Isomerization design for improving the efficiency of red thermally activated delayed fluorescence emitters based on pyridopyrazinophenanthroline acceptor

Developing novel acceptor unit with stronger electron absorption ability and deeper lowest unoccupied molecular orbital (LUMO) is of great significance for designing efficient red thermally activated delayed fluorescence (TADF) emitters. Herein, a new acceptor pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (PPP) was developed, and through isomer engineering, two red TADF isomers otriTPA-PPP and mtriTPA-PPP were designed, using triphenylamine (TPA) as donors and PPP as acceptor. Two emitters show small ΔE_ST, which is beneficial for achieving efficient reverse intersystem crossing (RISC) process. OLEDs devices based on otriTPA-PPP and mtriTPA-PPP exhibit red emission at 597 and 603 nm, respectively. Due to the introducing of intramolecular H-bonding, otriTPA-PPP-based device shows excellent performance with a maximum external quantum efficiency (EQE_max) of 24.8%, comparing to only 8.5% for the mtriTPA-PPP-based device. Furthermore, otriTPA-PPP displays a significant improvement in luminance, with a larger luminance of 13170 cd m⁻² compared to 3896 cd m⁻² for mtriTPA-PPP.