor cores (PDCN and PPDCN). Their TADF efficiencies and emission maxima (599–726 nm) are regulated by molecular isomer manipulation. Interestingly, the photoluminescence quantum yields (ΦPLs) of trans‐isomers T‐DA‐1 and T‐DA‐2 (78% and 89%) are remarkably higher than those of their corresponding cis‐isomers C‐DA‐1 and C‐DA‐2 (12% and 14%). Significantly increased ΦPL values can be explained by single crystal structures and theoretical simulation. As a result, a deep red TADF‐OLED based on T‐DA‐2 displays a maximum external quantum efficiency (EQE) of 26.26% at 640 nm. Notably, at a brightness of 100 cd m−2, the EQE value of T‐DA‐2‐based device still remains at an extremely high level of 23.95%, representing the highest value for reported red TADF‐OLEDs at the same brightness. These results provide a reasonable pathway to optimize optoelectronic properties and thereby construct efficient red TADF emitters through rational isomer engineering. Two pairs of isomers with thermally activated delayed fluorescence and obviously different photoluminescence efficiency are developed via rational isomer engineering. A highly efficient red thermally activated delayed fluorescence organic light‐emitting diode with a maximum external quantum efficiency up to 26.26% at a peak wavelength of 640 nm and low roll‐off is achieved.