An efficient aggregation‐enhanced delayed fluorescence luminogen created with spiro donors and carbonyl acceptor for applications as an emitter and sensitizer in high‐performance organic light‐emitting diodes

Organic light‐emitting diodes (OLEDs) fabricated using organic thermally activated delayed fluorescence materials as sensitizers have recently achieved significant advancements, but the serious efficiency roll‐offs are still troublesome in most cases. Herein, a tailor‐made multifunctional luminogen SBF‐BP‐SFAC containing 9,9′‐spirobifluorene (SBF) and spiro[acridine‐9,9‐fluorene] (SFAC) as electron donors and carbonyl as an electron acceptor is synthesized and characterized. SBF‐BP‐SFAC has the advantages of high thermal stability, aggregation‐enhanced delayed fluorescence, and balanced carrier transport ability, and prefers horizontal dipole orientation. Highly efficient OLEDs employing SBF‐BP‐SFAC as an emitter radiate intense cyan light with outstanding external quantum efficiencies (ηexts) of up to 30.6%. SBF‐BP‐SFAC can also serve as an excellent sensitizer for orange fluorescence, phosphorescence, and delayed fluorescence materials, providing excellent ηexts of up to 30.3% with very small efficiency roll‐offs due to the fast Förster energy transfer as well as exciton annihilation suppression by bulky spiro donors. These outstanding performances demonstrate the great potential of SBF‐BP‐SFAC as an emitter and sensitizer for OLEDs.


INTRODUCTION
Organic light-emitting diodes (OLEDs), as a hotpoint in displays and white illumination devices, have been gradually applied in recent years because of their flexibility, high color quality, and low energy cost, and the development of robust organic luminescent materials for OLEDs has become particularly important. [1] Thermally activated delayed fluorescence (TADF) luminogens generally consist of organic donor (D) and acceptor (A) with a highly twisted connection, which can utilize 100% singlet and triplet electro-generated excitons via a reverse intersystem crossing (RISC) process to achieve outstanding electroluminescence (EL) efficiencies in OLEDs. The small energy gap (∆E ST ) from the lowest excited triplet state (T 1 ) to the lowest excited singlet state (S 1 ) is the key for the occurrence of the RISC process, which can be facilely achieved by molecular design to sufficiently restrict the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). [2] So far, TADF emitters are emerging as third generation luminescent materials for OLEDs, [3] and have furnished excellent EL performances for full-color and white OLEDs. [4] Besides, some TADF materials can also be used as sensitizers for fluorescence, phosphorescence, and TADF emitters, providing new strategies for the fabrication of OLEDs with high efficiencies, [5] but the large efficiency roll-offs remain a difficult problem in most cases. As shown in Figure 1, the TADF sensitizers in these sensitized devices act as the main carrier recombination centers. The triplet excitons of TADF sensitizers are upconverted to singlet excitons and then transferred F I G U R E 1 Schematic of the energy transfer mechanisms in sensitized organic light-emitting diodes (OLEDs) for the fluorescence guest (A), phosphorescence guest (B), and thermally activated delayed fluorescence (TADF) guest (C) to the singlet excitons of the guest emitters via long-distance Förster energy transfer (FET) to realize a theoretical 100% exciton utilization efficiency. [6] In order to ensure high FET efficiencies in sensitized devices, choosing an appropriate sensitizer and guest emitter to maximize the spectral overlap between the photoluminescence (PL) spectrum of the sensitizer and the absorption spectrum of the guest emitter is of high importance. [7] Meanwhile, in this blended system of sensitizer and guest emitter, it is necessary to maintain sufficient intermolecular distances to suppress exchange and quenching of the triplet excitons via Dexter energy transfer (DET), which is usually realized by employing bulky substituents or lowering doping concertation. [8] In this way, it is envisioned that an efficient sensitized device can be achieved through a fast and efficient energy transfer process, balanced carrier transportation, and inhibited triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA) in the emitting layers (EMLs). [9] As discussed above, TADF sensitizers play a promising role in creating sensitized OLEDs with high EL efficiencies, while a majority of TADF sensitizers experience serious emission quenching and exciton annihilation, which greatly undermine device performance and practical application. To meet the above needs and challenges, in our previous work, we developed a multifunctional green luminogen SBF-BP-DMAC (where DMAC is 9,9-dimethyl-9,10-dihydroacridine) with evident aggregation-enhanced emission (AEE) and TADF characteristics, which achieved high external quantum efficiencies (η ext s) of 24.5% as an emitter and 26.8% as a sensitizer for Ir(tptpy) 2 acac. [10a] This achievement greatly encourages us to explore more efficient multifunctional luminogens for application as both emitters and sensitizers. In this contribution, a new luminogen SBF-BP-SFAC with a highly twisted asymmetric D-A-D′ structure is designed and synthesized, where 9,9′-spirobifluorene (SBF) and spiro[acridine-9,9-fluorene] (SFAC) work as electron donors and carbonyl works as an electron acceptor. The well-separated HOMO and LUMO, because of the twisted D-A framework of carbonyl and SFAC units, can achieve a small ∆E ST value for gaining delayed fluorescence. SBF is employed to improve hole injection and balance carrier transportation. Besides, the introduction of SBF and SFAC as bulky functional units cannot only enhance molecular rigidity and stability but also increase the intermolecular distance between the sensitizer and the dopant to alleviate emission quenching and exciton annihilation in the solid state. In comparison with SBF-BP-DMAC, the DMAC in SBF-BP-S C H E M E 1 Synthetic route and chemical structure of SBF-BP-SFAC DMAC is replaced by the rigid SFAC in SBF-BP-SFAC, which is demonstrated to be conducive to facilitating horizontal orientation of the emitting dipole [11] and increasing thermal stability. Meanwhile, because the electron-donating ability of SFAC is weaker than that of DMAC, the D-A interaction in SBF-BP-SFAC should be weakened, rendering blueshifted PL emission. The elaborate SBF-BP-SFAC shows cyan aggregation-enhanced delayed fluorescence and good bipolar carrier transport capacity. Relative to SBF-BP-DMAC, SBF-BP-SFAC can function as a better emitter with outstanding η ext s of up to 30.6% in doped devices and a better sensitizer for orange fluorescence, phosphorescence, and TADF emitters with excellent η ext s as high as 30.3% and small roll-offs, validating that it is a promising emitter and sensitizer for OLEDs.

Synthesis and characterization
The synthetic procedure and structure of the target molecule are presented in Scheme 1. The molecule SBF-BP-SFAC was synthesized by two-step reactions with 87% yield. The chemical structure was comprehensively confirmed, and the characterization data are listed in Supporting Information. Thermogravimetric analysis and differential scanning calorimetry measurements ( Figure 2A) reveal that it possesses a high decomposition temperature (T d , at 5% weight loss) of 442 • C and a high glass-transition temperature (T g ) of 163 • C, higher than those of SBF-BP-DMAC (T d = 369 • C; T g = 131 • C), [11a] indicating that the rigid spiro SFAC is To confirm the structure and investigate the intermolecular interactions of SBF-BP-SFAC, the single crystals were cultured by solvent vapor diffusion in a dichloromethane/nhexane mixture, and the crystal structure was determined by a single-crystal X-ray diffraction method. Figure 3A shows that SBF-BP-SFAC adopts a highly torsional geometry with a large dihedral angle of 82.22 • between the SFAC moiety and the adjacent phenyl group. Such a twisted conformation can effectively suppress the spatial overlap between the HOMO and LUMO to generate a small ∆E ST and contribute to inhibiting close molecular packing to alleviate energy dissipation in the aggregated state. Besides, according to the packing pattern of SBF-BP-SFAC in crystals ( Figure 3B), there are no obvious close ππ stacking interactions but abundant C−H⋅⋅⋅π interactions (2.690−3.183 Å) and C−H⋅⋅⋅O interactions (2.419 Å), which are beneficial for enhancing molecular rigidity and restricting intramolecular motion to reduce energy loss through nonradiative decay, thus promoting radiative decay in solid states.

Electronic structures and energy levels
Theoretical calculations were performed to investigate the geometrical structure and frontier molecular orbitals and predict ∆E ST of SBF-BP-SFAC. The optimized molecular structure of SBF-BP-SFAC is highly twisted, similar to the crystal structure. The HOMO is primarily distributed on the acridine segment, while the LUMO is mainly dispersed over the carbonyl core and adjacent phenyl and carbazole units ( Figure 3C). The completely separated distribution of HOMO and LUMO indicates a small theoretical ΔE ST of SBF-BP-SFAC. According to the optimized ground-state geometry, the ΔE ST value is calculated as small as 0.11 eV, ensuring the occurrence of the RISC process to generate delayed fluorescence. To obtain the electrochemical properties of SBF-BP-SFAC, cyclic voltammetry was conducted in the solutions ( Figure 2B). The initial potentials of oxidation and reduction against the Fc/Fc + redox couple are 0.48 and −1.87 V, respectively. Therefore, the energy levels of HOMO and LUMO were calculated to be approximately −5.28 and −2.93 eV, respectively, according to the onsets of oxidation and reduction waves ( where E ox and E re represent the onset potentials of oxidation and reduction potentials of SBF-BP-SFAC relative to Fc/Fc + , respectively). The appropriate energy levels are applicable for the injection of holes and electrons in OLEDs.

Photophysical properties
The photophysical properties of SBF-BP-SFAC were investigated in tetrahydrofuran (THF) solution with a concentration of 10 -5 M. As shown in Figure 4A, SBF-BP-SFAC possesses an absorption peak at approximately 328 nm assigned to the π-π* transition of the D-A framework. Meanwhile, a weak wide absorption tail at long wavelengths is observed till 410 nm, which can be attributed to the intramolecular charge transfer (ICT) state from the SFAC group to the carbonyl core. In THF solution, SBF-BP-SFAC shows weak green PL emission at 521 nm with a relatively low PL quantum efficiency (Φ PL ) of 22.5%. When a small volume of water (as a poor solvent) is added to the THF solution, the PL intensity is weakened to some extent. This result can be ascribed to the enhanced ICT effect with the increased polarity of the mixed solvents. But the PL intensity increases significantly as the water fraction (f w ) of the mixture exceeds 80 vol% ( Figure 4B). This phenomenon of AEE can be attributed to aggregate formation in an almost aqueous environment, which can restrict intramolecular vibrations and rotations to suppress nonradiative decay in the excited state. [10] In comparison with THF solution, the vacuum deposition film of SBF-BP-SFAC shows a bluer emission at 498 nm, with a high Φ PL of 71.1%. Similarly, the doped film of SBF-BP-SFAC in the bis-(diphenylphosphoryl)-dibenzo [b,d]-furan (PPF) host with a doping concentration of 30 wt% also shows a blueshifted emission peak at 506 nm with a higher Φ PL of 94.6%. The bluer and stronger PL emissions in both neat and doped films can be partially ascribed to the reduction of the polarity in the films and restriction of intramolecular motion, which further validates the AEE characteristic of SBF-BP-SFAC. SBF-BP-SFAC shows similar prompt fluorescence lifetimes in the range of 19.8-27.6 ns in THF solution and neat and doped films. But the latter two states exhibit much longer delayed fluorescence lifetimes of 23.8 and 17.4 μs, respectively, with much higher ratios of delayed components than THF solution (2.5 μs) (Table 1), indicating that SBF-BP-SFAC has promoted delayed fluorescence in vacuum deposition film. To further confirm the delayed fluorescence property of SBF-BP-SFAC, the transient PL behaviors under different temperatures were examined for the neat and doped films in a nitrogen atmosphere. As depicted in Figure 4C,D, the decay of PL intensity is apparently improved as the temperature rises from 77 to 300 K as a result of the promoted RISC process under thermal activation, and the delayed components become higher at higher temperatures than low temperatures (Table S1). These results clearly validate the typical TADF characteristic of SBF-BP-SFAC. [12] Combining the evident AEE and TADF properties, SBF-BP-SFAC possesses an interesting aggregation-enhanced delayed fluorescence nature. The low-temperature (77 K) fluorescence and phosphorescence spectra were collected ( Figure S1), and the triplet energy levels of SBF-BP-SFAC were calculated to be 2.618 and 2.637 eV in neat and doped films, respectively. Therefore, the ΔE ST values are as small as 0.028 and 0.024 eV for SBF-BP-SFAC in neat and doped films, which are conducive to accelerating the RISC process and acquiring efficient delayed fluorescence in aggregated states.

OLEDs with SBF-BP-SFAC as an emitter
To investigate the EL property of SBF-BP-SFAC, non-doped and doped OLED devices were fabricated via a vacuum deposition technique with the multilayer configuration of Figures 5A and S2, in which a neat film of SBF-BP-SFAC functions as an EML for the non-doped device (device I) and a doped film of 30 wt% SBF-BP-SFAC in the PPF host functions as an EML for the doped device (device II). The low turn-on voltages (V) at 3.0 V are obtained for both devices, and the devices illuminate strong cyan light at  approximately 506−510 nm with a maximum luminance (L) beyond 30,000 cd m −2 ( Table 2). For non-doped device I, the maximum current efficiency (η C ), power efficiency (η P ), and η ext are 65.0 cd A −1 , 61.7 lm W −1 , and 23.1%, respectively. For doped device II, the maximum η C , η P , and η ext are increased to 83.3 cd A −1 , 74.3 lm W −1 , and 30.6%, respectively, due to the higher Φ PL of SBF-BP-SFAC in the doped film. The η ext s of SBF-BP-SFAC in both non-doped and doped devices are superior to those of SBF-BP-DMAC (Table S3). To understand the outstanding EL performance, the ratio of the horizontal orientation emitting dipole (Θ // ) of SBF-BP-SFAC in the doped film is investigated by variableangle p-polarized PL measurement (Supporting Information), and the result is shown in Figure 5F. The Θ // of the SBF-BP-SFAC-doped film is fitted to be 84.5%, indicating that it prefers a horizontal dipole orientation. According to Θ // , the reflex indexes, and the thickness of the active layers, the optical outcoupling efficiency (η out ) of an optimized doped OLED with SBF-BP-SFAC as the emitter is calculated to be 34.9%. Therefore, the high Φ PL , η out , and exciton utilization account for the superb EL performance of SBF-BP-SFAC in the doped device.

Carrier transport capacity
Balanced carrier transport is of great importance to gain high EL performances of the devices. In view of the excellent EL efficiencies in non-doped and doped devices with SBF-BP-SFAC as an emitter, the electron-only device (EOD) and the hole-only device (HOD) were fabricated to measure its carrier mobility property via the space-charge-limited current (SCLC) method (Supporting Information). [13,14] Thin layers of TmPyPB with an electron mobility (μ e ) of approximately 10 −3 cm 2 V −1 s −1 [15] and TAPC with a hole mobility (μ h ) of approximately 10 −2 cm 2 V −1 s −1 [16] were used as the electron/hole-injection layer and hole/electron-blocking layer between the emissive layer and the electrodes. The current density versus the applied voltage of the EOD and HOD are shown in Figure 6A, and the electric field-dependent mobilities (μ) are illustrated in Figure 6C. The results reveal that SBF-BP-SFAC features noticeable bipolar carrier transport ability in the operational voltage region with very close μ e and μ h values of 1.96 × 10 −4 and 1.72 × 10 −4 cm 2 V −1 s −1 , respectively, at a practical electric field of 5.5 × 10 5 V cm −1 . It is beneficial to balance the carrier transportation, enhance EL performance, and suppress efficiency roll-off in OLED devices.

OLEDs with SBF-BP-SFAC as a sensitizer
With the excellent aggregation-enhanced delayed fluorescence property and balanced carrier transport capacity, the potential of SBF-BP-SFAC as a sensitizer for orange fluorescence, phosphorescence, and TADF emitters of TBRb, phosphorescent It(tptpy) 2 acac, and 4CzTPN-Ph, respectively, are evaluated ( Figure 7A). The UV-vis absorption spectra of guest emitters overlap well with the PL spectrum of SBF-BP-SFAC ( Figure 7B), which is favorable to achieve efficient FET from SBF-BP-SFAC to these emitters. Multilayer sensitized OLEDs were fabricated with the configurations illustrated in Figure 7C, where the EML is composed of doped films of PPF host containing 30 wt% SBF-BP-SFAC and 1 wt% TBRb for device III, 30 wt% SBF-BP-SFAC and 3 wt% It(tptpy) 2 acac for device IV, and 30 wt% SBF-BP-SFAC and 1 wt% 4CzTPN-Ph for device V. The energy-level diagrams of the sensitized OLED devices are depicted in Figure 7C, and the summarized EL performances are shown in Table 3. These devices can be turned on at low voltages of 2.9-3.2 V and provide orange lights with peaks at approximately 560 nm, indicating efficient charge injection and transportation as well as sufficient FET from the sensitizer to the emitters are successfully achieved in these devices. These devices afford peak luminance of 31,060, 140,700, and 50,990 cd m −2 and maximum η ext s of 13.6%, 30.3%, and 24.2%, respectively. The η ext of device III with TBRb as the emitter is enhanced by more than twofold relative to those of traditional devices, and the η ext of device V with 4CzTPN-Ph as the emitter is the highest η ext ever reported for 4CzTPN-Ph (Table S2). In addition, these devices still maintain good EL performance with η ext values of 10.0%, 25.6%, and 18.6% at 1000 cd m −2 , respectively, indicative of small efficiency roll-offs. The performance of SBF-BP-SFAC as a sensitizer is also better than that of SBF-BP-DMAC (Table S4). All of the above outstanding EL performances demonstrate that SBF-BP-SFAC can function as an excellent sensitizer for orange emitters.

Mechanistic study of sensitized OLEDs
In view of the satisfactory EL performance and small efficiency roll-offs of these sensitized OLEDs, it is meaningful to further explore the process of energy transfer and the mechanism of efficiency attenuation. It can be found from the schematic energy levels in Figure 7C that the HOMO  level of SBF-BP-SFAC is much shallower than that of the PPF host, indicating that SBF-BP-SFAC can trap holes to comfine charge recombination within the sensitizer instead of the emitters. The sensitizer SBF-BP-SFAC can harvest 100% excitons by converting triplet excitons to singlet excitons and then excite the guests through an efficient long-distance FET process to acquire guests with high EL efficiencies. To investigate the exciton behavior of these sensitized OLEDs, the exciton recombination zones of devices III−V were studied. The FET radius (R 0 ), theoretical FET rate (k FET ), and FET efficiency (Ф FET ) were determined according to a previously reported method (Supporting Information). [17] All the calculated data are presented in Table 4. The k FET s in the EMLs of the three sensitized OLEDs are calculated as 1.81−5.95 × 10 8 s −1 , and the Ф FET s are 83.3%−93.4%, demonstrating the fast rate and high efficiency of FET. In addition, the distance between the sensitizer and the doped emitters is in the range 2.22−3.11 nm, indicating that the low concentration doping technique has weakened the DET, whose effective transmission distance is generally considered as 1-1.5 nm. [18] Therefore, the long-distance FET from the sensitizer SBF-BP-SFAC to the emitters predominantly contributes to the outstanding EL efficiencies of the sensitized OLEDs. These above sensitized OLEDs exhibit not only high efficiencies but also small efficiency roll-offs at high current density. Actually, there are many factors leading to efficiency degradation of OLEDs, such as TTA and TPQ. To deeply understand the efficiency decay mechanism of these sensitized devices, the curves of η ext and J of devices III−V are fitted by the TTA model [19] and TPQ model. [20] The TTA simulation can be described as Equation (1), where η, η 0 , and J 0 represent η ext in the presence of TTA, initial η ext in the absence of TTA (TTA quenching is ideally negligible at very low current density), and the current density at the halfmaximum of η ext , respectively. The TPQ simulation can be described as Equation (2), where η 0 is η ext in the absence of TPQ and C is a constant that is related to parameters such as the dielectric constant, carrier mobility, TPQ rate constant, and decay lifetime. To obtain a good fitting result, it requires that l equals to 1, which means that the charge transport in the device should follow the SCLC characteristic. (2) As depicted in Figure 8, the results of TTA fitting coincide well with the practical curves for these sensitized devices, even in the region of high current density, indicating that the TTA process is the prime factor for the efficiency decline of the sensitized devices at high current density. In this sense, the small efficiency roll-off under high current density can be attributed to the alleviation of triplet exciton aggregation via a fast FET process and long intermolecular distances due to space-demanded bulky spiro donors. On the other hand, the failure of TPQ model fitting implies well-balanced charge injection and transportation in these sensitized devices, especially at high current density, which can be partially attributed to the well-balanced hole and electron transport ability of SBF-BP-SFAC.

CONCLUSION
In summary, a new multifunctional luminogen, SBF-BP-SFAC, with the highly twisted configuration of an electronaccepting carbonyl core and two electron-donating spiro groups was successfully synthesized and characterized. SBF-BP-SFAC exhibits high thermal stability, bipolar carrier transport, and aggregation-enhanced delayed fluorescence with an excellent Φ PL of 94.6%. Owing to the presence of the spiro acridine donor, SBF-BP-SFAC prefers horizontal orientation with a high Θ // value of 84.5%, leading to a large F I G U R E 8 Curves of η ext and J and the corresponding triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) fitting curves of devices III−V η out of 34.9%. The non-doped and doped OLEDs employing SBF-BP-SFAC as emitters provide outstanding maximum η ext values of up to 23.1% and 30.6%, respectively. In addition, SBF-BP-SFAC can also act as an efficient sensitizer for orange fluorescence, phosphorescence, and TADF materials, providing excellent EL performance with η ext s of up to 30.3%. The bulky spiro donors can effectively suppress close molecular packing and alleviate exciton annihilation, and the fast FET rates (10 8 s −1 ) and high FET efficiencies (93.4%) from the SBF-BP-SFAC sensitizer to the guest emitters also efficiently decrease the density of triplet excitons. Both factors work synergistically to result in reduced efficiency roll-offs of the OLEDs. The superior comprehensive EL performance indicates that SBF-BP-SFAC has great application potential in OLEDs as an emitter and sensitizer.

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 (21788102) and the Natural Science Foundation of Guangdong Province (2019B030301003).

C O N F L I C T O F I N T E R E S T
The authors declare that there are no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
None.