Rational molecular design of TADF emitters towards highly efficient yellow electroluminescence with a nearly 30% external quantum efficiency and low roll-off

Developing a feasible strategy to realize thermally activated delayed fluorescence (TADF) emitters with high efficiency and tunable emission colors is essential for organic light-emitting diodes (OLEDs). In this study, based on a rigid planar acceptor of 2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and phenoxazine-derived donors, two new TADF emitters, namely PXZBO1 and PXZBO2, featuring tunable emission were designed and prepared for highly efficient OLEDs. The donor engineering by attaching additional electron-donating tert-butyldiphenylamine groups to the donor of PXZBO1 successfully results in PXZBO2 with significantly redshifted emission. PXZBO1 and PXZBO2 are characterized by highly twisted molecular structures to achieve a very small singlet-triplet splitting of 0.01 eV and enable efficient TADF, with high photoluminescence quantum yields of over 94% and fast reverse intersystem crossing rates of over 10(6) s(-1) in doped films. As a result, TADF OLEDs using PXZBO1 and PXZBO2 as emitters achieved maximum external quantum efficiencies of 25.9% and 29.7%, peaking at 503 and 563 nm, respectively, with roll-off efficiencies of only similar to 11% at a luminance of 1000 cd m(-2). The excellent device performance renders PXZBO2 one of the most efficient yellow TADF emitters ever reported.