Molecular design to regulate the photophysical properties of multifunctional TADF emitters towards high-performance TADF-based OLEDs with EQEs up to 22.4% and small efficiency roll-offs

The photophysical properties of four new quinoxaline derivatives featuring both AIE and TADF characteristics were controlled to give high EQEs.


Introduction
In the past few years, thermally activated delayed uorescence (TADF) emitters have attracted intensive interest in the eld of organic light-emitting diodes (OLEDs). [1][2][3][4][5] Owing to the effective triplet exciton up-conversion process, they can achieve 100% internal quantum efficiency (IQE). The efficiencies of TADFbased uorescent OLEDs have become comparable to those of OLEDs based on phosphorescent complexes. [6][7][8][9] However, similar to phosphorescent OLEDs, TADF-based OLEDs also suffer from triplet-exciton-involved annihilation processes, which hamper the efficiency improvement and also inict serious efficiency roll-off, especially for TADF-based orange/red OLEDs. Even though many efforts have been devoted, the current development of orange/red TADF emitters is far from satisfactory due to their low efficiency and serious efficiency roll-offs. As is known, it is inherently difficult for longwavelength TADF emitters to simultaneously achieve a high uorescence radiative rate (k r S ) and a small singlet-triplet energy gap (DE ST ). A restricted orbital overlap is in favor of a small DE ST , but it is not conducive to a high k r S . According to the energy-gap law, 10 the non-radiative internal conversion rate (k IC ) is signicantly enhanced with an increase of emission wavelength, while k r S is usually not high enough to overcome k IC which results in a low photoluminescence (PL) quantum efficiency (F PL ). [11][12][13] Therefore, the most critical factor for the development of efficient red/orange TADF emitters is to simultaneously achieve a small DE ST and a high uorescence radiative rate k r S , which still requires unremitting endeavors.
To overcome the above-mentioned dilemma, the design of emitters integrating both TADF and aggregation-induced emission (AIE) features may be a wise approach, which not only can utilize 100% IQE, but can also effectively relieve the exciton quenching, 14 especially at high brightness. Herein, we managed to respectively introduce 9,9-dimethyl-9,10dihydroacridine (DMAC) and 10H-phenoxazine (PXZ) as donor units into a quinoxaline-based acceptor. As demonstrated, DMAC and PXZ have large spatial conformations, 15,16 which are conducive to promote the separation of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) and thus achieve a small DE ST in the donor-acceptor (D-A) system. We anticipate that the different electron-donating capabilities of DMAC and PXZ along with the amount of donor unit regulate the degree of intramolecular charge transfer (ICT) and consequently obtain orange/red TADF emitters with improved F PL . We also anticipate that the possible TADF-AIE emitters enable highperformance OLEDs with high efficiencies and low efficiency roll-offs.

DFT calculations and electrochemical properties
In order to investigate the impacts of different electrondonating groups, the frontier molecular orbitals (FMOs) and energy levels of all the compounds were calculated using the B3LYP/6-31g(d) level. Their HOMOs and LUMOs are mainly distributed on the donor unit and the quinoxaline acceptor fraction, respectively. As is known, the value of DE ST is proportional to the exchange interaction integral between the HOMO and the LUMO wavefunction in a molecule. 19 As shown in Fig. 1, the four molecules display structures with large steric hindrance. For instance, the dihedral angles between the donor unit connected to the 6-position of quinoxaline and the central quinoxaline plane are 87 , 88 , 84 and 83 for SBDBQ-DMAC, DBQ-3DMAC, SBDBQ-PXZ and DBQ-3PXZ, respectively. The dihedral angles between the two benzene rings and the donor units are 88 /87 for DBQ-3DMAC, and 75 /77 for DBQ-3PXZ. 20 Consistent with the very twisted structures, the theoretical DE ST s are about 0.24 eV for SBDBQ-DMAC, 0.04 eV for DBQ-3DMAC, 0.07 eV for SBDBQ-PXZ and 0.04 eV for DBQ-3PXZ (Table S1, ESI †). It is worth noting that DBQ-3DMAC and DBQ-3PXZ based on multi-donor substitution show a smaller DE ST than their mono-donor substituted analogues.

Photophysical and AIE properties
The UV-vis absorption spectra, uorescence and phosphorescence spectra of the four compounds in lm are shown in Fig. S3 and S4. † Their strong absorption peaks at about 340 nm are attributed to the p-p* transition of the donor unit, and the weak absorption around the 400-550 nm range is due to the intramolecular charge transfer (ICT) process from the electron donor unit to the acceptor center. As the electron-donating ability increases, the spectra gradually exhibit bathochromic shi. In lm, the uorescence emission peaks of SBDBQ-DMAC, DBQ-3DMAC, SBDBQ-PXZ and DBQ-3PXZ are 541, 551, 594 and 618 nm, respectively, exhibiting a wide range of color tuning from green to yellow to orange to red. The phosphorescence spectra at 77 K of all the compounds are structureless, illustrating that the phosphorescence derives from the charge transfer (CT) state radiation transition of the triplet excitons. According to their uorescence and phosphorescence spectra, the DE ST s of SBDBQ-DMAC, DBQ-3DMAC, SBDBQ-PXZ and DBQ-3PXZ are 0.06, 0.06, 0.07, and 0.03 eV, respectively, which are basically consistent with the results of the theoretical calculations. The thermal, photophysical and electrochemical data related to the compounds are summarized in Table S2, (ESI †). To probe the possible AIE phenomenon, we measured the PL spectra of these compounds in THF/water with various water fractions from 0 to 99%. As shown in Fig. 2, the PL intensities are abruptly enhanced as the water ratio reaches 90%, indicating a prominent AIE feature.

TADF characterization
To verify their TADF characteristics, we tested the transient PL spectra in 10 À5 M toluene solution. As exemplied in Fig. 3a-d, when oxygen is present, each of the four compounds only shows a prompt uorescence emission. 21 Aer bubbling with argon in toluene, all the compounds distinctly display prompt and delayed components with lifetimes of 23 ns/25.7 ms for SBDBQ-DMAC, 28 ns/33 ms for DBQ-3DMAC, 20 ns/2.86 ms for SBDBQ-PXZ and 8.7 ns/1.38 ms for DBQ-3PXZ. Obviously, the DMACbased compounds exhibit much longer delayed uorescence lifetimes than the PXZ-based compounds. In the neat lm, their transient photoluminescence (PL) curves also exhibit doubleexponential decay. Moreover, the delayed components are gradually intensied with the increase of ambient temperature from 100 to 300 K (Fig. 4), demonstrating the typical thermally activated nature. 1,22 To further evaluate the TADF performances of the four compounds as emitters in the host-guest system, we also investigated the transient PL decays of the 10% emitters doped into CBP (4,4 0 -N,N 0 -dicarbazole-biphenyl), which consist of a nanosecond-scale prompt component and a microsecondscale delayed component (Fig. S5 †). Similar to the case in toluene, the delayed uorescence lifetimes of the DMAC-based emitters are much longer than those of the PXZ-based emitters in the doped lm. The F PL s of SBDBQ-DMAC, DBQ-3DMAC, SBDBQ-PXZ and DBQ-3PXZ in the doped lms are 74%, 84%,   73% and 76%, respectively. As shown in Table 1, the radiative decay rate constants (k r S s) from the S 1 to S 0 transition and the rate constants (k RISC s) for RISC between the S 1 and T 1 states can be reasonably estimated as 2.1 Â 10 7 /1.3 Â 10 5 s À1 (SBDBQ-DMAC), 2.0 Â 10 7 /1.9 Â 10 5 s À1 (DBQ-3DMAC), 9.7 Â 10 6 /8.4 Â 10 5 s À1 (SBDBQ-PXZ) and 1.1 Â 10 7 /1.2 Â 10 6 s À1 (DBQ-3PXZ). 23 These above-mentioned experimental results reveal that DBQ-3DMAC possesses the highest F PL and DBQ-3PXZ features the maximum k RISC , which suggest that they could achieve better electroluminescent performance.

Device characterization
The satisfactory F PL s and rate constants inspired us to investigate the potential applications in doped OLEDs (device A for  a The total and delayed uorescence quantum yield, respectively. b The rate constant for prompt and delayed uorescence, respectively. c k r S represents the radiative decay rate constant from S 1 to S 0 transition. d The rate constants for intersystem crossing (ISC) and reverse intersystem crossing (RISC) between the S 1 and T 1 states, respectively. light emission peaking at 532 and 536 nm, respectively. Meanwhile, the devices with SBDBQ-PXZ (device C) and DBQ-3PXZ (device D) show orange light emission with identical peaks at 572 nm (Fig. S6 †). For the green TADF devices, the device A based on DBQ-DMAC achieves an EQE max , a CE max and a PE max of 13.0%, 45.0 cd A À1 and 39.9 lm W À1 , respectively. The device B based on DBQ-3DMAC exhibits the highest EL performance with an EQE max of 22.4%, a CE max of 80.3 cd A À1 and a PE max of 64.1 lm W À1 without any light out-coupling technique, which is signicantly superior to device A, owing to the relatively high F PL (84%) and k RISC (1.9 Â 10 5 s À1 ). It is worth noting that both devices A and B display comparatively low efficiency roll-offs (Table 2). 24 Furthermore, the orange devices C and D based on SBDBQ-PXZ and DBQ-3PXZ not only exhibit considerably high efficiencies, but also achieve ultralow efficiency roll-offs compared to any other reported orange TADF devices. As shown in Fig. 5 and Table 2, the device C achieves an EQE max of 11.1%, a CE max of 29.1 cd A À1 and a PE max of 23.4 lm W À1 , and the efficiencies maintain at 11.0%/10.1%, 28.8/26.6 cd A À1 and 20.8/12.9 lm W À1 , at a brightness of 100 and 1000 cd m À2 , respectively. This corresponds to the ultra-low efficiency rolloffs of 0.9% and 9.0%, respectively. Comparatively, the DBQ-3PXZ-based device D obtains higher efficiencies with an EQE max of 14.1%, a CE max of 36.1 cd A À1 and a PE max of 28.1 lm W À1 . Inspiringly, when the brightness is 100 and 1000 cd m À2 , the EQE, CE and PE are still as high as 13.9%/11.1%, 35.3/28.4 cd A À1 and 22.9/12.4 lm W À1 with efficiency roll-offs of only 1.4% and 21.3%, respectively. The AIE nature and TADF property endows these target compounds with excellent solid-state PL efficiency and a high utilization of excitons under electrical excitation, meanwhile the relatively low efficiency roll-off probably results from the favourable charge injection, transport, and recombination ability at the high brightness. To the best of our knowledge, the excellent device performance and extremely low efficiency roll-offs are among the highest values for TADF-based OLEDs ever reported, especially for orange TADF devices (Table S3, ESI †). In terms of the relationship between the molecular structures and device performance, we can draw conclusions as below: (i) as predicted by theory, the multi-substituted quinoxaline derivatives, namely DBQ-3DMAC  and DBQ-3PXZ, display better device efficiencies, which are related to their own F PL , k r S and k RISC ; (ii) the efficiency roll-offs of the devices with PXZ as the donor unit are smaller than those of the device with DMAC as the donor unit, attributed to the shorter lifetime, which is benecial to suppress triplet-excitoninvolved quenching, such as singlet-triplet annihilation (STA) and triplet-triplet annihilation (TTA) processes, and in a certain extent to alleviate the efficiency roll-offs. 25 In addition, the unique TADF-AIE features enlightened us to further explore non-doped devices, and a common structure was fabricated with the congurations of ITO/MoO 3 (10 nm)/ TAPC (50 nm)/mCP (10 nm)/TADF emitter (20 nm)/Bphen (45 nm)/LiF/Al (SBDBQ-DMAC for device E, DBQ-3DMAC for device F, SBDBQ-PXZ for device G and DBQ-3PXZ for device H). As shown in Fig. S7, † both devices E and F display yellow emission peaking at 544 and 548 nm, respectively. In contrast, devices G and H exhibit orange (peaking at 608 nm) and red (peaking at 616 nm) emission, respectively, which are consistent with their PL spectra in the neat lms. Eminently, the best results are obtained by device F based on the multi-substituted DBQ-3DMAC, featuring an EQE max of 12.0%, a CE max of 41.2 cd A À1 and a PE max of 45.4 lm W À1 . Besides, device E based on the mono-substituted SBDBQ-DMAC achieves high performance with an EQE max of 10.1%, a CE max of 35.4 cd A À1 and a PE max of 32.7 lm W À1 (Fig. 6 and Table 3). To the best of our knowledge, these efficiencies are among the highest reported for non-doped yellow OLEDs (Table S4, ESI †). As expected, the long-wavelength emissive devices G and H achieve an EQE max of 5.6% and 5.3%, respectively, which all exceed the theoretical limit of 5% for traditional uorescent emitters. Most importantly, all the nondoped devices exhibit low efficiency roll-offs, which may be attributed to their unique TADF-AIE nature.

Conclusion
In conclusion, we developed a series of new asymmetric quinoxaline derivatives with donor units of DMAC and PXZ.

Conflicts of interest
The authors declare no competing nancial interest. a At a luminance of 1 cd m À2 . b The maximum value and the corresponding values at a brightness of 100 and 1000 cd m À2 , respectively.