New Wine in Old Bottles: Fully Substituted Arylthio Effect Realizes High‐Efficiency Purely Organic Phosphorescence Light‐Emitting Diode With Single and Ultra‐Stable Spectra Under 2000 cd m⁻²

ABSTRACT Organic light‐emitting diodes (OLEDs) based on purely organic room‐temperature phosphorescence (RTP) materials often encounter issues of relatively low efficiency and spectral instability. To overcome these limitations, three (arylthio)benzene derivatives (4S, 5S, and 6S) with gradually increased RTP component are designed and compared. Theoretical calculation and photophysical investigation reveal that the fully substituted arylthio effect could enhance the aggregation‐induced phosphorescence, enlarge the spin orbital coupling, and reduce the energy gap between S 1 and T 1 as much as possible. As a result, 6S can exhibit single spectra in films with a high phosphorescence efficiency of up to 76.7%, and its doped RTP‐OLED furnishes a high maximum external quantum efficiency (EQE) of 15.3% and ultra‐stable spectra with the brightness raised from 30 to 2000 cd m −2 . Furthermore, serving 6S as the sensitizer, the RTP‐sensitized‐fluorescent OLEDs based on fluorescence dopant TBRb and multiple resonance type dopant BN3 both show near three times improvement in electroluminescence performance, with EQE values of 11.3% and 25.6%, respectively. These results demonstrate the feasibility of fully substituted arylthio effect in designing RTP materials and could advance the development of high‐performance RTP OLEDs.