Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter.

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λ PL : 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕ PL : 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ // : 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λ EL : 398 nm; CIE x,y : 0.161, 0.040), but also show a record maximum external quantum efficiency (EQE max ) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.