Enhancing electroluminescence performance of ultra-deep-blue through-space charge transfer emitters with CIE_y ≈ 0.05 via methyl-modification

Abstract

Achieving efficient solution-processed ultra-deep-blue OLEDs remains a challenge. Herein, a methyl-modification strategy is proposed to overcome weak intramolecular charge transfer and the large energy gap between the singlet and triplet states (ΔE_ST) of ultra-deep-blue through-space charge transfer (TSCT)-thermally activated delayed fluorescence (TADF) emitters. In this way, the reverse intersystem crossing (RISC) process is found to be effective from T₁ to S₁ and can be accelerated with the assistance of T₂. As a result, the ultra-deep-blue TSCT emitter 3MeCz-BO exhibits a minimized ΔE_ST of 0.02 eV, and an enhanced RISC rate of 3.71 × 10⁵ s⁻¹. Additionally, this modification can improve the solubility, enabling the fabrication of solution-processed organic light-emitting diodes (OLEDs). The maximum external quantum efficiency of the 3MeCz-BO-based solution-processed OLED reaches 10.1%, with a Commission Internationale de L'Eclairage (CIE) coordinates of (0.151, 0.051) and a luminance of 1334 cd m⁻². This work is the first instance of developing high-performance ultra-deep-blue solution-processed TSCT-TADF OLEDs, which show comparable performance to vacuum-deposited OLEDs. Furthermore, the 3MeCz-BO-based OLED fits well within the standard Red Green Blue (sRGB) of CIE coordinates (0.15, 0.06), and is close to the CIE coordinates (0.131, 0.046) for the Rec. 2020 standard, implying its potential application in colorful display devices.

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Introduction

Thermally activated delayed fluorescence (TADF) emitters represent the third generation of light-emitting materials for organic light-emitting diodes (OLEDs), achieving nearly 100% exciton utilization without any precious heavy metals.^1–3 Through-space charge transfer (TSCT) allows for the spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), resulting in a small energy gap between singlet and triplet excited states (ΔE_ST).^4–7 In 2020, π-stacked molecules with space-confined face-to-face donors and acceptors were proposed by Jiang et al. to fabricate high-efficiency TSCT-TADF OLEDs.⁸

In recent years, high-performance TSCT-TADF materials have been widely developed and used in blue,^8–17 green,^18–27 and red^28–31 OLEDs. Although TSCT is advantageous for reducing the ΔE_ST of TADF materials, achieving a small ΔE_ST for deep-blue TSCT-TADF materials with CIE_y ≤ 0.1 still remains challenging, due to weak intramolecular charge transfer (ICT). Therefore, it is hard to develop high-performance ultra-deep-blue TSCT-TADF materials. Oxygen-bridged boron was first reported for OLED materials³² and has been widely utilized to fabricate deep-blue OLEDs.^33–36 Based on this, our group previously developed deep-blue TSCT materials, QAc-BO and Cz-BO, but they possess high ΔE_ST values of 0.20 and 0.29 eV.¹³ The high ΔE_ST limits the performance of the OLEDs. Additionally, Zhao et al. developed violet TSCT materials with extremely low CIE_y (<0.03).³⁷ However, the significant ΔE_ST (≈0.3 eV) makes TADF characteristics nearly unnoticeable. For ultra-deep-blue TADF materials with low CIE_y, controlling ΔE_ST becomes increasingly difficult. Due to weak ICT, these materials generally display a large bandgap, resulting in a high-energy singlet charge transfer state (¹CT). The triplet charge transfer state (³CT) is generally positioned near ¹CT, with both states residing at high energy levels. In contrast, the triplet local excited state (³LE) is primarily determined by the molecular segment with the lowest energy, which could be the donor, the acceptor, or the bridge connecting them, and this state is typically much lower in energy than ³CT. As a result, the ΔE_ST of these materials, mainly determined by ¹CT and ³LE, tends to increase.^38–40 This challenges the reverse intersystem crossing (RISC) process in ultra-deep-blue TSCT-TADF materials.

To address the issue of large ΔE_ST in ultra-deep-blue TSCT-TADF materials, this work innovatively modified methyl groups at three positions in the donor; referring to the previously developed fluorescent material Cz-BO, three new TSCT-TADF materials were prepared: 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO. The molecular design concept is shown in Fig. 1. Due to the electron-rich nature of methyl groups, they can enhance ICT and thus reduce the energy levels of ¹CT and ³CT, while ³LE remains unaffected. This leads to a reduction in ΔE_ST. In addition, the energy gap between ³CT and ³LE (ΔE(³CT − ³LE)) was also reduced, leading to the mixing and hybridization of ³CT and ³LE. Therefore, both T₁ and T₂ have certain LE characteristics. This could enhance the spin–orbit coupling (SOC) values of T₁–S₁ and T₂–S₁, allowing RISC to occur from T₁ and T₂ and to be accelerated with the assistance of T₂. In this methyl-modification design, the optimized 3MeCz-BO shows ultra-deep-blue emission at 432 nm, achieving a small ΔE_ST of 0.02 eV and an enhanced RISC rate of 3.17 × 10⁵ s⁻¹. Additionally, the methyl-modified TSCT-TADF materials show good solubility and are suitable for fabricating OLED devices by solution processing. As a result, the EQE_max of solution-processed 3MeCz-BO-based OLEDs is 10.1%, with a full width at half maximum (FWHM) of 50 nm, and a high luminance of 1334 cd m⁻². Furthermore, the notable Commission Internationale de L'Eclairage (CIE) coordinates of 3MeCz-BO (0.151, 0.051) fit well within the standard Red Green Blue (sRGB) of CIE coordinates (0.15, 0.06), and are close to the CIE coordinates (0.131, 0.046) for the Rec. 2020 standard. This suggests its potential application in colorful display devices.

Fig. 1 Molecular design concept and the chemical structures of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO.

Results and discussion

Synthesis and characterization

The chemical structures of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO are shown in Fig. 1. The synthetic routes are provided in the ESI (Scheme S1†). The donor units consist of the 4-methylphenyl group (or without methyl group) and dimethyl-carbazole (or carbazole) by nucleophilic substitution reactions. Unlike Cz-BO,¹³ the tert-butyl groups on the acceptors are replaced with methyl groups, to reduce the steric hindrance caused by tert-butyl groups and generate parallel D–A arrangements. The Suzuki–Miyaura cross-coupling reaction was performed to couple the acceptor and the bridge units to retain the intermediate compound 4. The final emitters are prepared through lithium–bromine exchange of the donor and exchange reaction with compound 4. The emitters show good solubility in organic solvents such as dichloromethane and chlorobenzene, owing to their external methyl groups. The chemical structures of all intermediates and final products were characterized by ¹H NMR, ¹³C NMR, high-resolution mass spectrometry, and elemental analysis (Fig. S1–S19†).

To evaluate the thermal stabilities of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, their thermal decomposition temperatures (T_ds) were measured by thermogravimetric analysis (TGA) under a nitrogen atmosphere (Fig. S20†). The T_d values for 5% weight loss are 415 °C for 1MeCz-BO, 413 °C for 2MeCz-BO, and 412 °C for 3MeCz-BO. The excellent thermal stability arises from the rigid molecules, which is beneficial for the fabrication of OLEDs. Cyclic voltammetry (CV) measurements were conducted to determine the oxidation–reduction potentials of the materials (Fig. S21a†). The HOMO values of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO are estimated to be −5.41, −5.29, and −5.23 eV, respectively. 1MeCz-BO shows the deepest HOMO level, while 3MeCz-BO shows the shallowest HOMO level, which results from the number of electronic-rich methyl groups. The LUMO values are measured as −2.85, −2.84, and −2.82 eV for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively. The similar structure of their acceptors indicates similar LUMO energy levels.

The calculated HOMO–LUMO energy gap (E_g) values are 2.56, 2.45, and 2.41 eV, respectively, suggesting an enhanced ICT and slight redshift emission with the increase in the methyl groups on the donor. Moreover, the pseudo-reversible redox processes are validated by CV measurements over multiple cycles (Fig. S29b–d†), suggesting good device stability when charges are injected repeatedly into the emitter.

Single crystals and theoretical calculations

To unequivocally investigate the molecular spatial structure of the compounds, single crystals of 1MeCz-BO (CCDC 2402467), 2MeCz-BO (CCDC 2402468), and 3MeCz-BO (CCDC 2402469) were obtained by slow evaporation from a mixture of dichloromethane and methanol (v/v = 1 : 4) at room temperature. The spatial structures were obtained from X-ray diffraction analysis, as shown in Fig. 2a and b from the side and top view. Besides this, X-ray crystallographic data are summarized in Table S1,† the packing modes in the single crystals of the compounds are shown in Fig. S22,† and the ORTEP diagrams of the compounds are shown in Fig. S23–S25.† To distinguish between the donor and acceptor units in different compounds, we designate the donors as D1, D2, and D3, and the acceptors as A1, A2, and A3, in 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively. Variations in distances and angles between the donor and acceptor units in the three compounds arise from the spatial effects of methyl groups. The measured D–A distances are as follows: D1 to A1 is 3.175 Å, D2 to A2 is 3.101 Å, and D3 to A3 is 3.192 Å. The dihedral angles of D1–A1, D2–A2, and D3–A3 are measured as 10.2°, 18.8°, and 14.7°, showing that they all have an aligned and parallel D–A spatial arrangement. This configuration can help enhance the radiative transitions of the compounds. The angle between D1 and the bridge is 84.4°, while those of D2–bridge and D3–bridge are 88.9° and 88.3°, respectively. Both D2 and D3 adopt nearly orthogonal conformations to the bridge, due to the locking effect of spiro-fluorene and the spatial hindrance of the methyl groups near the bridge. In contrast, D1, which has only one methyl group, forms a larger angle with the bridge, making it less conducive to the spatial alignment of D1 and A1. Due to the large steric hindrance imposed on the acceptors by the immobilized donor, the angles of A1, A2, and A3 relative to the bridge are measured to be 74.2°, 77.1°, and 73.6°, respectively. Nevertheless, they can still establish an effective face-to-face parallel arrangement between the donor and acceptor to ensure optimal π-orbital overlap. The observation of intramolecular noncovalent interactions is further corroborated by reduced density gradient (RDG) analysis.⁴¹ The RDG iso-surfaces are colored on a blue-green-red scale based on the sign(λ₂)ρ, as shown in Fig. 2c. The iso-surfaces colored in green and brown between D and A units illustrate the π–π interactions and steric hindrance. For three compounds, large green and brown iso-surfaces are present between the donor and acceptor, suggesting significant π–π interactions that support the potential occurrence of the TSCT process. Furthermore, the steric hindrance between donor and acceptor units creates considerable rigidity, which restricts molecular vibrations and inhibits non-radiative transitions, leading to a narrow FWHM. In addition, in D2 and D3, the top methyl group introduces steric hindrance with the acceptors, as shown by the red dotted circle in Fig. 2c, which is not found between D1 and A1. This additional steric hindrance further dampen the vibration of the acceptor unit, which helps to further inhibit the process of non-radiative transition. According to the scatter diagrams of the RDG analysis (Fig. 2d), all three compounds show numerous and dense spikes in the green and brown regions, demonstrating the effective π–π interaction between the D and A units.

Fig. 2 (a) Side and (b) top views of the single crystal structures for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO. The distribution of RDG isosurfaces (c) with the scatter plots of RDG versus sign(λ₂) (d).

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed using the B3LYP-D3/6-31G(d) method to optimize the ground state configurations and analyze frontier molecular orbitals (FMOs) along with electron–hole distribution.⁴² According to DFT simulations (Fig. S26†), we observed that the HOMO and LUMO mainly distribute on the donor and acceptor units, respectively, illustrating the ICT characteristics of the compounds. The HOMO energy levels were determined to be −5.01, −4.95, and −4.87 eV, while the LUMO energy levels were determined to be −1.45, −1.47, and −1.45 eV. Thus, the calculated E_g values of 3.56, 3.48, and 3.42 eV, for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively, align well with the observed trend in electrochemical measurements. It was further confirmed that the ICTs improve progressively with more methyl groups on the donor. The rigidity of the molecules helps reduce the energy loss that occurs during structural relaxation. Therefore, 2MeCz-BO and 3MeCz-BO present similar radiative decay oscillator strengths (f) (0.0178 for 3MeCz-BO and 0.0168 for 2MeCz-BO), whereas 1MeCz-BO shows an f of 0.0064, which is much lower than that of 3MeCz-BO and 2MeCz-BO owing to its relatively weaker structural rigidity and weaker ICT. Thus, the higher f will provide 3MeCz-BO and 2MeCz-BO with fast radiation transition rates.

The TD-DFT calculations show the excited-state energy levels, natural transition orbital (NTO) distributions, and CT proportion, as shown in Fig. 3. All three compounds exhibit CT-dominated S₁ states with 99.6% CT proportions. The energy levels of the S₁ states decrease slightly as the number of methyl groups in the donor unit increases, showing values of 2.998, 2.901, and 2.848 eV. This change occurs due to the enhancement of ICT caused by the additional electron-rich methyl groups in the donor. For 1MeCz-BO, the weak ICT results in a large bandgap, leading to high energy levels for both the ¹CT and ³CT. However, the ³LE is mainly influenced by the molecular segment with the lowest energy, which is the acceptor unit. Thus, the T₁ of 1MeCz-BO shows a 7.6% CT proportion and an energy level of 2.826 eV. Although the value of 〈S₁|Ĥ_SOC|T₁〉 (0.393 cm⁻¹) is large, the large ΔE_ST (172 meV) makes the RISC process extremely difficult. In addition, 1MeCz-BO exhibits a significant ΔE(³CT − ³LE) (134 meV), making the RISC process inaccessible with the assistance of T₂. This suggests a slow RISC process for 1MeCz-BO. For 2MeCz-BO, the additional methyl group in the donor (compared to 1MeCz-BO) can slightly decrease the energy level of ¹CT and ³CT states, resulting in lower S₁ and T₂ (CT: 70.6%, 2.889 eV) energy levels than those of 1MeCz-BO. However, the additional methyl groups in the donor have minimal impact on the ³LE energy level, resulting in the T₁ (CT: 27.6%, 2.818 eV) of 2MeCz-BO similar to that of 1MeCz-BO. Thus the ΔE_ST of 2MeCz-BO is significantly reduced to 83 meV. Furthermore, because of the distinct LE characteristics of T₁, a high value of 〈S₁|Ĥ_SOC|T₁〉 of 0.330 cm⁻¹ can be achieved, suggesting that an effective RISC process can occur from T₁ to S₁. In addition, the ΔE(³CT − ³LE) (71 meV) of 2MeCz-BO is also reduced, facilitating the mixing of ³LE and ³CT, as shown in Fig. 3d. Thus, the mixing of T₁ and T₂ allows the RISC process to be assisted by T₂. With the further addition of methyl groups, 3MeCz-BO presents the lowest S₁, resulting in a minimized ΔE_ST of 51 meV. Additionally, the mixing of T₁ (CT: 63.8%, 2.797 eV) and T₂ (CT: 34.7%, 2.857 eV) is reinforced due to the further reduction of ΔE(³CT − ³LE) (60 meV), suggesting more effective assistance from T₂. The smaller ΔE_ST with the high enough 〈S₁|Ĥ_SOC|T₁〉 (0.201 cm⁻¹) ensures a faster RISC process of 3MeCz-BO.

Fig. 3 Excited-state energy levels and natural transition orbital distributions for 1MeCz-BO (a), 2MeCz-BO (b), and 3MeCz-BO (c). (d) Schematic diagram of interaction between ³LE and ³CT excited states.

Photophysical properties

To investigate the photophysical properties of these emitters, UV-Vis absorption spectra, fluorescence spectra in dilute toluene solution (10⁻⁵ mol L⁻¹), and photoluminescence spectra in doped films were measured at 298 K, as shown in Fig. 4a. In the films, emitters (20 wt%) are doped in 10 wt% poly(N-vinylcarbazole) (PVK, M_w ≥ 50 000) and 70 wt% 1,3-bis(N-carbazolyl)benzene (mCP). As seen in the UV-Vis absorption spectra, all three emitters show two strong absorption bands at 280–320 nm and 340–410 nm. The band at 280–320 nm is attributed to the local π–π* transitions, and the band at 340–410 nm includes the characteristic absorption band of oxygen-bridged boron acceptor and ICT between D and A. The optical band-gap values are calculated from the onset of the absorption spectra, which are 3.00, 2.99, and 2.98 eV for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively. The PL emission peaks in toluene solution are located at 416, 428, and 432 nm for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, with narrow FWHM values of 42, 41, and 39 nm, respectively. Compared to the PL emission in the toluene solution, the doped films exhibit more redshifted emission (423, 434, and 441 nm), and larger FWHM values (52, 51, and 49 nm). As expected, as the number of methyl groups in the donor increases, the emitters exhibit more redshifted emission, due to the electron-rich nature of the methyl groups. The fluorescence and phosphorescence spectra at 77 K in toluene were recorded to calculate the energy levels of S₁ and T₁ states, shown in Fig. 4b. The energy levels of S₁ states are determined to be 3.08, 3.01, and 2.95 eV, while the energy levels of T₁ states are determined to be 2.95, 2.95, 2.93 eV, for 1MeCz-BO, 2MeCz-BO, 3MeCz-BO, respectively. The increase in the number of methyl groups in the donor is beneficial for improving the electron-donating ability of the donor, which in turn enhances the ICT process and reduces the energy level of the S₁ states with CT characteristics. By contrast, these three emitters have similar T₁ energy levels, since T₁ states are mainly distributed on the same acceptor units. In particular, because the T₁ state of 3MeCz-BO exhibits more CT characteristics, it shows a relatively low T₁ energy level. Therefore, the corresponding ΔE_STs of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO are calculated to be 0.13, 0.06, and 0.02 eV, respectively. 3MeCz-BO shows extremely small ΔE_ST, leading to the most effective RISC process. Fig. 4c illustrates the PL spectra of three emitters in different solvents at 298 K, showing obvious solvatochromism and highlighting the distinct CT characteristics of the S₁ states.

Fig. 4 (a) The UV-Vis absorption and fluorescence spectra in toluene (10⁻⁵ mol L⁻¹, 298 K), and photoluminescence spectra in the doped films. (b) The fluorescence and phosphorescence spectra at 77 K in toluene (10⁻⁵ mol L⁻¹). (c) The photoluminescence spectra in different solvents at 298 K. (d) The transient photoluminescence decay curves of the doped films in air and in a vacuum for 3MeCz-BO. (e) The temperature-dependent transient photoluminescence decay spectra of 3MeCz-BO. (f) Comparison of the efficiencies (ϕ) of PL, PF, DF, and the rate constants (k) of radiation, non-radiation, and RISC.

To investigate the TADF properties of these emitters, the transient photoluminescence decay curves of the doped films in air and in a vacuum were measured, as shown in Fig. 4d (for 3MeCz-BO) and Fig. S27† (for 1MeCz-BO and 2MeCz-BO). They show noticeable delayed fluorescence and display a clear difference between the decay curves in a vacuum and in air, confirming that triplet excitons are involved in delayed fluorescence processes. Fig. S28† shows the decay curves over a larger range. Then the temperature-dependent transient PL decay spectra of the three emitters were measured in the doped films, as shown in Fig. 4e (for 3MeCz-BO) and Fig. S29† (for 1MeCz-BO and 2MeCz-BO). The enhanced lifetimes and proportions of delayed fluorescence as the temperature increases from 100 to 300 K further confirm the thermally activated mechanism of the three emitters. It can be seen that temperature has the greatest effect on the decay curve of 1MeCz-BO, followed by 2MeCz-BO with the least impact observed for 3MeCz-BO. This phenomenon is closely related to their ΔE_STvalues. For 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO films, the prompt lifetimes (τ_p) are measured as 22.9, 22.5, and 20.2 ns, while the delayed lifetime (τ_d) are measured as 14.1, 7.32, and 4.73 μs, respectively. Furthermore, the photoluminescence quantum yields (PLQYs) of doped films were measured in a vacuum (by converting the PLQYs of doped films in air, using the integral ratio of the steady-state emissions of the corresponding films in a vacuum to those in air). 1MeCz-BO, 2MeCz-BO and 3MeCz-BO display PLQYs of 83%, 86%, and 88%, respectively. All three compounds exhibit the highest PLQY at a doping ratio of 20 wt%, which may be attributed to the formation of rigid π-stacked structures¹⁷ (Fig. S30†). To quantitatively compare the exciton kinetics of the emitters, the rate constants of radiation (k_r), non-radiation (k_nr), and RISC (k_RISC) (Fig. 4f) were precisely calculated according to a previously reported method.⁴³ The k_r values for 1MeCz-BO, 2MeCz-BO and 3MeCz-BO are 2.75 × 10⁷, 2.81 × 10⁷, and 2.92 × 10⁷ s⁻¹, respectively. High k_r values demonstrate the face-to-face D–A arrangements and short D–A distances. Meanwhile, the k_RISC values of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO are measured to be 9.5 × 10⁴, 1.87 × 10⁵, and 3.17 × 10⁵ s⁻¹, respectively. With the modification of methyl groups, the RISC rate gradually increases, which is consistent with the results obtained by the NTO analysis. Moreover, the values of k_nr have been calculated to be 5.75 × 10⁶, 4.38 × 10⁶, and 3.87 × 10⁶ s⁻¹ for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively. It has been demonstrated that the increased molecular rigidity from the methyl groups can inhibit non-radiative transitions. The photophysical data of three emitters are given in Tables 1 and S5.†

Table 1 Photophysical properties of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO

Compound	λ PL [nm]	FWHM^b [nm]	S1/T1^c [eV]	ΔEST^d [eV]	HOMO/LUMO^e [eV]	E g [eV]	PLQY^g [%]	k r [10^7 s^−1]	k RISC [10^5 s^−1]
a The photoluminescence spectra in toluene (10^−5 M) solution and doped films at 298 K. b The full-width-at-half maximum of PL spectra in toluene solution and doped films. c Determined from the onset of fluorescence and phosphorescence spectra in toluene solution at 77 K. d ΔEST = S1 − T1. e Estimated from the oxidation and redox potential in cyclic voltammetry. f The energy gap (Eg) values calculated from the HOMO–LUMO levels. g The photoluminescence quantum yields of doped films in a vacuum. h The radiative rate constants. i The rate constants of reverse intersystem crossing.
1MeCz-BO	416, 423	42, 52	3.08/2.95	0.13	−5.41/−2.85	2.56	83	2.75	0.95
2MeCz-BO	428, 434	41, 51	3.01/2.95	0.06	−5.29/−2.84	2.45	86	2.81	1.87
3MeCz-BO	432, 441	39, 49	2.95/2.93	0.02	−5.23/−2.82	2.41	88	2.92	3.17

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Electroluminescence properties

Promised by the minimized ΔE_ST and enhanced RISC process of the ultra-deep-blue emitter 3MeCz-BO, the corresponding OLED devices were fabricated to investigate the electroluminescence (EL) performances of the emitters with the following structure: ITO/PEDOT:PSS (30 nm)/PVK (10 nm)/mCP: 20 wt% emitters and 10 wt% PVK (40 nm)/DPEPO (5 nm)/TmPyPB (35 nm)/LiF (0.8 nm)/Al (100 nm). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was used as the hole-injection and hole-transporting layer, lithium fluoride (LiF) was used as the electron-injection layer, and 1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB) was used as the electron-transporting layer. PVK and bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) were used as the electron- and hole-blocking layers, respectively, while mCP was used as the host material. “nMeCz-BO” refers to the series of these emitters: 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO. The energy level diagram of the materials used in OLEDs is shown in Fig. 5a, and the EL properties are summarized in Table 2. The EL properties at other doping ratios are shown in Fig. S31–S33 and Tables S6–S8.†

Fig. 5 (a) Device architecture and energy levels of the used materials. (b) EL spectra of 1MeCz-BO-, 2MeCz-BO-, and 3MeCz-BO-based OLEDs. (c) Current efficiency and power efficiency versus luminance of the devices. (d) Current density–voltage–luminance (J–V–L) characteristics of the devices. (e) EQE versus luminance of the devices. (f) EQE_maxs with CIE_y obtained in this work and the reported TSCT-TADF emitters (CIE_y < 0.5).

Table 2 Summary of electroluminescence performances

Materials	λ EL [nm]	EQEmax^b [%]	EQE100^c [%]	EQE500^d [%]	CEmax^e [cd A^−1]	PEmax^f [lm W^−1]	L max [cd m^−2]	FWHM^h [nm]	CIE^i [x, y]
a The peak value of electroluminescence. b Maximum external quantum efficiency. c External quantum efficiency at 100 cd m^−2. d External quantum efficiency at 500 cd m^−2. e Maximum current efficiency. f Maximum power efficiency. g Maximum luminance. h Full width at half-maximum of the electroluminescence peak. i Coordinates of Commission Internationale de L'Eclairage.
1MeCz-BO	430	6.1	5.5	4.2	11.7	10.2	1024	49	0.162, 0.048
2MeCz-BO	436	8.6	7.5	6.0	17.1	14.9	1277	49	0.160, 0.050
3MeCz-BO	442	10.1	9.0	7.1	19.6	16.9	1334	50	0.151, 0.051

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For three emitters, the highest efficiency of devices can be obtained at a doped ratio of 20 wt%. Devices based on 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO display EL peaks (λ_EL) at 430, 436, and 442 nm with FWHM values of 49, 49, and 50 nm, respectively (Fig. 5b). The corresponding CIE coordinates show ultra-deep-blue colors of (0.162, 0.048), (0.160, 0.050), and (0.151, 0.051) for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively. The emissions from these devices show a slight redshift as the number of methyl groups in the donor increases, which is consistent with the results of molecular simulations and photophysics. All three solution-processed devices exhibit narrow emission, which aligns with the measured FWHM values in photophysics. The rigid molecular structures effectively reduce vibrational relaxation, leading to narrow emissions. This is advantageous for the color purity of ultra-deep-blue OLEDs.

Furthermore, the CIE coordinates of 1MeCz-BO and 2MeCz-BO are (0.162, 0.048) and (0.160, 0.050), respectively, exhibiting more violet emission. While the CIE coordinates of 3MeCz-BO (0.151, 0.051) fit well within the sRGB of CIE (0.15, 0.06), and are close to the CIE (0.131, 0.046) for the Rec. 2020 standard. This suggests its potential application in colorful display devices. As shown in Fig. 5c, the current efficiencies (CEs) of 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO-based OLEDs are 11.7, 17.1, and 19.6 cd A⁻¹, respectively. Their power efficiencies (PEs) are 10.2, 14.9, and 16.9 lm W⁻¹. This suggests that the enhanced electrical properties of the emitters are due to the methyl-modification. In terms of device performance, 3MeCz-BO exhibits a maximum EQE of 10.1%, which is higher than that of 2MeCz-BO (8.6%) and 1MeCz-BO (6.1%). The EQEs at 100 cd m⁻² are 5.5%, 7.5%, and 9.0%, and the EQEs at 500 cd m⁻² are 4.2%, 6.0%, and 7.1% for 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO, respectively, showing the suppressed efficiency roll-offs. Additionally, the maximum luminance of the devices is measured as 1024, 1277, and 1334 cd m⁻², indicating that these devices exhibit high brightness as ultra-deep-blue solution-processing OLEDs. The criss-crossed stacking structures (Fig. S22†) prevent triplet–triplet annihilation of the materials, leading to the highest EQE at relatively high doping ratios (20 wt%). The high concentration of emitters provides higher brightness for the OLEDs, which is particularly challenging for ultra-deep-blue or deep-blue solution-processed OLEDs. These results indicate that the methyl groups on the donor suppress the non-radiative transitions and the enhanced ITC accelerates the RISC, thereby significantly enhancing the device performance of ultra-deep-blue TSCT materials. A summary of device data for face-to-face TSCT-TADF emitters (CIE_y < 0.5) is shown in Fig. 5f.^8–27 It can be seen that this work fills the gap in the violet to deep blue region of TSCT materials. For comparison, thermal evaporated OLEDs based on 3MeCz-BO were also tested, as shown in Fig S34 and Table S9.† As a solution-processed device, it does not demonstrate significant disadvantages compared to the thermally evaporated device. Additionally, the limited broadening in the EL spectrum (compared to the PL in doped films) indicates the potential use of these materials in narrowband ultra-deep-blue OLEDs.

Conclusions

In summary, we proposed a strategy of methyl-modification in the donor to enhance the ICT and accelerate the RISC process of ultra-deep-blue TSCT-TADF emitters, solving the challenging issue of large ΔE_ST values and low k_RISC values in these materials. Due to the electron-rich nature of methyl groups, the ICT was enhanced and the energy levels of the ¹CT and ³CT states were reduced, while the ³LE remains unaffected by the methyl group. In this way, the ΔE_ST and ΔE(³CT − ³LE) of the materials can be minimized. This enables effective RISC processes from T₁ to S₁, which can be accelerated with the assistance of T₂. The prepared emitters 1MeCz-BO, 2MeCz-BO, and 3MeCz-BO display k_RISC values of 9.5 × 10⁴, 1.87 × 10⁵, and 3.17 × 10⁵ s⁻¹, respectively. In addition, ultra-deep-blue solution-processed OLED devices based on these materials exhibit good overall performance. The EQE_max of 3MeCz-BO is 10.1%, with CIE coordinates of (0.151, 0.051) and a high luminance of 1334 cd m⁻². This work developed the first high-performance ultra-deep-blue solution-processed TSCT-TADF OLEDs. Additionally, their colour fits well within the sRGB of CIE coordinates of (0.15, 0.06), and is close to the CIE coordinates of (0.131, 0.046) for the Rec. 2020 standard.

Data availability

The experimental procedures, characterization data, crystals data and device data generated in this study are provided in the ESI.†

Author contributions

Z. Ren and Q. Li designed the experiments. Q. Li synthesized the molecules, carried out the photophysical measurements and theoretical calculations, fabricated OLEDs and wrote the manuscript draft. H. Zhao and J. Zhao helped fabricate the OLEDs. Z. Cao and C. Yu helped synthesize the molecules. S. Yan and Z. Ren wrote the manuscript and supervised this study.

Conflicts of interest

There is no conflict of interest to report.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 52273164) and the Shandong Provincial Natural Science Foundation (ZR2022ZD37).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2402467–2402469 contain the crystallographic data for 1MeCz-BO, 2MeCz-BO and 3MeCz-BO. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc08094g

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