High‐efficiency thermally activated delayed fluorescence materials via a shamrock‐shaped design strategy to enable OLEDs with external quantum efficiency over 38%

To achieve highly‐efficient organic light‐emitting diodes (OLEDs), great efforts have been devoted into constructing thermally activated delayed fluorescence (TADF) with high horizontal dipole ratios (Θ//). Here, we proposed a design strategy by integrating a rigid electron‐accepting oxygen‐bridged boron core with triple electron‐donating groups, which exhibited a “shamrock‐shape”, namely BO‐3DMAC and BO‐3DPAC. Benefiting from the rigid and large‐planar skeletons brought by shamrock‐shaped design, BO‐3DMAC and BO‐3DPAC exhibit high Θ// of 84%/70% and 93%/94% in neat/doped films, respectively, and finally furnish excellent external quantum efficiencies (EQEs) of up to 28.3% and 38.7% in 20 wt% doped OLEDs with sky‐blue emission, as well as adequate EQEs of up to 21.0% and 16.7% in nondoped OLEDs. This work unveils a promising strategy to establish high‐Θ// TADF emitters by constructing large‐planar molecular structures using shamrock‐shaped design.

outcoupling efficiency and is usually limited to the range of 20%-30% for multilayer OLEDs without extra light extraction techniques. [6,7]Obviously, how to enhance IQE and η out simultaneously should be taken into thoughtful consideration for achieving extremely high EQEs.
Since singlet and triplet excitons are created by the charge carries recombination with a ratio of 25% and 75%, respectively, the best use of triplet excitons is critical to build highly-efficient OLEDs.As the 1st generation of OLED materials, fluorescence emitters suffer from low EQEs (<5%) without breaking spin-forbidden rule to harvest triplet excitons.Subsequently, phosphorescence-based OLEDs using organometallic compounds as emitters (the 2nd generation) can make full use of triplet excitons with theoretical 100% of IQE relying on effective spin-orbit coupling.10][11] To address these issues, purely organic thermally activated delayed fluorescent (TADF) emitters were first applied as promising OLED materials (the 3rd generation) by Adachi in 2012. [12]Based on the premise of small singlet-triplet energy splitting (ΔE st ), TADF emitters are capable of a theoretical 100% exciton harvesting efficiency via reverse intersystem crossing (RISC) process. [13][16][17][18][19][20][21] Besides IQE, the η out is another bottleneck hampering further development of TADF-based OLEDs with extremely high EQE over 30%. [22]n order to overcome pre-existing issues with OLED performance, some rational strategies should also be implemented to enhance η out , among which, raising the horizontal dipole ratios (Θ // ) of emitters is an effective and low-cost approach.25][26][27] In the past decade, a kind of shamrock-shaped donoracceptor (D 3 -A) design have emerged in TADF-OLED field.[30][31][32][33][34][35][36][37][38][39][40][41][42] For example, in 2021, Adachi and coworkers developed a shamrock-shaped emitter mBDPA-TOAT, and it achieves a high EQE max of 17.3% with narrow red emission. [37]n 2022, Eli Zysman-Colman and coworkers reported a similar shaped molecule namely 3Cz-DiKTa, which can reach an EQE max of 24.4%, [41] and further, developed two new molecules with enhanced EQE max values over 30% using the identical design in the same year. [40]Nevertheless, with regard to the shamrock-shaped design, most related studies pay more attention to achieving solutionprocessable feature, [29-33, 36, 37] while the great potential to construct high-Θ // molecules are usually neglected. [39,40,42]he shamrock-shaped design is able to endow molecules a dish-like geometry with large effective-planarity to maximize the Θ // , which is quite promising to achieve super-high EQE (EQE max ≥ 35%). [43]Meanwhile, the huge-increasing MW brought by the multi-substituted structure is also helpful for the high-Θ // construction. [26]Additionally, it is found that TADF molecules with such a shamrock-shape can readily realize some surprising features such as aggregation-induced emission (AIE), low efficiency roll-off, and narrowband through the introduction of appropriate D&A groups. [37,39,40]ith this hypothesis, we proposed a design strategy that the intrinsically-planar oxygen-bridged boron acceptor core (BO) with triple substituent sites were selected, and then the donors with different lengths were attached to further enlarge the planarity two-dimensionally for optimized Θ // (Figure 1).
Herein, enlightened by shamrock-shaped molecular design, two rigid and large-plane TADF emitters BO-3DMAC and BO-3DPAC were designed, as shown in Figure 1.The planar BO unit and acridine-series (AC) unit are selected to act as the electron-acceptor and electrondonor, respectively.The BO acceptor holds two main advantages of high rigidity and wide planarity to ensure suppressed intramolecular motions and high Θ // . [44]Meanwhile, the triple AC donors with the shamrock-shaped structure widely extend the plane of structures from three different directions and increase the MW, which drives the Θ // of the two emitters up to 70-94%.Owing to the shamrock-shaped D-A and rigid structures, BO-3DMAC and BO-3DPAC exhibit significant TADF characteristics with high PLQYs up to 86.5% and 99.6%, respectively.Benefiting from these factors, BO-3DMAC-and BO-3DPAC-based sky-blue TADF-OLEDs are fabricated, achieving high EQE max of 28.3% and 38.7% by adopting host-guest doping technique, respectively.

Theoretical calculations
The synthetic routes and details of BO-3DMAC and BO-3DPAC are provided in Scheme S1, and structural characterizations are explained in Figures S1-S9 via nuclear magnetic resonance spectroscopy, high-resolution mass spectrum and elemental analyses.
To explore the frontier orbital distributions of BO-3DMAC and BO-3DPAC, theoretical calculations using time-dependent density functional theory (TD-DFT) with PBE0/6-31G (d,p) level were performed based on singlecrystal geometries.As shown in Figure 2, the highest occupied molecular orbitals (HOMOs) are mainly dispersed on one of the side-wing AC donors, while the lowest unoccupied molecular orbitals (LUMOs) are mainly localized on the BO acceptor core, which features well-separated distributions and contributes to small theoretical ΔE st values of 0.03 and 0.06 eV for BO-3DMAC and BO-3DPAC, respectively.Thereby, it can be predicted that the efficient RISC process will be realized for BO-3DMAC and BO-3DPAC to construct distinct TADF characteristics.Furthermore, natural transition orbital analyses were implemented using TD-DFT based on Multifwn software [45] to examine the excited states of the two molecules (Figures S10 and S11).The holes and electrons distributions of both singlet (S 1 ) and triplet (T 1 ) states are well separated on the side-wing donor and acceptor cores respectively, suggesting distinct intramolecular charge transfer (ICT) properties of the excited-states.These results indicate that the charge separation is still effective for shamrock-shaped substitution with the connection of donors and acceptors.

F I G U R E 2
Highest occupied molecular orbitals (HOMOs)/lowest unoccupied molecular orbitals (LUMOs) distributions and calculated ΔE st of BO-3DMAC (left) and BO-3DPAC (right).

Photophysical properties
The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra of BO-3DMAC and BO-3DPAC in dilute toluene (1 × 10 −5 M) solution are illustrated in Figure 3.The similar intense absorption peaks of BO-3DMAC and BO-3DPAC below 335 nm arise from π-π* transition, while the weak peaks at the range of 400-450 nm are assigned to the ICT transition from donor to acceptor.
To further verify the ICT property, solvatochromic effects of the two emitters were also measured (Figures S12 and  S13).With increasing polarity of solvents, the emission spectra of BO-3DMAC and BO-3DPAC both exhibit apparent bathochromic shift accompanied by broadened full-width at half-maximum (FWHM), while the absorption spectra in each solvent are hardly changed.Meanwhile, the PL spectra of BO-3DMAC and BO-3DPAC in toluene show narrow emissions with FWHMs of 57 and 54 nm, respectively, which can be ascribed to the rigidity of BO acceptor core. [46]urthermore, the ΔE st of BO-3DMAC and BO-3DPAC are calculated to be 22.0 and 21.8 meV respectively based on the onsets of low-temperature fluorescence and phosphorescence spectra (Figure S14).Such small ΔE st values of the two emitters can facilitate the up-conversion of excitons from T 1 to S 1 via RISC process.Intriguingly, both BO-3DMAC and BO-3DPAC exhibit AIE characteristics (Figure S15-S16).In the tetrahydrofuran (THF)/water mixed solution systems, with the increasing of water content, the PL intensity decreases at first when the water fraction (f w ) ranges from 10 to 60% and then rapidly boosts under the f w ≥ 70%.The decreased emission intensities under f w ≤ 60% are assigned to the polarity increasement of the mixed solvents while the enhanced emission intensities under higher f w are due to the formation of nanoaggregates to restrict the molecular motions and exclude the high polarity solvent. [46]To further confirm the existence of nanoaggregates, the effective diameters of BO-3DMAC and BO-3DPAC were measured to be 164 and 152 nm respectively by dynamic light scattering test (Figure S17).In further set of experiments, transient PL (TRPL) spectra of the emitters in THF/water mixtures (f w = 0% and 90%) were also implemented (Figure S18).The microsecondscale delayed components of the two compounds increase significantly when the f w changes from 0% to 90%.According to the results above, it is evident that BO-3DMAC and BO-3DPAC show great potential to possess AIE-TADF characteristic, thus avoiding the concentration quenching in aggregated state.These results demonstrate that the ICT and AIE feature can be easily integrated in a molecule via the shamrock-shaped design.
Furthermore, single crystals of BO-3DMAC and BO-3DPAC were grown by slow solvent evaporation to evaluate the aggregation features.The crystal structures of these two molecules are shown in Figure 4.For BO-3DMAC, relative to the BO acceptor core, the two side-wing AC donors display a nearly orthogonal arrangement (80.35 • /89.92 • ), while the tail AC donor displays a boat configuration with the angle of 123.56 • (C-N-C) (Figure S19).On the other hand, in the packing mode, BO-3DMAC forms a dense π-π stacking dimer in an antiparallel manner with a very short plane-plane (O-B-O plane) distance of 3.350 Å (Figure 4C).Moreover, two pairs of C-H⋅⋅⋅O (a: 2.553 Å) and C-H⋅⋅⋅π (b: 2.830 Å) interactions are also observed (Figure S20).[49][50] For BO-3DPAC, it can be observed visually that DPACs provide a wider molecular plane due to the introduction of peripheral phenyls, which makes BO-3DPAC more inclined to align horizontally to the substrate than BO-3DMAC.All three twisting angles between the AC donors and the BO acceptor core are close to 90 • (80.11 • /84.91 • /74.68 • ) (Figure 4B), which is conducive to the effective separation of HOMO and LUMO.Intriguingly, no π-π interaction (Figure 4D) but only two pairs of C-H⋅⋅⋅π (a: 2.797 Å; b: 2.897 Å) interactions are found in the crystalline packing because of the large steric hindrance from highly-twisted structure of the peripheral donors (Figure S20).This kind of relatively incompact packing mode may be responsible for suppressing concentration-caused quenching of luminescence. [51,52]It should be noted that the packing modes can be feasibly regulated by different donors to achieve high Θ // with better alignment at molecular level.
In order to verify the TADF features, the photophysical features were evaluated in both doped and neat films.The low temperature PL and TRPL spectra of  3C) were slightly red-shifted and broadened compared to those in toluene solutions, which related to the polarity of the host materials and the self-aggregations. [53]Similar to that of the diluted solution, the ΔE st of both emitters in doped films still keep small values of 17.5 and 12.6 meV for BO-3DMAC and BO-3DPAC, respectively.As depicted in Figure 3D, both emitters exhibit biexponential decays, which can be divided into a nanosecond-scale prompt component and a microsecond-scale delayed component.The prompt lifetime (τ p )/ delayed lifetime (τ d ) of BO-3DMAC and BO-3DPAC are determined to be 75.1 ns/2.5 μs and 72.6 ns/2.4 μs, respectively.Furthermore, oxygen-sensitive PL spectra were also examined (Figure S21).The luminescence intensities of both emitters are enhanced in vacuum compared to that in air, indicating the participation of triplet excitons.In addition, the small ΔE st and lifetimes with biexponential decay features were also observed in the neat films for BO-3DMAC and BO-3DPAC (Figure S22).Meanwhile, temperature-dependent transient decays from 100 to 300 K were recorded (Figure S23).As the temperature increases, the proportion of delayed components gradually grows, revealing that the up conver-sions from T 1 to S 1 can be stepwise activated by thermal energy.These results perfectly verify the TADF characteristic of BO-3DMAC and BO-3DPAC in both doped and neat films, which will be accessible to doped/nondoped TADF-based OLEDs fabrications.
The PLQYs of the doped films are measured to be 86.5% and 99.6% for BO-3DMAC and BO-3DPAC, respectively.Furthermore, the key rate constants were obtained from the transient decay curves in doped films.BO-3DPAC doped film exhibits a rate constant of nonradiative decay (k nr ) of 3.80 × 10 4 s −1 which is two-orders magnitude lower than that of BO-3DMAC (1.34 × 10 6 s −1 ).Unexpectedly, the other rate constants such as radiative decay, ISC and RISC (k r , k ISC , and k RISC ) show no significant differences between BO-3DMAC and BO-3DPAC.In order to verify the mechanism of lower k nr in BO-3DPAC, reorganization energy (λ) analyses are performed and depicted in Figure 4E,F.The total λ of BO-3DMAC and BO-3DPAC are 5818 cm −1 and 1352 cm −1 , respectively, indicating less geometry reorganization of the excited state in BO-3DPAC.Detailed normal mode analysis of the two emitters demonstrates that the primary vibrations of the emitters are induced by the donor moieties.Due to the large steric hindrance of the phenyl rings, BO-3DPAC exhibits much smaller excited-molecular motions and thus affords a lower k nr and a higher PLQY.The related  photophysical data of BO-3DMAC and BO-3DPAC in neat/doped films are summarized in Table S1/Table 1.

Horizontal dipole ratio
To assess the Θ // of BO-3DMAC and BO-3DPAC, transition dipole moment (TDM) calculations were conducted from the ground state (S 0 ) to S 1 with the optimized S 1 state structures. [45]As shown in Figure 5A,B, the TDM vectors of the emitters exhibit major x and y components with minor z components, implying that the directions of TDMs mainly locate in the x-y plane.The surface distance projection maps in the x-y plane are further provided in Figure 5C,D. [45]The introduction of phenyl rings in BO-3DPAC enlarges the diameter of the molecule in plane but no obvious changes in the vertical axis.From the perspective of isolated molecule, the BO acceptor cores of the two emitters bring about major planarity due to the rigid and planar structure.Moreover, the peripheral phenyl rings endow BO-3DPAC with a more extensive plane which facilitates the overlap of TDM and molecular axis in x-y plane, portending a higher Θ // in BO-3DPAC than BO-3DMAC.To confirm our assumption, the Θ // values were measured using p-polarized angle-dependent PL spectra.As shown in Figure 5E-H, BO-3DPAC in both neat and 20 wt% DPEPO doped films achieves high Θ // of over 93%, which is very close to the maximum Θ // value of 100%.On the other hand, BO-3DMAC neat film reaches a high Θ // of 84%, which is 70% in the 20 wt% CBP doped film probably due to the conformation change in the doped system.Still, these values are higher than those in common molecules with random dipole orientation.These experimental results not only verify the assumption above, but also indicate that the compounds with rigid and large-planar structures based on the shamrockshaped design hold an enormous potential on the high Θ // construction.To further estimate the electroluminescent (EL) potential of BO-3DMAC and BO-3DPAC, the theoretical maximum EQE is also calculated.Based on the tested Θ // , the η out is simulated to be 34%/28%, and 40%/40% for BO-3DMAC (neat/doped film) and BO-3DPAC (neat/doped film) by Setfos 5.1 (Figure S24), respectively. [54]Furthermore, the theoretical EQE max values are calculated to be 28.2%/25.2%for nondoped/doped BO-3DMAC devices, and 17.6%/39.3%for nondoped/doped BO-3DPAC devices via equation ( 1)), respectively.Encouragingly, such high theoretical EQE max values imply the excellent EL performance in BO-3DMAC and BO-3DPAC based OLEDs.

Thermal and electrochemical properties
The thermodynamic properties of the emitters were evaluated by thermogravimetric analysis and differential scanning calorimetry (Figure S25).BO-3DMAC and BO-3DPAC exhibit excellent thermal stability with the decomposition temperatures (T d , corresponding to 5 wt% loss) of 433 and 488 • C, respectively.Besides, BO-3DPAC shows a high glass transition temperature (T g ) of 263 • C while no apparent T g is observed for BO-3DMAC.Such satisfactory thermal stabilities imply that both emitters are suitable for vacuum evaporation to fabricate OLED devices.Meanwhile, cyclic voltammetry was performed to study the electrochemical properties of the two compounds (Figures S26 and S27).The HOMO energy levels of BO-3DMAC and BO-3DPAC are calculated from the onset of the oxidation curves (versus a ferrocene/ ferrocenium reference), which are −5.27 and −5.40 eV, respectively.According to the calculated bandgaps (E g ) based on the onset of their UV-vis spectra in dichloromethane solution, the LUMO energy levels are calculated to be −2.72 and −2.74 eV, respectively.The data of thermal and electrochemical properties are summarized in Table S2.

Electroluminescent properties
Due to the perfect luminescence features of BO-3DMAC and BO-3DPAC in doped and neat films, doped and nondoped OLEDs were fabricated to evaluate their EL performances with simple three organic-layer configurations: ITO/poly (  (16 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TP Bi) (40 nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS, mCP and TPBi serve as the hole-injection layer, holetransporting layer, and electron-transporting layer, while ITO and Al acted as anode and cathode, respectively.In the EML, CBP and DPEPO were selected to be the host materials for BO-3DMAC and BO-3DPAC, respectively, because of corresponding matched energy levels and superior experimental results (Figure S29 and Table S3).The energy level diagram and the molecular structures of the materials adopted in the devices are depicted in Figure 6A,B, and key data are summarized in Table 2.
The current density-voltage-luminance curves (Figure S28) of BO-3DMAC-based OLEDs present higher current densities and luminance than that in BO-3DPAC ones, probably due to smoother carrier mobility, which is in accord with single crystal analysis.
As shown in Figure 6C, for BO-3DMAC system, the nondoped device achieves a competitive EQE of 21.0% with the green emission peak at 532 nm.The experimentally obtained EQE max of BO-3DMAC nondoped device is 7.2% lower than the theoretical ones while the rest devices are well-matched.It can be attributed to excitons quenching brought by the bielectron involved short-ranged Dexter energy transfer process due to the large π-π overlaps between adjacent molecules as shown from Figure 4C, leading to the lower EUE. [55]To further enhance the EL performance, the CBP host doped EML was applied in OLED construction.Impressively, the EQE is increased up to as high as 28.3% with the blueshifted and narrowed emission at 495 nm accompanied with FWHM of 72 nm.Noticeably, BO-3DMAC-based devices also realize small efficiency roll-off, and the EQEs maintain to be 15.6%/23.0%at the luminance of 1000 cd m −2 in the nondoped and doped devices respectively, corresponding to 26%/18% roll-offs.
On the other hand, the EQE-luminance curves of BO-3DPAC system are illustrated in Figure 6D.Despite of a remarkable Θ // value of 93%, the BO-3DPAC nondoped device reaches an adequate EQE of 16.7%, which is limited by the modest PLQY of 44.0% probably due to adverse intermolecular action in aggregated state.By contrast, it can be seen that BO-3DPAC in the DPEPO: 20 wt% doped device achieves an outstanding EQE of 38.7% with sky-blue emission peak at 484 nm (FWHM = 71 nm).Such an encouraging EQE should be attributed to the concerted effect of small ΔE st , excellent PLQY, and superior Θ // .To our knowledge, this EL efficiency is comparable to the state-of-the-art performance of sky-blue TADF-based OLEDs.The EQEs of sky-blue TADF-based OLEDs (EL: 470-500 nm) reported from 2017 to 2022 were collected in Figure S30-S31 and Table S4, in which BO-3DPAC takes the absolute leading position.These results suggest that BO-3DMAC and BO-3DPAC are both highly-efficient TADF emitters to construct OLEDs with simple structures.

CONCLUSION
In summary, two TADF emitters (BO-3DMAC and BO-3DPAC) were designed and synthesized based on the shamrock-shaped D-A structure.The two emitters exhibit amusing AIE feature, significant TADF characteristic with high PLQY and outstanding Θ // , which boost EQEs in both doped and nondoped OLEDs.Moreover, the presence of the phenyl rings surrounding BO-3DPAC further suppresses nonradiative pathways and allows higher Θ // , thus affording the OLED with a much higher EQE of 38.7% which is amongst the highest values reported so far in sky-blue region.This work bears out the fact that high Θ // and other surprising features of emitters can be realized concurrently via shamrock-shaped design with rational selection of donors and acceptors, providing a clear way for the development of high-performance OLEDs.

A C K N O W L E D G M E N T S
This work was financially supported by the National Natural Science Foundation of China (NSFC; grant numbers: 51733010 and 52073316), and Guangdong Basic and Applied Basic Research Foundation (grant numbers: 2022B1515020052 and 2021A1515110119).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

F I G U R E 1
Molecular design of conventional strategy and shamrock-shaped strategy proposed in this work (left).Molecular structures of BO-3DMAC and BO-3DPAC (right).

TA B L E 1
Summary of the photophysical properties of BO-3DMAC and BO-3DPAC.

F I G U R E 5
The calculated transition dipole moments (A and B), surface distance projection maps (color bar represents z axis) (C and D) and measured p-polarized PL intensity in nondoped and doped films (E and H) of BO-3DMAC (left) and BO-3DPAC (right).

F
I G U R E 6 (A) The energy diagram of the devices and (B) chemical structures of the used materials.External quantum efficiencies (EQEs)−luminance curves (Inset: electroluminescence spectra) of (C) BO-3DMAC and (D) BO-3DPAC-based devices.TA B L E 2 EL performances of nondoped and doped OLEDs.
Note: a Absorption peak in toluene solutions (10 −5 M). b PL emission peak in toluene solutions (10 −5 M). c Singlet-triplet energy splitting based on spectra data from doped films.d Prompt fluorescence lifetime component in doped films.e Delayed fluorescence lifetime component in doped films.f Prompt component in doped films.g Delayed component in doped films.h Rate constant of radiative decay.i Rate constant of ISC.j Rate constant of RISC.k Rate constant of nonradiative decay for the singlet excited state.l Absolute PLQY measured in doped films.m

] EQE max / EQE 1000 d [%] λ EL e [nm] FWHM f [nm]
Abbrevitions: EL, electroluminescent; CE: current efficiency; PE: power efficiency; EQEs, external quantum efficiencies; OLEDs, organic light-emitting diodes; a Maximum luminescence.b Maximum current efficiency.c Maximum power efficiency.d Maximum external quantum efficiency/external quantum efficiency at 1000 cd m −2 .e Maximum EL wavelength.f Full width at half maximum of EL.