INTRODUCTION

Ultraviolet (UV) luminescent materials have garnered significant research attention owing to their promising applications in medical phototherapy, environmental purification, excitation light sources, and high‐density data storage.[ 1 , 2 , 3 , 4 , 5 ] Compared to conventional mercury lamps and inorganic light‐emitting diodes (LEDs), organic light‐emitting diodes (OLEDs) offer distinctive advantages such as self‐emission, low power consumption, lightweight flexibility, and designable molecular structures, making them ideal candidates for next‐generation UV light sources.[ 6 , 7 , 8 , 9 , 10 , 11 ] However, in contrast to their red, green, and blue counterparts, the variety and quantity of high‐performance UV emitters remain scarce, and electroluminescence (EL) efficiencies of ultraviolet organic light‐emitting diodes (UV‐OLEDs) still lag. As delineated by the expression for external quantum efficiency (EQE): where γ is the charge recombination efficiency (ideally 100%). η out is the light out‐coupling efficiency (20%–30% for bottom‐emitting OLEDs), η r is the radiative exciton probability, which characterizes the efficiency of excitons, ΦPLQY is the photoluminescence quantum yield (PLQY) of the emitter, respectively. The construction of UV emitters with high PLQY is one of the critical factors for achieving high‐performance UV‐OLEDs with a high EQE. However, high PLQY typically requires extended π‐conjugation lengths to enhance electron delocalization, whereas short‐wavelength UV emission necessitates short π‐conjugation lengths in emitters. This inherent contradiction poses significant challenges in designing UV emitters that simultaneously achieve high PLQY and short‐wavelength UV emission. Recent efforts to enhance PLQY have leveraged a diverse array of molecular design strategies, including using bulky substituents to suppress π–π stacking, designing planarized intramolecular charge transfer state emitters based on donor′–acceptor–donor (D′–D–A) architecture, constructing rigid coplanar emitter with medium‐range CT state, developing aggregation‐induced emission enhancement (AIEE)‐active emitters, and utilizing “crossed long‐short axis” molecular strategy.[ 12 , 13 , 14 , 15 , 16 ] Although the aforementioned strategies have achieved partial enhancement in PLQY, reported UV emitters rarely achieve PLQY values exceeding 80% in the film state.

(1) EQE=γ×ηout×ηr×ΦPLQY,

The effective utilization of 75% of electrically generated triplet excitons constitutes another critical factor for achieving high EQE in pure organic EL materials. To date, various triplet‐harvesting material systems have been developed, including triplet‐triplet annihilation (TTA), thermally activated delayed fluorescence (TADF), room‐temperature phosphorescence, and hybridized local and charge‐transfer (HLCT) materials.[ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ] In particular, HLCT materials enabling triplet exciton harvesting via high‐lying reverse intersystem crossing (hRISC) demonstrate exceptional promise in UV‐OLEDs.[ 4 , 12 , 23 , 26 , 27 , 28 , 29 , 30 ] In this molecule system, localized excited (LE) state component effectively preserves higher exciton energy and fast radiative decay process, achieving high PLQYs, while CT state component facilitate efficient hRISC from high‐energy triplet states for utilizing dark‐state triplet excitons.[ 13 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ] There are a few reported high‐efficiency OLEDs that have achieved a maximum EQE of over 10% and stable UV emission below 400 nm. For example, Zhang et al. reported a long‐short axis UV emitter 2BuCz‐CNCz, exhibiting an EL peak at 396 nm alongside a maximum EQE as 10.79%. [38] Zhang et al. developed a linear D–A–D type emitter 9,9'‐(2,2',5,5'‐tetrafluoro‐[1,1'‐biphenyl]‐4,4'‐diyl)bis(3,6‐di‐tert‐butyl‐9_H_‐carbazole) (CDFDB), achieving UV emission at 398 nm with an EQE of 12.0%. [6] Li et al. harnessed the embedding of bis‐B–O units into non‐linear polycyclic frameworks to develop BO‐bPh, which elicits EL peaking at 394 nm with a maximum EQE of 11.3%. [36] Zhan et al. engineered the rigid, planar double B–O–N polycyclic architecture of emitter BO–N, which, as the doped device, yields an EL peak at 399 nm and a maximum EQE of 18.6%. [37] Although the above‐mentioned UV‐OLEDs have achieved certain breakthroughs in maximum EQE, their emission peaks are predominantly concentrated in the 390–400 nm range, and the efficiency roll‐off at high luminance levels is particularly severe, with EQE dropping below 5% at 500 cd m−2 (Figure 1a).

FIGURE 1: (a) Reported UV emitters with high EL performance. (b) Schematic of the molecular design principle of D3CZ4F. EL, electroluminescence; UV, ultraviolet.

Hence, we present a straightforward yet highly efficient donor‐acceptor‐donor (D–A–D) type HLCT UV emitter, 3,3′‐(perfluoro‐1,4‐phenylene)bis(9‐phenyl‐9_H_‐carbazole) (D3CZ4F), which was synthesized via a one‐step Suzuki coupling reaction (Figure 1b). The enhanced intramolecular non‐covalent C–H⋯F interactions promote more planarized conformation from the ground state to the lowest singlet excited state, enhancing the structural rigidity and π‐electron delocalization. This endows D3CZ4F with a high oscillator strength and fluorescence efficiency while maintaining short‐wavelength UV emission. Accordingly, D3CZ4F exhibits a UV emission with peak and PLQY of 388 nm and 98% in solution, and 379 nm and 86% in doped film, respectively. The doped device achieved a maximum EQE of 8.97% with an EL peak at 388 nm, a full‐width at half‐maximum (FWHM) of 42 nm, and Commission Internationale de I’Eclairage (CIE) coordinates of (0.165, 0.025). The EQE remained at 8.15% and 6.63% at 500 and 1000 cd m−2. Owing to the high horizontal dipole orientation of the D3CZ4F neat film, the D3CZ4F‐based non‐doped OLED achieved a maximum EQE of 7.33%, an EL peak at 390 nm, and CIE coordinates of (0.164, 0.027), respectively.