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 (phi(PL)s) oftrans-isomers T-DA-1 and T-DA-2 (78% and 89%) are remarkably higher than those of their correspondingcis-isomers C-DA-1 and C-DA-2 (12% and 14%). Significantly increased phi(PL)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.