Delocalizing Electron Distribution in Thermally Activated Delayed Fluorophors for High-efficiency, Long-lifetime Blue Electroluminescence

doi:10.21203/rs.3.rs-4025018/v1

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Delocalizing Electron Distribution in Thermally Activated Delayed Fluorophors for High-efficiency, Long-lifetime Blue Electroluminescence

https://doi.org/10.21203/rs.3.rs-4025018/v1

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Thermally activated delayed fluorescent (TADF) emitters are promising for the next generation organic light-emitting diodes (OLEDs), yet their efficiency and stability still cannot meet the requirements for commercialization. Here, we establish a design rule for highly efficient and stable TADF emitters by introducing an auxiliary acceptor to delocalize electron distributions, not only enhancing the molecular stability in the negative polaron state but also accelerating the triplet-to-singlet up-conversion and the singlet radiative processes simultaneously. Proof-of-the-concept TADF compounds, based on a multi-carbazole-benzonitrile structure, exhibit near-unity photoluminescent quantum yields, short-lived delays, and improved photo- and electroluminescent stabilities. Deep-blue OLED utilizing one of these molecules as the sensitizer for a multi-resonance emitter achieves a remarkable LT95 (time to 95% of initial luminance) of 221 h at an initial luminance of 1000 cd m^-2, together with a maximum external quantum efficiency of 30.8% and Commission Internationale de l'Eclairage coordinates of (0.14, 0.17). This work would unlock the potential of TADF emitters for practical applications.

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

Physical sciences/Chemistry/Materials chemistry/Optical materials

Physical sciences/Materials science/Materials for devices/Electronic devices

Green and red phosphorescent emitters based on iridium complexes have been successfully used in organic light-emitting diodes (OLEDs) to reduce power consumption for more than a decade, while the research for efficient and stable blue phosphorescent emitters remains challenging.¹ Alternative approaches to harness all the excitons generated by electroexcitation in low-cost pure organic emitters have also been actively explored.^2–5 Adachi et al. pioneered the study of thermally activated delayed fluorescence (TADF) emitters, in which the non-emissive triplets can be effectively upconverted to the emissive singlets and 100% internal quantum efficiency (IQE) can be achieved in OLEDs thereof.^6–8 However, most of the efficient TADF emitters still face the stability issue, and studies on improving the molecular stability of TADF compounds are urgently needed.^9–13 Bimolecular interactions involving triplet excitons, such as exciton-exciton annihilations and exciton-polaron annihilations, have been recognized as the main causes of degradation in TADF compounds. In these processes, the generated high-energy species would induce bond cleavage and produce radical fragments, which then participate in further radical addition reactions, forming additional degradation products.^14,15

The solution to improve molecular stability, from a kinetic point of view, should naturally involve accelerating exciton consumption to reduce the chance of unwarranted chemical reactions. Noteworthily, multiple D-A charge-transfer (CT)-type TADF molecules could accelerate the reverse intersystem crossing (RISC) process with the assistance of intermediate triplet excited states from the partial molecular structures of these molecules.^16,17 As a result, the RISC process with rate constants exceeding 10⁶ s^-1 has been reported and afforded a bluish-green device with an LT90 (time to 90% of initial luminance) around 500 h at an initial luminance (L₀) of 1,000 cd m^-2, representing the cutting-edge operation stability among TADF devices.¹⁸ Additionally, further acceleration of exciton consumption can be achieved by using TADF compounds as sensitizers for a fluorescent final emitter, termed TADF-sensitized fluorescence (TSF) or hyperfluorescence.^19,20 In this way, a “one-way” rapid Förster energy transfer from the TADF sensitizer to the final emitter can reduce the number of spin-flip transition cycles between singlet and triplet in the TADF molecules and thereafter quicken the exciton consumption for a prolonged device lifetime.

On the other hand, recent attention has also been given to improving the intrinsic stability of TADF molecules from a thermodynamic perspective. An elegant tactic is to utilize a deuteration strategy to lower the vibrational energy of molecules. This strategy has been recently introduced into blue TADF systems, resulting in a three-fold improvement in device lifetime.²¹ Another way is to enlarge the bond dissociation energies (BDEs) of the weakest bonds in TADF molecules, particularly the carbon-nitrogen (C-N) bonds.^22–25 Previous studies have confirmed that, compared with the molecule in the excited and positive states, the C-N bond is more fragile when the molecule is in the negative polaron states.²⁶ Through enlarging BDE(-)s of green TADF emitters, Lee et al. have successfully realized nearly twenty-fold enhancement in device lifetimes.²⁷ Here we also calculated the BDE values of the C-N bonds in representative TADF emitters in positive (BDE(+)), negative (BDE(-)), and neutral (BDE(n)) states, validating that BDE(-)s are the lowest ones among all the states, as shown in Supplementary Fig.1-2 and Supplementary Table 1.Therefore, negative polaron-related states would be a fatal short-slab for the intrinsic stability of TADF molecules and should be given particular attention.

To construct blue TADF emitters favoring long operation device stability both thermodynamically (stable negative polaron states) and kinetically (fast exciton consumption), we establish a design rule for the promising multi-carbazole-benzonitrile (CzBN) TADF compounds by introducing an auxiliary acceptor to delocalize electron distributions. This approach not only enhances the molecular stability in the negative polaron state but also shortens the delayed lifetime due to the simultaneously accelerated RISC and radiative processes. The proof-of-the-concept sky-blue molecules were stable under electrical excitation, affording exceptionally long device lifetimes and high efficiency when used as both emitters and sensitizers, particularly for the deep-blue OLEDs.

Molecular design and intrinsic stability analysis

Fig.1a presents the structure of the target molecules, all derived from our previously reported multiple donors-acceptor charge-transfer-type TADF molecule, 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN).¹⁶ The CzBNs have been demonstrated as one of the most promising families of stable TADF emitters and continuous works have been devoted to modifying the molecule structure and achieve cutting-edge device stabilities in literature.^28,29 For instance, Adachi et al. established a hetero-donor strategy to modify 5CzBN by using carbazole derivatives with slightly different locally-excited (LE) triplet states to enhance the RISC process, as exampled by 3Cz2DPhCzBN, realizing a marked improvement in device operational stability.^30,31 However, though their precedence regarding device stability at present, the reported CzBNs still suffer from a relatively large ΔE_ST of > 0.1 eV, resulting in a moderate RISC rate in the range of 10⁵ s^-1. Moreover, despite numerous efforts to modify donor units, few attempts have been made to stabilize the BDE(-)s of CzBN compounds, which limits the further improvement of device performances. With this consideration in mind, we proposed a new design rule for CzBNs, that is introducing an auxiliary acceptor group at the para-position of CN units to delocalize the electron distributions. Such a concept could well establish, on one hand, a more delocalized distribution of the lowest unoccupied molecular orbital (LUMO), which favors reducing the ΔE_ST value while maintaining a large oscillator strength (f). On the other hand, the more dispersed negative charge distribution will naturally reduce the electron density of the central benzene unit and thus favor reducing the chemical reaction activity of the molecule. Regarding the auxiliary acceptor group, benzonitrile (PhCN) and 2,4,6-triphenyl-1,3,5-triazine (TPTRZ) groups were chosen as they were the only ones that had displayed acceptable stability as acceptors in TADF molecules. Three target molecules, namely 4CzBN-PhCN, 4tCzBN-PhCN, and 4tCzBN-TPTRZ were thereafter constructed. It should be mentioned that such molecular design was expected to exert a limited impact on the emission color compared with 5CzBN as both PhCN and TPTRZ are weaker in the electron-deficit ability than the CN group and the number of the donor groups is reduced meanwhile.

Firstly, we studied the distributions of the frontier molecular orbitals for these compounds. Unlike their similar highest occupied molecular orbital (HOMO) distributions, which are mainly on the multi-carbazole units, clear differences in the LUMO distributions are observed between the target and reference compounds, as illustrated in Supplementary Fig.3.As expected, the LUMOs of the target molecules extend to the auxiliary acceptor segment, being more delocalized than that of the reference compound, where the LUMO is solely located on the PhCN group. The BDEs of the C-N bonds in the target compounds are calculated and summarized in Fig.1b and Supplementary Table 2 with 3Cz2DPhCzBN as the reference. All target compounds show improved BDE(-) values of 3.04, 3.24, and 3.19 eV for 4CzBN-PhCN, 4tCzBN-PhCN, and 4tCzBN-TPTRZ, respectively, compared with that of 3Cz2DPhCzBN, which is 2.75 eV. The enhanced BDE(-) can be attributed to the increased electron affinity of the molecule. According to Hess’s law, BDE(-)=BDE(n)+EA_m-EA_x, where EA_m and EA_x represent the electron affinity of the radical anion after dissociation and the intact molecule, respectively.²⁶ Introducing secondary acceptor groups would not only enhance electron-withdrawing ability but also extend the conjugation length, favoring the enlargement of EA_m and ultimately resulting in a large BDE(-).

Furthermore, we experimentally evaluated the photoluminescent (PL) stabilities of these materials in pristine thin films (thickness of 100 nm) using ultra-violet (UV) irradiation with an emission peak of 360 nm and a power density of about 1 mW cm^-2. As illustrated in Fig.1c and Supplementary Fig. 4, 4CzBN-PhCN and 4tCzBN-PhCN remained almost unchanged during the measurement, exhibiting excellent molecular intrinsic stability, followed by 4tCzBN-TPTRZ. In contrast, 3Cz2DPhCzBN shows the worst result. Under UV excitation, these molecules are in the neutral states, which are considered to be biradical states, namely a radical cation and a radical anion couple.³² In the CT excited state, the donor loses an electron to form a radical cation (D⁺) while the acceptor acquires an electron to form a radical anion (A^-). The stability of radical species should be similar to the corresponding polarons, and thus, the good durations of A^- should matter more than those of D⁺ in determining the overall molecular stability. Referring to the results from theoretical predictions, the better photo-aging behaviors of the target molecules should be attributed to the better long-term stable A^- species compared to the references. Besides, the large electron delocalization ranges in the target molecules will also lower the nucleophilicity of A^-, preventing further radical addition reactions and the formation of degradation products, even if radical segments are formed.

The stability of these molecules in positive and negative states can be experimentally demonstrated under electrical excitation using hole- and electron-only devices (HODs/ EODs) with device structures of ITO/ HATCN (5 nm)/ NPB (30 nm)/ EML (30 nm)/ HATCN (5 nm)/ Al and ITO/ Cs₂CO₃ (1 nm)/ DPPyA (30 nm)/ EML (30 nm)/ DPPyA (30 nm)/ LiF (0.5 nm)/ Al, respectively. Here, HATCN stands for 1,4,5,8,9,11-hexaazatriphenylenehexacabonitrile, NPB stands for N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diaminem, and DPPyA stands for 9,10-Bis(6-phenylpyridin-3-yl) anthracene. The carrier-only devices were aged under the operating conditions of current stress with a constant current density of 20 mA cm^-2 and the voltage changes were recorded. Previous works have pointed out that under electrical stress, the molecular degradation products will act as the fixed charge sites to repulse the vicinal charge and thus yield a voltage rise.^33–35 According to Fig. 1d-e, the voltages of EODs of all the samples rise faster than their HODs, indicating relatively better stability of the molecules in cationic states than in anionic states, which is in agreement with the theoretical results. More importantly, the EODs of 4CzBN-PhCN, 4tCzBN-PhCN, and 4tCzBN-TPTRZ exhibited only inconspicuous voltage rise, but the voltage rise of the EOD of 3Cz2DPhCzBN is much more evident. It can thus be reasonably concluded that the target molecules exhibit better chemical stabilities in anionic states than the reference.

Photophysical analysis

The photophysical properties of these compounds are studied in dilute toluene solutions (10^-5 M). Fig.2a shows the UV-Vis absorption and the PL spectra of these compounds. All of them exhibit similar absorption peaks around 290 nm and 340 nm that arise from the π-π* or n-π* transition of carbazole moieties. Broad bands with wavelengths above 400 nm are also recorded, arising from the CT transitions. Among them, 4tCzBN-PhCN exhibits the strongest absorption intensity, suggesting its largest f value of the S₀-S₁ transition. PL spectra of these emitters show emission maxima at 497 nm for 4tCzBN-PhCN, 492 nm for 4tCzBN-TPTRZ, and 486 nm for 4CzBN-PhCN. From the onset of these fluorescence spectra, similar S₁ energies in the range of 2.72-2.84 eV are thereafter obtained, rationalizing the direct comparison of their stabilities. The phosphorescent spectra of these compounds are also recorded under 77 K with a 10 ms delay, as illustrated in Supplementary Fig. 5, obtaining triplet energies of 2.63-2.76 eV. Small ΔE_STs< 0.05 eV are calculated for the target compounds while it is 0.21 eV for 3Cz2DPhCzBN. As mentioned earlier, reducing ΔE_ST is a challenge for CzBN molecules by only modulating donors, restricting the further improvement of the RISC rate. Our work here validates that extending acceptor groups is a more feasible and effective approach to minimize ΔE_ST.The photoluminescence quantum yields (PLQYs) of these compounds in toluene are measured to be 0.12-0.15 before degassing and can be improved to near unity after bubbling nitrogen.

The transient PL decay curves of these emitters are further recorded at an excitation wavelength of 360 nm, and clear bi-exponential decay characteristics are observed, as shown in Fig. 2b. Compared with 3Cz2DPhCzBN, whose delayed lifetime (τ_D) is 9.6 μs, all three target compounds exhibited obvious shorter τ_Ds of <6.5 μs, especially for 4tCzBN-PhCN (2.4 μs). To reveal the origin of their rapid exciton consumption, the rate constants of TADF processes are calculated and presented in Table 1. Of particular note, compared with the reference, the three target molecules exhibited both higher rate constants of radiative decay (k_r) and RISC (k_RISCs), with k_rs over 10⁷ s^-1 and k_RISCs over 10⁶ s^-1, which should account for their relatively shorter-lived delayed components. The balanced k_rs and k_RISCs of the target molecules should benefit from the molecular design as the more delocalized LUMO distributions could reduce the ΔE_ST and increase the transition dipole moment for a larger f value compared with the reference. The maintained multiple-donors structures also favor a densemanifold of triplet states with hybrid CT and LE characters, as confirmed by hole-electron analysis (Supplementary Fig.6) and the large coefficients of the spin-orbital coupling (λ_SOCs) between T₂ and S₁ are obtained to be 0.73, 0.71 and 0.66 cm^-1 for 4CzBN-PhCN, 4tCzBN-PhCN and 4tCzBN-TPTRZ, respectively, greatly promoting spin-flip transitions without sacrificing the singlet radiative decays. As mentioned above, rapid exciton consumption is beneficial to prolong the operational stability of OLEDs. Therefore, besides thermodynamically stabilizing the molecules in both excited and negative polaron states, introducing auxiliary acceptors also kinetically favors enhancing molecular stability by balancing singlet radiation and triplet up-conversion to prevent exciton quenching.

Device analysis

OLEDs of these molecules as TADF emitters are fabricated with the following architectures: ITO/ HATCN (5 nm)/ NPB (30 nm)/ SiCzCz (10 nm)/ SiCzCz: SiTrzCz2: TADF emitter (24 nm, 0.48: 0.32: 0.20 w/w/w)/ SiTrzCz2 (5 nm)/ DPPyA: Liq (30 nm, 1:1 ) / LiF (0.5 nm)/ Al (150 nm). The materials used in device fabrication are presented in Supplementary Fig. 7. A proved stable exciplex-forming system consisting of 9-(3-(triphenylsilyl)phenyl)-9H-3,9′-bicarbazole (SiCzCz) and 9,9′-(6-(3-(triphenylsilyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9H-carbazole) (SiTrzCz2) was adopted for stability evaluation.¹ Pristine SiCzCz and SiTrzCz2 films are inserted between transporting layers and emitting layer (EML), to confine charge recombination within the EML.The energy level diagram and the device properties are shown in Fig.3 and summarized in Table 2. The doping concentration of the TADF emitters is optimized to be 20 wt%, as shown in Supplementary Fig. 8-10. Electroluminescent (EL) spectra of these devices at a luminance of 1,000 cd m^-2 are provided in Fig. 3b. Similar to the PL results, emission peaks in the range of 489-501 nm are obtained. The EL spectra of 4CzBN-PhCN and 3Cz2DPhCzBN-based devices are almost the same, as are those of 4tCzBN-PhCN and 4tCzBN-TPTRZ-based devices, which makes it reasonable to compare the stabilities of the molecules. The external quantum efficiency (EQE)-luminance characteristics are depicted in Fig.3c. All devices exhibited high maximum EQE values ranging from 25.9% to 37.1%. To confirm the origin of the high efficiencies, we further characterize the angle-dependent PL intensities of the p-polarized light emitted from the 30 nm-thick EMLs to study the emitting dipole orientation (EDO) of these emitters. As illustrated in Fig.3d and Supplementary Fig.11, they show relatively high ratios of the horizontal EDO (Ө_//s) in the range of 77.5% to 82%, which would lead to high outcoupling efficiencies of devices and therefore high EQEs.^36–39

Next, we assessed the operational stability of these devices at an initial luminance of 5,000 cd m^-2 under a constant current density, as shown in Fig. 3e. Long LT95s of 26.7, 37.6, and 16.4 h are measured for devices based on 4CzBN-PhCN, 4tCzBN-PhCN, and 4tCzBN-TPTRZ, respectively, all longer than the LT95 (only 6.9 h) of the device based on 3Cz2DPhCzBN. The transient EL decay curves of those devices are taken at a luminance of 1,000 cd m^-2 as shown in Fig.3f. Devices based on the target molecules of 4CzBN-PhCN, 4tCzBN-PhCN and 4tCzBN-TPTRZ all show relatively faster decay curves, compared with that of the reference, similar to the trend observed for the transient PL decays. 4tCzBN-PhCN exhibited the best operational stability due to its combination of a stable molecular structure and fast exciton consumption. Noteworthily, lifetime of the OLED based on 4tCzBN-TPTRZ is shorter than the 4CzBN-PhCN one, despite the similarity in BDE values and rates of photophysical processes between 4tCzBN-TPTRZ and 4CzBN-PhCN. It was speculated that the stability of the auxiliary acceptors would also affect the stability of the TADF emitters. To validate this, we selected a reported stable molecule 5Cz-TRZ as a comparison with the structure shown in Supplementary Fig. 12, which possessed also multi-carbazole donors while only a triazine acceptor.¹⁸ The photophysical and device characterizations of 5Cz-TRZ are shown in Supplementary Fig. 13-14. 5Cz-TRZ shows a large BDE(-) value of 3.14 eV of C-N bond, rapid exciton consumption with a delayed lifetime of 2.2 μs and a k_RISC of 1.42×10⁷ s^-1. However, the OLED based on 5Cz-TRZ shows an even shorter lifetime than the 4tCzBN-TPTRZ-based one, using the same device structure. Previous works have also theoretically predicted that the triazine group would undergo a ring fission process in the excited state, making them undesirable in the EML.⁴⁰ Therefore, the cyano group would be better than triazine in constructing stable TADF emitters.

Due to the superior performances of 4CzBN-PhCN and 4tCzBN-PhCN, we further evaluated their effectiveness as sensitizers for a deep blue MR emitter t-BuCz-DABNA.⁴¹ The devices are noted as TSF-DB and TSF-SB for those using 4CzBN-PhCN and 4tCzBN-PhCN as sensitizers, respectively. It is interesting to note that, though t-BuCz-DABNA possesses a blue-shifted emission peak compared with both sensitizers, large overlaps were observed between the sensitizers’ emission and the emitter’s absorption spectra as illustrated in Supplementary Fig.15. The Förster energy transfer from 4CzBN-PhCN and 4tCzBN-PhCN to t-BuCz-DABNA shows large radii (R₀) of 3.59 and 3.06 nm, respectively. This is reasonable as recent work has demonstrated that sensitizers with a larger 0-0 band than that of the final emitter could guarantee efficient energy transfer.⁴² As depicted in Fig.4a, sharp blue emission spectra peaking at 470 nm were obtained from both devices with small full widths at half maximum (FWHMs) of merely 18 and 21 nm for TSF-DB and TSF-SB, corresponding to Commission Internationale de l'Eclairage (CIE) coordinates of (0.14, 0.17) and (0.16, 0.27). Compared with sensitizers, both TSF devices show much blue-shifted emission, mainly arising from the narrowband spectra of the final emitter. Different from TSF-DB, which shows a deep-blue emission, TSF-SB shows a sky-blue emission owing to the incomplete energy transfer, reflected by the tail at the long wavelength region. This phenomenon has also been observed in previous works and the emission tail could be effectively reduced in top-emitting devices, taking advantage of the microcavity effect.^9,11,12 Fig.4b provides the EQE-luminance characteristics and high EQE_max values of 30.8% and 27.8% are attained for TSF-DB and TSF-SB, which remain 26.3% and 24.2% at 1,000 cd/m² and 20.0% and 19.4% at 5,000 cd/m², respectively. Both devices are measured to exhibit a Lambertian distribution as shown in Supplementary Fig.16, ensuring that the EQE values are not overestimated.

We tested the stability of the blue OLEDs and remarkable LT95s of 221 and 454 h were obtained for TSF-DB and TSF-SB at an initial luminance of 1,000 cd m^-2 as provided in Fig.4c. To the best of our knowledge, TSF-SB is one of the most stable blue devices ever reported among OLEDs with EQE of 20% and CIE_y< 0.3 as summarized in Fig. 4d. Particularly, TSF-DB even outperforms the recently reported stabledeep-blue phosphorescent OLEDs based on Platinum (Pt) complex with an EQE_max of 25.4%, an LT95 of 150 h and CIEcoordinates of (0.141, 0.197).¹ Recently, our group reported a stable blue TSF device with an LT95 of 189 h and CIEcoordinates of (0.15, 0.20) by using a perdeuterated sensitizer.²¹ TSB-DB here showed not only a longer LT95 but also a smaller CIE_y value. It is believed that the performances of the TADF emitters here can be further enhanced by perdeuteration. We also noticed that Kyulux claimed at SID Display week 2022 that blue devices with LT95 of about 450 h with CIE_y of 0.09 have been obtained. However, the details of the materials they adopted were not published and those results were obtained from top-emitting devices, different from our bottom-emitting ones.

We proposed a feasible molecule design strategy to stabilize negative polaron states of CzBN-type TADF emitters by introducing an auxiliary acceptor, resulting in a combination of high efficiency and long operational lifetime in blue OLEDs. It is experimentally unveiled that our strategy not only thermodynamically strengthens the intrinsic molecular stability in both excited and anionic states, but also kinetically speeds up exciton consumption by balancing triplet-up-conversion and singlet-radiation processes. The proof-of-the-concept emitters exhibited excellent intrinsic stability under photo/electrical-aging tests and short-delayed lifetimes owing to their balanced singlet radiation (k_r > 10⁷ s^-1) and triplet up-conversion (k_RISC > 10⁶ s^-1) processes. Those molecules are adopted as sensitizers for a deep blue final emitter, exhibiting stable OLEDs with LT95s of 221 and 454 h, together with EQE_max of 30.8% and 27.8%, and CIE_y of 0.17 and 0.27, respectively. The observed lifetimes are the cutting-edge values for devices based on TADF molecules, particularly for the blue ones, and the color purity of the TSF OLEDs could be further improved by using MR emitters with BT.2020 emission.^43–45 Our work here demonstrates that through the judicious design of TADF sensitizers, it is possible to achieve not only high efficiency and color purity but also long device lifetimes simultaneously. To speak further, organic semiconductors are often better hole transporters than electron transporters in consideration of both stability and efficiency, and it is evident that our molecular design strategy may thus benefit not only TADF emitters but also other organic electron-related functional materials, including host and electron-transporting materials.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (Grant Nos. 52222308 and 22135004), the National Key Research and Development Program (2022YFB3603002 and 2023YFE0203300) and the Guangdong Major Project of Basic and Applied Basic Research (Grant No. 2019B030302009). The authors would like to thank Prof. Juan Qiao and Mr. Qingyu Meng from Tsinghua University for their help in calculation of BDE values and valuable suggestions. The authors would like to thank Prof. Lixiang Wang and Prof. Shumeng Wang from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, for their help in providing the MR-TADF material t-BuCz-DABNA. We would also like to thank Dr. C. Li and Dr, X. Cao from Sunera Technology Co.,Ltd. for their help in device fabrication and operational lifetime measurement.

Author Contributions Statement

D.L. and Z.D.D conceived and supervised this work. Z.D.D proposed the molecule design concept. H.T.Y. carried out the quantum-chemical calculations, synthesized the materials and carried out the aging measurements and photophysical measurements. H.T.Y., Z.H., W.Q., X.Y.Y. and Z.Y.W. fabricated OLEDs. D.L, Z.D.D and H.T.Y. discussed the results and wrote the manuscript with input from all authors.

Competing interests Statement

The authors declare no competing interests.

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Materials synthesis and characterization. All commercially available reagents were used as received unless otherwise stated. All reactions were carried out using Schlenk techniques under a nitrogen atmosphere. NMR spectra were measured on a JEOL 400/600 MHz spectrometer with the internal standard of tetramethylsilane (TMS). Mass spectra were recorded on a Shimadzu MALDI-TOF mass spectrometer. Full details of the synthesis can be found in Supplementary Information.

Quantum chemistry calculation methods. Density functional theory (DFT) calculations were conducted on the M06-2x/6-31+G(d) level with Grimme’s dispersion correction (GD3) using the Gaussian 16 program.⁴⁶ The thermodynamic properties of optimized structures were verified by frequency analysis. Time-dependent DFT (TDDFT) was used to simulate the excitation properties on the B3LYP/6-31+G(d) level. Spin-unrestricted DFT was used to optimize the electronic structure and calculate properties in a triplet excited state. The bond dissociation energies (BDEs) in polaronic(+/-) and neutral states were derived by frequency simulations of the entire molecules and the fragments. Spin-orbital coupling constants were calculated by using the ORCA 5.0 program.⁴⁷ Hole-electron analysis was performed by using the Multiwfn 3.8 program.^48,49

Electrochemical measurements. The electrochemical measurements were performed with a Potentiostat/Galvanostat Model 283 (Princeton Applied Research) electrochemical workstation by using Pt as the working electrode, platinum wire as the auxiliary electrode, and an Ag wire as the reference electrode standardized against ferrocene/ferrocenium. The oxidation potentials were measured in a dichloromethane (CH₂Cl₂) solution containing 0.1 M n-Bu₄NPF₆ as the supporting electrolyte at a scan rate of 100 mV s^-1. The reduction potentials were measured in N, N-Dimethylformamide (DMF) solution containing 0.1 M n-Bu₄NClO₄ as the supporting electrolyte at a scan rate of 100 mV s^-1.

Photophysical measurements. Organic films for optical measurements were fabricated by thermal evaporation under high vacuum onto clean quartz substrates. UV-vis absorption spectra were recorded by an Agilent 8453 spectrophotometer. Fluorescence and phosphorescence spectra at steady state were recorded by Hitachi F-7000 Fluorescence Spectrometer. Fluorescence lifetime measurement was carried out with an Edinburgh fluorescence spectrometer (FLS1000) using a nanosecond pulsed diode laser under the excitation at 365 nm. Photoluminescence quantum yields were measured by a Hamamatsu absolute PL quantum yield spectrometer (C9920-02G) with an integrating sphere. The dipole orientation of the doped film was determined by angle-resolved and polarization-resolved PL measurements. A doped film with a thickness of 30 nm was deposited onto a fused silica-based half-cylindrical lens. A continuous-wave He: Cd laser (375 nm) with a fixed angle of 45° to the substrate was employed as the excitation source. The p-polarized emission light was detected at the PL peak wavelength of dopants.

X-ray structural analysis. Single crystals of the compounds were obtained by sublimation in vacuum, respectively. Intensity data were collected on an Oxford Gemini S Ultra system (Cu Kα). Absorption corrections were applied by using the program CrysAlis (multi-scan). The structure was solved by direct methods and non-hydrogen atoms except solvent molecules and counter anions were refined anisotropically by least-squares on F2 using the SHELXTL program. The diffuse electron densities resulting from the residual solvent molecules were removed from the data set using the Olex2 solvent mask. The corresponding CCDC reference numbers (2232729, 22327230, 2232731) and the data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Device fabrication and characterization. The OLEDs were fabricated by vacuum-deposition processing (pressure < 1×10⁻⁴ Pa) using a Trovato 450C system. The current density, voltage, luminance, external quantum efficiency, electroluminescent spectra, angle-dependent EL intensities and other characteristics were measured with Keithley 2400 sourcemeter and the absolute EQE measurement system with Lambertian approximation. The EQE measurement system is Hamamatsu C9920-12, which is equipped with Hamamatsu PMA-12 Photonic multichannel analyzer C10027-02 whose longest detection wavelength is 1100 nm. All the device fabrication and characterization steps were carried out at room temperature under ambient laboratory conditions.

Methods-only references

46. Frisch, M. J. et al. Gaussian 16 Revision B.01. (2016).

47. Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2, 73–78 (2012).

48. Lu, T. & Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J. Comput. Chem. 33, 580–592 (2012).

49. Liu, Z., Lu, T. & Chen, Q. An sp-hybridized all-carboatomic ring, cyclo[18]carbon: Electronic structure, electronic spectrum, and optical nonlinearity. Carbon 165, 461–467 (2020).

Table 1 Summary of photophysical characteristics of TADF compounds in dilute toluene solutions

	λ_PL^a [nm]	S₁ [eV]	T₁ [eV]	ΔE_ST [eV]	PLQY^b [a.u.]	τ_p [ns]	τ_d [μs]	k_r [10⁷ s^-1]	k_RISC [10⁵ s^-1]
4CzBN-PhCN	486	2.82	2.76	0.06	0.12/0.98	7.7	6.5	1.56	12.5
4tCzBN-PhCN	497	2.72	2.70	0.02	0.13/0.99	9.0	2.4	1.22	37.5
4tCzBN-TPTRZ	492	2.73	2.72	0.01	0.14/0.97	7.1	6.0	1.55	14.6
3Cz2DPhCzBN	479	2.84	2.63	0.21	0.15/0.99	12.6	9.6	0.95	8.6

^a) λ_PL stands for the peak wavelength of photoluminescence in dilute toluene. ^b)PLQY measured in aerated/O₂-free toluene solution.

Table 2 Characteristics of the TADF and TSF devices.

Device Type	Emitters/ Sensitizers	λ_EL^a [nm]	EQE^b [%]	FWHM [nm]	CIE^c (x, y)	LT95@ 5,000 cd m^-2 [h]	LT95@ 1,000 cd m^-2 [h]
TADF	4CzBN-PhCN	489	28.0/26.6/21.9	80	0.21, 0.40	26.7	-
	4tCzBN-PhCN	501	37.1/36.4/34.2	74	0.23, 0.48	37.6	-
	4tCzBN-TPTRZ	496	27.9/27.6/24.5	77	0.23, 0.44	16.4	-
	3Cz2DPhCzBN	490	25.9/24.7/18.6	79	0.20, 0.38	6.9	-
TSF-DB	4CzBN-PhCN	470	30.8/26.2/20.0	18	0.14, 0.17	-	221
TSF-SB	4tCzBN-PhCN	470	27.8/24.2/19.4	21	0.16, 0.27	-	454

^a) λ_EL stands for the peak wavelength of electroluminescent devices recorded at 1,000 cd m^-2.^b) Efficiency values recorded at maxima, 1,000 and 5,000 cd m^-2. ^c) CIE values recorded at 1,000 cd m^-2.

There is NO Competing Interest.

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Delocalizing Electron Distribution in Thermally Activated Delayed Fluorophors for High-efficiency, Long-lifetime Blue Electroluminescence

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