Judicious Heteroatom Doping Produces High‐Performance Deep‐Blue/Near‐UV Multiresonant Thermally Activated Delayed Fluorescence OLEDs

Abstract Two multiresonant thermally activated delayed fluorescence (MR‐TADF) emitters are presented and it is shown how further borylation of a deep‐blue MR‐TADF emitter, DIDOBNA‐N, both blueshifts and narrows the emission producing a new near‐UV MR‐TADF emitter, MesB‐DIDOBNA‐N, are shown. DIDOBNA‐N emits bright blue light (Φ _PL = 444 nm, FWHM = 64 nm, Φ _PL = 81%, τ _d = 23 ms, 1.5 wt% in TSPO1). The deep‐blue organic light‐emitting diode (OLED) based on this twisted MR‐TADF compound shows a very high maximum external quantum efficiency (EQE _max ) of 15.3% for a device with CIE _y of 0.073. The fused planar MR‐TADF emitter, MesB‐DIDOBNA‐N shows efficient and narrowband near‐UV emission ( λ _PL = 402 nm, FWHM = 19 nm, Φ _PL = 74.7%, τ _d = 133 ms, 1.5 wt% in TSPO1). The best OLED with MesB‐DIDOBNA‐N, doped in a co‐host, shows the highest efficiency reported for a near‐UV OLED at 16.2%. With a CIE _y coordinate of 0.049, this device also shows the bluest EL reported for a MR‐TADF OLED to date.