New-record efficiency in non-doped ultraviolet OLEDs: Hot-exciton emitters by a crossed long-short axis design boosting EQE up to 8.3% and emission peak close to 380 nm

doi:10.21203/rs.3.rs-4916381/v1

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New-record efficiency in non-doped ultraviolet OLEDs: Hot-exciton emitters by a crossed long-short axis design boosting EQE up to 8.3% and emission peak close to 380 nm

https://doi.org/10.21203/rs.3.rs-4916381/v1

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Organic light-emitting diodes (OLEDs) radiating ultraviolet (UV) emission are highly desirable for their unique applications in anti-counterfeit, healthcare, industry, etc. However, high-performance UV-OLEDs using a simplified non-doped process are rarely reported because organic emitters usually face the emission red-shift and quenching problems in aggregates. Herien, two new UV hot-exciton emitters with crossed long-short axis structures, abbreviated p-Cz and m-Cz, are designed by altering the para- and meta- patterns at the long-axis skeleton. Theoretical calculations combined with photophysical measurements indicate that the m-Cz can display a bluer emission because of its shorter π-conjugation and effectively alleviate the negative effects in aggregates. Consequently, a doped UV-OLED is realized based on m-Cz with an electroluminescence peak (λ_EL) at 382 nm and a maximum external quantum efficiency (EQE_max) of 10.6%. What's more, the non-doped UV-OLED based on m-Cz exhibits an λ_EL at 382 nm and an EQE_max up to 8.3% and maintains an EQE of 7.0% at 1000 cd m^− 2, representing a new record of efficiency in the field. Furthermore, the device shows the longest reported operational lifetimes and can be successfully applied to an excitation light source.

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Organic LEDs

Physical sciences/Optics and photonics/Optical materials and structures

organic light-emitting diode

ultraviolet emission

non-doped device

hot-exciton

Ultraviolet (UV) emission plays a key role in the development of synthetic photochemistry and photocatalysis. The common equipment to realize UV emission includes poisonous mercury lamps and light-emitting diodes (LEDs)^1–4. In comparison, organic LEDs (OLEDs) have been regarded as a new generation of display and illumination technologies due to their lightweight and flexible characteristics, thus potentially becoming a new carrier for UV light sources. UV organic emitters serve as the core functional layer in OLEDs, which require limited conjugation and structural rigidity in molecular design. However, their inherent wide bandgap is not conducive to carrier injection and recombination. Meanwhile, the strong intermolecular interactions in aggregates originating from the structural rigidity may lead to a remarkably red-shift and thus deviate from the UV region. To overcome these issues, UV emitters are always doped into bipolar hosts. This will face new issues: (i) the doping process is relatively complex; (ii) the high triplet energy level of UV emitters has a rigorous demand for host materials. Therefore, the development of non-doped UV-OLEDs is significant and imperative.

The pioneering work of Wong et al. reported the UV emitters based on the fluorene group in 2005, and the corresponding non-doped UV-OLEDs show maximum external quantum efficiencies (EQE_max) up to 3.6%⁵. In the following decade, UV-OLEDs' development was mainly based on traditional fluorescent emitters, but their electroluminescence (EL) efficiencies were below 5% due to the limitation of 25% singlet-exciton utilization^6–11. Therefore, some researchers began to introduce triplet exciton utilization mechanisms into UV emitters. For example, Lu et al. reported the first UV emitter CZ-MPS with the thermally activated delayed fluorescence (TADF) mechanism in 2020¹². The corresponding doped UV-OLED exhibits a peak maximum (λ_max) at 389 nm and an EQE_max of 9.3%. However, its low-lying intramolecular charge-transfer (CT) emissive state and slow reverse intersystem crossing (RISC) process make the device poor color-purity and large efficiency roll-off. In 2021, our group reported the first hot-exciton UV emitter 2BuCz-CNCz, whose low-lying locally excited (LE) emissive state and relatively fast RISC process endow its doped device with high color-purity and improved efficiency roll-off, and the corresponding λ_max and EQE_max are 396 nm and 10.8%, respectively¹³. In 2023, Romanov et al. reported a series of carbene-metal-acetylides (CMAc) complexes, and the doped near-UV OLED showed a λ_max at 404 nm and EQE_max of 1.1%, respectively. In addition, they evaluated the device stability and the best operational lifetime showing LT₅₀ of ca. 20 min at 10 nits brightness¹⁴. In recent years, multi-resonance TADF materials have emerged rapidly because their unique frontier molecular orbitals distribution effectively solves the poor color-purity of traditional TADF. Xu et al. designed UV multi-resonance TADF emitters CzP2PO and tBCzP2PO with narrowband emission. The doped UV-OLED (λ_max: 384 nm) achieved record-high EL efficiency with EQE_max of 15.1%¹⁵. However, all high-efficiency UV-OLEDs above are doped devices (Fig. 1), and most reported EQEs of non-doped ones are still less than 5%^16–21.

Constructing limited intramolecular π-conjugation and suppressing strong intermolecular aggregation is essential to fabricating non-doped OLEDs using short-wavelength emitters^22–26. On the other hand, some hot-exciton emitters feature the separation of the triplet-to-singlet conversion channel from the radiation transition channel, which is proven to have advantages in achieving UV-OLEDs with high efficiency^27–30. On these bases, two tailor-made hot-exciton-type UV emitters, abbreviated p-Cz and m-Cz, are designed based on our previously proposed crossed long-short axis (CLSA) strategy, where small torsion angles (θ₁ and θ₂) in the long-axis conjugated skeleton while a large torsion angle (θ₃) in the short-axis donor-acceptor (D-A) structure can be found (Fig. 1)^{13,27,28,30,31–33}. Theoretical calculations and photophysical measurements show that meta-pattern in m-Cz can shorten the π-conjugation degree for blue-shift emission and increase steric hindrance for suppressing close aggregation. Consequently, compared with p-Cz, m-Cz can not only display a larger UV₄₀₀ (the area proportion of the UV emission, λ ≤ 400 nm) in the EL spectra, but also maintain a λ_max smaller than 400 nm in the non-doped device. Fascinatingly, the m-Cz-based non-doped UV-OLED achieves a record-high EQE of 8.3% with a λ_max at 382 nm, representing a breakthrough in this field. Moreover, the operational lifetimes of the doped and non-doped UV-OLEDs based on m-Cz are ca. 3.0 and 0.3 hours at an initial current density of 10 mA/cm², which are the longest results among the reported UV-OLEDs.

p-Cz and m-Cz were obtained in high yields according to the synthetic routes described in Scheme S1 and S2 and verified by ¹H and ¹³C NMR and mass spectra (Fig. S1 and S2). The electrochemical properties of p-Cz and m-Cz are investigated by cyclic voltammetry (CV) measurement (Fig. S3). The ionization potentials (IP_CV) and electron affinities (EA_CV) are estimated to be 5.53 eV and 2.29 eV for p-Cz and 5.57 eV and 2.29 eV for m-Cz, respectively. The thermogravimetric analysis and differential scanning calorimetry reveal their high thermal stability with the decomposition temperature (T_d, corresponding to 5% weight loss) of 490 ℃ and undetectable glass-transition temperature (T_g) for p-Cz, and T_d of 501 ℃ and T_g of 157 ℃ and for m-Cz (Fig. S4).

The root-mean-square displacements (RMSDs) of p-Cz and m-Cz are calculated to be 0.472 Å and 0.633 Å, which indicates that p-Cz shows higher structural rigidity than m-Cz and might have higher photoluminescence (PL) efficiency in single-molecule level (Fig. 2). Nevertheless, the S₀ configuration of m-Cz shows a more pronounced twisted conformation than p-Cz from the top view (Fig. S5), which could suppress the close intermolecular interactions and is conducive to high PL efficiency in aggregates. The lowest unoccupied molecular orbitals (LUMOs) of both p-Cz and m-Cz are distributed on the central carbazole and methyl-phenylcyano group. Different from p-Cz with the highest occupied molecular orbital (HOMO) delocalized over the whole long-axis, the HOMO of m-Cz is only located on the two meta-linked phenyl carbazoles. The almost completely separated HOMO and LUMO implies that m-Cz could have a more bipolar transport ability. The natural transition orbitals (NTO) simulation (Fig. S6 and S7) shows that the holes and particles of p-Cz and m-Cz in S₁→S₀ are overlapped on the long-axis, which is in accord with the CLSA design where the long-axis skeleton is responsible for the emission. Compared to p-Cz, m-Cz with mate-pattern shows an elevation of the S₁ energy level from 3.39 eV to 3.61 eV. The T₁ energy levels are also calculated to be 2.53 eV and 2.57 eV for p-Cz and m-Cz. It can be noted that the energy gaps (ΔE_ST) between the high-lying triplet states (T₂, T₃, and T₄) and S₁ states are relatively small for both p-Cz and m-Cz. This kind of energy-level distribution could facilitate the triplet-to-singlet conversion through the hot-exciton channels. In addition, for both p-Cz and m-Cz, the spin-orbital coupling (SOC) values between S₁ and T₃ (〈S₁ĤSOT₃〉) are remarkably larger than 〈S₁ĤSOT₂〉 and 〈S₁ĤSOT₄〉, which implies that T₃→S₁ might be dominant in the hot-exciton channels (Table S1 and S2).

Absorption spectra of p-Cz and m-Cz in dilute toluene solutions (10^− 5 M) are composed of two main bands peaked at 294 and 344 nm for p-Cz and 293 and 326 nm for m-Cz, corresponding to π-π* transition of carbazole units and extended π-conjugation of the long-axis skeleton, respectively (Fig. 3a and Table 1). Compared to p-Cz, the blue-shift long-axis absorption of m-Cz should be caused by the shortened conjugation length. p-Cz shows a narrowband UV emission peaking at 380 nm and 398 nm with a full width at half maximum (FWHM) values of 38 nm. For m-Cz, PL peaks at 375 nm and 392 nm with an FWHM value of 44 nm were observed. The slightly smaller FWHM of p-Cz originated from its more rigid structure (smaller RMSD value in theoretical simulation). Further, the phosphorescence emissions with peaks at 499 and 458 nm were observed in frozen toluene solutions, and the S₁ and T₁ energy levels were estimated to be 3.27 and 2.49 eV for p-Cz and 3.47 and 2.86 for m-Cz, corresponding to large ΔE_S1T1 values of 0.78 and 0.61 eV, respectively. The solvatochromic effects in different solvents with varied polarity were measured to better understand the excited-state characteristics (Fig. 3b and S9, Table S3 and S4). The absorption spectra of p-Cz and m-Cz are independent of solvents, but their PL spectra red-shift by 24 and 12 nm from low-polar n-hexane to high-polar acetonitrile. Compared to p-Cz, the maintained fine structures and smaller redshifts in PL spectra for m-Cz indicated a decreased CT component. The Lippert–Mataga model of p-Cz and m-Cz was further evaluated^34,35. It can be seen that p-Cz shows two-section fitted lines between stokes shift (v_a–v_f) and solvent polarity (f) with corresponding dipole moments (µ_e) values of 6.6 D and 16.1 D, which are assigned to the nonequivalent excited states with LE-dominant in low-polarity solvents and CT-dominant in high-polarity solvents, respectively. In comparison, m-Cz only shows one fitted line with moderate µ_e of 10.4 D, indicative of a quasi-equivalent hybridization in excited state^32,36–38.

The PL quantum yields (PLQYs) of p-Cz and m-Cz were tested at 93.8% and 65.1%, respectively. The smaller PLQY of m-Cz is caused by limited π-conjugation and reduced structural rigidity. After being fabricated into neat films, the PLQY values of p-Cz and m-Cz were recorded as 59.4% and 57.6%, respectively. The smaller decrease of PLQY in m-Cz neat

Table 1

Optical, thermal, and electronic properties of p-Cz and m-Cz.
	λ_abs [nm]	λ_em [nm]				η_PL [%]		τ [ns]		E_S1/E_T1/ΔE_S1T1 [eV]	EA/IP [eV]
	λ_abs [nm]	soln	FWHM	film	FWHM	soln	film	soln	film	E_S1/E_T1/ΔE_S1T1 [eV]	EA/IP [eV]
p-Cz	342	380, 398	38	414	69	93.8	59.4	1.4	1.2	3.27/2.49/0.78	2.31/5.79
m-Cz	340	375, 392	44	387	56	65.1	57.6	4.6	7.8	3.47/2.86/0.61	2.31/5.84

soln = in toluene solution; film = in neat film; η_PL = absolute PLQY; τ = fluorescence lifetime.

film is due to the effective suppression of strong aggregation, which is consistent with the calculation results. The lifetimes of p-Cz and m-Cz in solutions and neat films are nanosecond-scaled (Fig. 3c and Table 1), demonstrating no TADF characteristic in these two emitters. Furthermore, the horizontal dipole orientation (Θ_//) of p-Cz and m-Cz neat films were estimated, and both p-Cz and m-Cz show high Θ_// values of 87.0% and 80.5%, respectively (Fig. 3d).

To evaluate the potential of p-Cz and m-Cz for application to UV-OLEDs, the doped devices were first fabricated with configuration of ITO (180 nm)/ hexaazatriphenylenehexacabonitrile (HATCN) (5 nm)/ 1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (50 nm)/ tris[4-(carbazol-9-yl)phenyl]amine (TcTa) (5 nm)/EML (20 nm)/ 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) (40 nm)/ lithium fluoride (LiF) (1 nm)/Al (120 nm), in which ITO is the highly conductive transparent anode, HATCN is the efficient hole injection material, TAPC acts as the hole-transporting layer, TcTa serves as the electron blocking layer, TPBi functions as the electron-transporting layer, LiF and Al are the cathode, 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) matrix with high T₁ is used as host, and the optimized doping concentration is 3 wt% (Fig. S10). As expected, The doped devices of both p-Cz and m-Cz exhibit UV emissions with peaks at 387 and 382 nm, respectively. The small FWHMs of 44 and 38 nm make the devices high color-purity with the corresponding CIE coordinates of (0.167, 0.027) and (0.175, 0.042), respectively. The doped devices based on p-Cz and m-Cz achieve high EQE_max values of 8.8% and 10.6%, which are outstanding efficiencies among the UV-OLEDs (Fig. S11 and Table S5). The m-Cz-based doped device shows a slightly larger CIE_y value due to the very weak electroplex emission in the range of 550 ~ 650 nm, which is a common phenomenon in short-wavelength materials^39,40. In fact, the spectra of currently reported UV-OLEDs are composed of the UV (λ ≤ 400 nm) component and partial visible-light (λ > 400 nm) component². To evaluate the purity of UV-OLED, we proposed the parameter UV₄₀₀, which is defined as the area proportion of the UV emission. It can be seen that the doped OLED based on m-Cz has a larger UV₄₀₀ (57.5%) than that of p-Cz (42.8%), manifesting a purer UV-OLED.

Further, the non-doped devices based on p-Cz and m-Cz were fabricated with the same configuration. As most reported UV emitters undergo red-shift and deviation from the UV region after aggregation, the p-Cz exhibits a near-UV emission with a peak at 406 nm and a decreased UV₄₀₀ of 16.3% in the non-doped device (Fig. 4)^16,20,41. Excitingly, the non-doped OLED based on m-Cz can maintain a UV emission peaking at 382 nm, with a nearly unchanged FWHM of 39 nm and corresponding CIE coordinates of (0.165, 0.028). Different from p-Cz, the UV₄₀₀ of m-Cz-based non-doped devices is still as large as 59.6%. It is worth noting that m-Cz can furnish an excellent EQE_max of 8.3% and keep an EQE of 7.0% at 1000 cd m^− 2 (Fig. 4b, Fig. S12, and Table 2), representing a new record of EL efficiency for non-doped UV-OLEDs (Fig. S1 and Table S6). Because UV emission can be absorbed by functional layer materials in OLED, the optical coupling output efficiencies (η_out) of UV-OLEDs are relatively smaller than the OLEDs with other colors. With the data of Θ_//, reflex index, and active layer thickness, the p-Cz- and m-Cz- based non-doped OLEDs show small η_out values of 18.5% and 17.1%, respectively. In this case, the corresponding exciton utilization efficiencies (EUE) were calculated to be ~ 68.4% and ~ 84.5%, which exceeds the upper limitation of EUE in TTA emitters. Besides, the linear dependences of current density-luminance curves at the current density smaller than 10 mA cm^− 2 indicate that the EQE_max is not caused by TTA up-conversion (Fig. S13). The transient EL spectra of p-Cz- and m-Cz- based non-doped OLEDs demonstrate that the delayed component (delay%) from TTA are less than 1.6% and 2.0% (Fig. 4d and S14, Table S7-S8), which further exclude the contribution of TTA mechanism^42,43. Therefore, the high-efficiency non-doped UV-OLED should be attributed to the hot-exciton mechanism.

Encouraged by the low efficiency roll-off and high UV₄₀₀, the operational stability and application scenario of m-Cz-based OLEDs were investigated (Fig. 4e and Fig.s S15). The doped and non-doped UV-OLEDs exhibit an LT₅₀ (time to 50% of the initial current density) of 3.0 h and 0.3 h at an initial current density of 10 mA m^− 2. By applying an acceleration factor of 1.6, the LT₅₀ values at an initial luminance of 10 cd m^− 2 were simulated as 52.7 h and 3.9 h, which are 158 and 12 times longer than the operational lifetime of recently reported CMAc-based UV-OLEDs¹⁴, respectively. Furthermore, the non-doped UV-OLED based on m-Cz could serve as a UV excitation light source to validate the RMB anti-counterfeiting labels,

Table 2

EL performances of the devices based on p-Cz and m-Cz.
Device type	Emitter	λ_max [nm]	FWHM [nm]	UV₄₀₀ [%]	V_on [V]	L_max [cd/m²]	CE_max [cd/A]	PE_max [lm/W]	EQE_max/1000 [%]	CIE (x, y)	LT₅₀ [hour]
doped OLEDs	p-Cz	391	44	42.8	4.2	1109	0.74	0.55	8.8/3.4	0.167, 0.027	0.1
doped OLEDs	m-Cz	382	38	57.5	4.0	476	1.20	0.90	10.6/-	0.175, 0.042	3.0
non-doped OLEDs	p-Cz	406	47	16.3	3.8	2732	1.02	0.72	7.5/7.1	0.162, 0.024	249.3
non-doped OLEDs	m-Cz	382	39	59.6	4.4	1706	0.62	0.43	8.3/7.0	0.165, 0.028	0.3

V_on = turn-on voltage; L_max = maximum luminance; CE_max = maximum current efficiency; PE_max = maximum power efficiency; EQE_max/1000 = maximum EQE/EQE at 1000 cd m^− 2; CIE (x, y) = commission internationale de l'éclairage, recorded at 10 mA cm^− 2; LT₅₀ = initial current density at 10 mA cm^− 2.

excite RGB emitters, and display the phenomenon of aggregation-induced emission (AIE) (Fig. 4f).

In summary, two UV hot-exciton emitters of p-Cz and m-Cz were obtained based on CLAS molecular design. Compared to p-Cz, m-Cz with meta-pattern at long-axis conjugated skeleton features limited π-conjugation and increased steric hindrance, which can facilitate a bluer emission and alleviate the negative effects of strong aggregation. As a result, m-Cz can maintain both UV PL and EL in neat film and nondoped OLED, respectively. Combined with high EUE from hot-exciton mechanism, the non-doped UV-OLED (λ_max: 382 nm) based on m-Cz achieves a record-high EQE_max of 8.3% with large UV₄₀₀ and CIE coordinates of 59.6% and (0.165, 0.028). Moreover, the non-doped UV-OLED exhibits a small efficiency roll-off with EQE₁₀₀₀ of 7.0% and an operational lifetime 12-fold longer than the reported longest device. We have also successfully applied the non-doped UV-OLED as a UV excitation light source. These results could provide a referable molecular design for non-doped UV-OLEDs and promote the progress of UV-OLEDs towards practical application.

Synthesis and Characterization

The compounds HATCN, TAPC, TcTa, TPBi, and CzSi were purchased from commercial sources. The p-Cz and m-Cz were synthesized according to previously reported methods.

4-(2,7-bis(4-(9H-carbazol-9-yl)phenyl)-9H-carbazol-9-yl)-3-methylbenzonitrile (p-Cz): 3-Methy-4-Fluoro benzonitrile (4.05 g, 30 mmol), 2,7-Dibromo-9H-Carbazole (8.13 g, 25 mmol) and K2CO3 (10.37 g, 75 mmol) were stirred for 30 min in deoxygenated and dehydrated N, N-dimethylformamide (50 mL) under nitrogen at room temperature; Then the reaction mixture was heated up to 150°C and reacted for 12 h. After cooling to room temperature, the reaction mixture was poured into a large amount of water and extracted twice with dichloromethane. After the solvent was evaporated under reduced pressure, the residue was purified by column chromatography on silica-gel (dichloromethane/petroleum) to afford a white solid of 2Br-CNMCz in 76% yield.

A mixture of compound 2Br-CNMCz (1.14 g, 2.6 mmol), (4-(9H-carbazol-9-yl)phenyl)boronic acid (2.24 g, 7.8 mmol), Pd(PPh3)4(0.3 g, 0.26 mmol)and potassium carbonate (1.24 g, 9 mmol) was added in 250 mL two-neck bottle under nitrogen. Then, a mixed solvent system of toluene, C2H5OH, and H2O (v/v/v = 8:1:1) was injected into the bottle, and the reaction mixture was refluxed for 12 h. After cooling to room temperature, the mixture was poured into water, extracted twice with dichloromethane, and dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated under reduced pressure and the residue was purified by silica-gel column chromatography (dichloromethane/petroleum). White solid of p-Cz was obtained in 76% yield.

¹H NMR (400 MHz, CD₂Cl₂) δ 8.34 (d, J = 8.1 Hz, 2H), 8.17 (d, J = 7.7 Hz, 4H), 7.90 (d, J = 8.2 Hz, 5H), 7.86–7.78 (m, 2H), 7.67 (d, J = 8.4 Hz, 7H), 7.46 (dd, J = 14.3, 7.5 Hz, 8H), 7.36–7.27 (m, 5H), 2.19 (s, 3H). ¹³C NMR (101 MHz, CD₂Cl₂) δ 140.81 (s), 128.79 (s), 127.25 (s), 125.99 (s), 123.36 (s), 121.10 (s), 120.12 (d, J = 25.4 Hz), 109.76 (s), 108.08 (s). MALDI-TOF-MS (m/z): calcd for C₅₆H₃₆N₄, 764.294; found, 764.330 [M⁺].

4-(2,7-bis(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-9-yl)-3-methylbenzonitrile (m-Cz): 2Br-CzMCN is prepared by referring to the method reported in the literature. A mixture of compound 2Br-CNMCz (1.14 g, 2.6 mmol), (3-(9H-carbazol-9-yl)phenyl)boronic acid (2.24 g, 7.8 mmol), Pd(PPh3)4(0.3 g, 0.26 mmol)and potassium carbonate (1.24 g, 9 mmol) was added in 250 mL two-neck bottle under nitrogen. Then, a mixed solvent system of toluene, C2H5OH, and H2O (v/v/v = 8:1:1) was injected into the bottle, and the reaction mixture was refluxed for 12 h. After cooling to room temperature, the mixture was poured into water, extracted twice with dichloromethane, and dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated under reduced pressure and the residue was purified by silica-gel column chromatography (dichloromethane/petroleum). White solid of m-Cz was obtained in 76% yield. ¹H NMR (500 MHz, CD₂Cl₂) δ 8.99 (d, J = 8.1 Hz, 2H), 8.88 (d, J = 7.8 Hz, 4H), 8.55 (d, J = 1.6 Hz, 2H), 8.47 (d, J = 9.4 Hz, 3H), 8.43–8.37 (m, 5H), 8.28 (d, J = 8.1 Hz, 3H), 8.18–8.10 (m, 8H), 8.03–7.99 (m, 4H), 7.96 (s, 2H), 2.24 (s, 3H). ¹³C NMR (126 MHz, CD₂Cl₂) δ 144.80 (s), 143.11 (s), 142.18 (s), 140.20 (s), 139.49 (s), 137.02 (s), 131.64 (s), 127.82 (s), 127.22 (d, J = 17.6 Hz), 124.60 (s), 122.42 (s), 121.72–121.60 (m), 121.60–121.20 (m), 111.08 (s), 109.50 (s), 18.87 (s). MALDI-TOF-MS (m/z): calcd for C₅₆H₃₆N₄, 764.294; found, 764.256 [M⁺].

Density functional theory (DFT) calculations

All density functional theory (DFT) calculations are carried out using the Gaussian 16 package. The optimized S₀ geometry and the single point properties at S₀ are calculated by the DFT method at the M06-2X/6-31G (d,p) level^[1]. The S₁ geometry is optimized by time-dependent DFT (TD-DFT) at the M06-2X/6-31G (d,p) level. Natural transition orbitals (NTO) and energy levels of the first five singlet and triplet states are performed based on S₁ geometry at the M06-2X/6-31G(d,p) level to better understand the excited-state properties. The NTOs are carried out using Multiwfn, which is an electron wave function analysis software.

General characterization

UV-vis absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer. PL spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. PL quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus_QY. Transient PL decay spectra were measured using Edinburgh Instruments FLS1000 spectrometer. Cyclic voltammetry (CV) was measured on a CHI 610E A14297 in a solution of tetra-n-butylammonium hexafluorophosphate (Bu₄NPF₆) (0.1 M) in dichloromethane or dimethylformamide at a scan rate of 100 mV s^‒1, using a platinum wire as the auxiliary electrode, a glass carbon disk as the working electrode and Ag/Ag⁺ as the reference electrode, the calibrated value was performed by the redox couple ferricenium/ferrocene (Fc/Fc⁺). The Ionization Potential (IP_CV) and Electron Affinities (EA_CV) of these molecules were calculated by the following formula, Ionization Potential (IP_CV) = [E_ox − E_1/2(Fc/Fc⁺) + 4.8] eV, Electron Affinities (EA_CV) = [E_red − E_1/2(Fc/Fc⁺) + 4.8] eV, where E_ox and E_red represent the onset oxidation potential and the reduction potential relative to Fc/Fc⁺ (4.8 eV), respectively.

OLED fabrication and measurement

The glass substrates precoated with a 180 nm layer of ITO with a sheet resistance of 15 to 20 ohms per square are successively cleaned in an ultrasonic bath of acetone, isopropanol, detergent, and deionized water, respectively, taking 10 min for each step. Then, the substrates are completely dried in a 70°C oven. Before the fabrication processes, to improve the hole injection ability of ITO, the substrates are treated with O₂ plasma for 6 min. The vacuum-deposited OLEDs are fabricated under a pressure of < 5 × 10^− 4 Pa in the Suzhou Fangsheng FS-380 vacuum deposition system. Organic materials, LiF, and Al are deposited at rates of 0.5 to 1.5 A s^− 1, 0.1 A s^− 1, and 3 A s^− 1, respectively. The effective emitting area of the device is 9 mm², determined by the overlap between the anode and cathode. The luminance-voltage-current density and external quantum efficiency are characterized by a dual-channel Keithley 2614B source meter and a PIN-25D silicon photodiode. The EL spectra are obtained via an Ocean Optics USB 2000 + spectrometer, along with a Keithley 2614B source meter. For WOLEDs, the luminance-voltage-current density characteristics were measured by a Konica Minolta CS-200 Color and Luminance Meter. All the characterizations are conducted at room temperature in ambient conditions without any encapsulation, as soon as the devices are fabricated.

Author details

¹ State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology (SCUT), Guangzhou 510640, China. ² AIE Institute, Guangzhou 510640, China. ³ School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong Shenzhen (CUHK-Shenzhen), Guangdong 518172, China. ⁴ Department of Chemistry, Department of Chemical and Biological Engineering and the Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China.

Conflict of interest

The authors declare no competing interests.

Supporting Information

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Author Contributions

J.L. and X.G. synthesized and characterized the compounds; J.L., X.G., and H.Z. collected and analyzed the spectroscopic data; J.L. and X.G. performed the theoretical calculation. J.L. fabricated and tested the OLEDs; Y.C. and D.Y. measured the horizontal dipole orientation. B.L., J.H., X.H., and Y.H. provided expertise and feedback. All authors discussed the results and commented on the manuscript. H.Z., B.Z.T., and Z.W. directed the project.

Acknowledgements

J. L. and X. G. equally contributed to this work. We are grateful for financial support from the National Natural Science Foundation of China (21975077), Natural Science Foundation of Guangdong Province (2022B1515020084), Guangdong Basic and Applied Basic Research Foundation (2023B1515040003), Key Project of Yunnan Provincial Department of Science and Technology (202303AC100021), Independent Research Project of State Key Lab of Luminescent Materials and Devices (SCUT) (Skllmd-2024-10), Shenzhen Key Laboratory of Functional Aggregate Materials (ZDSYS20211021111400001), Science and Technology Program of Guangzhou (Grant No. 2023A04J0988), and the Science Technology Innovation Commission of Shenzhen Municipality (KQTD20210811090142053), and the Innovation and Technology Commission (ITC-CNERC14SC01).

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The authors declare no competing interests.

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New-record efficiency in non-doped ultraviolet OLEDs: Hot-exciton emitters by a crossed long-short axis design boosting EQE up to 8.3% and emission peak close to 380 nm

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