Objective Organic light-emitting diode (OLED) is a new generation of display technology following LCD display, which has many advantages and has received widespread attention from researchers. OLED has been developing over thirty years, and emitters based on the three primary colors of red, green, and blue have met the requirements of commercialization. However, the latest Broadcast Television 2020 (BT.2020) display standard defines the color coordinates of blue emission as (0.131, 0.046), indicating higher requirements for the saturation of blue emission. Commercialized blue emitters are no longer able to meet the requirements of the BT. 2020 display standard. Some studies have shown that color purity is not only related to the emission wavelength, but also to the full width at half maximum (FWHM) of the emission spectrum. Therefore, deep blue materials with narrow emission are the object of current research workers' pursuit, which is crucial for achieving ultra-high definition and full color with wide color gamut displays. Moreover, ultraviolet light, as the extension of deep blue toward the short-wavelength direction, is also widely concerned by researchers because of its scarcity. Therefore, developing high-efficiency near ultraviolet and ultraviolet luminescent materials with narrow half peak width is of great significance for academic research and commercial applications

Methods All chemicals and reagents were purchased from commercial sources and used as received. The final product underwent vacuum sublimation to improve purity. All density functional theory (DFT) calculations were performed using the Gaussian 16 package. The theoretical calculation was completed at the M06-2X/6-31G (d,p) level. Solutions with a concentration of 1×10⁻⁵ M were prepared for the solution measurements. All organic films used for the photoluminescence measurements were deposited onto clean quartz substrates via thermal evaporation at 1–1.5 Å s⁻¹ under high vacuum with a base pressure of < 10⁻⁵ torr. Ultraviolet-visible absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer. PL spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. PLQYs were measured using a Hamamatsu absolute PL quantum yield spectrometer (C11347 Quantaurus_QY). Transient PL decay curves were measured using an Edinburgh Instrument FLS1000 spectrometer.

Cyclic voltammetry was conducted on a CHI 610E A14297 using a solution of tetra-n-butylammonium hexafluorophosphate (Bu₄NPF₆) (0.1 M) in dichloromethane or dimethylformamide at a scan rate of 100 mV s⁻¹. A platinum wire was used as the auxiliary electrode, a glass carbon disk as the working electrode, and Ag/Ag⁺ as the reference electrode, with the redox couple ferricenium/ferrocene (Fc/Fc⁺) serving as the calibration standard. The ionization potential (IPCV) and electron affinities (EACV) of these molecules were calculated using the following formulas: IPCV = (E_ox − E_1/2(Fc/Fc⁺) + 4.8) eV and EACV = (E_red − E_1/2(Fc/Fc⁺) + 4.8) eV, where E_ox and E_red represent the onset oxidation potential and reduction potential relative to Fc/Fc⁺ (4.8 eV), respectively. Thermogravimetric analysis was performed on a Netzsch TG 209 under nitrogen flow at a heating rate of 10℃ min⁻¹. Differential scanning calorimetric was performed on a Netzsch DSC 200 F3 under nitrogen flow at a heating rate of 10 ℃ min⁻¹.

The glass substrates, precoated with a 90 nm layer of ITO with a sheet resistance of 15 to 20 ohms per square, were successively cleaned in ultrasonic bath of acetone, isopropanol, detergent, and deionized water, respectively, with each step lasting 10 min. Then, the substrates were completely dried in a 70 ℃ oven. To improve the hole injection ability of ITO, the substrates underwent O₂ plasma treatment for 6 min before fabrication. The vacuum-deposited OLEDs were fabricated under a pressure of < 5 × 10⁻⁴ Pa in the Suzhou Fangsheng FS-380 vacuum deposition system. Organic materials, LiF, and Al were deposited at rates of 0.5 to 1.5 A, 0.1, and 3 A s⁻¹, respectively. The effective emitting area of the device was 9 mm², determined by the overlap between the anode and cathode. The luminance-voltage-current density characteristics and EL spectra were obtained via a PIN-25D silicon photodiode and Luminance Meter and an Ocean Optics USB 2000+ spectrometer, along with a Keithley 2400 Source Meter.

Results and Discussions Based on the O/B/O multiple resonance molecular framework, benzocarbazole derivatives were introduced in the para position of the central benzene ring connected to B atom. By introducing methyl groups on the side of the intermediate benzene bridge, the emissions of the TB-PhCz, TB-MePhCz, and TB-2MePhCz were shifted from near ultraviolet to ultraviolet. Theoretical calculations indicate that a gradual decrease in the electron delocalization degree of S₁ states of TB-PhCz, TB-MePhCz, and TB-2MePhCz with an increase in the number of methyl groups introduced, exhibiting a characteristic of multiple resonance distribution. This is consistent with the gradual blue shift of their fluorescence emission peak positions, shifting from near ultraviolet to ultraviolet. Their doped devices all exhibit excellent electroluminescence performance, the maximum external quantum efficiency of TB-MePhCz based OLED device reaches 8.05% and thedevice has a high color purity (the full width at half maxima is 23 nm); TB-2MePhCz based OLED exhibits the deepest violet emission, thepeak of which is at 396 nm (the full width at half maxima is 20 nm), the CIE coordinates are (0.169, 0.012), and the CIEy value approaches 0.01, which is currently the lowest CIEy value reported in the literature, and the external quantum efficiency of the device reaches 7.81%, indicating that this molecular design strategy can effectively limit molecular conjugation and achieve blue shift of luminescent molecules, providing guidance for the development of more high-performance ultraviolet emitters.

Conclusions Three ultraviolet-emitting molecules, TB-PhCz, TB-MePhCz, and TB-2MePhCz were designed and synthesized by tuning molecular torsion based on tert-butyl boron-oxygen multiple resonance skeletons. These molecules exhibit multiple resonance characteristics, achieving emission blue shift from near-ultraviolet to ultraviolet region. High-efficiency narrowband violet OLEDs were fabricated using the three molecules, respectively.