A novel design strategy for deeper blue and more stable thermally activated delayed fluorescent emitters

Journal Pre-proof A novel design strategy for deeper blue and more stable thermally activated delayed fluorescent emitters Hua Sun, Xiao Tan, Shenglong Sang, Qian Liu, Po Sun, Jing Zhang, Xiao-Chun Hang, Fei Chen, Zhi-Kuan Chen PII:

S1566-1199(19)30637-8

DOI:

https://doi.org/10.1016/j.orgel.2019.105610

Reference:

ORGELE 105610

To appear in:

Organic Electronics

Received Date: 18 October 2019 Revised Date:

24 December 2019

Accepted Date: 27 December 2019

Please cite this article as: H. Sun, X. Tan, S. Sang, Q. Liu, P. Sun, J. Zhang, X.-C. Hang, F. Chen, Z.-K. Chen, A novel design strategy for deeper blue and more stable thermally activated delayed fluorescent emitters, Organic Electronics (2020), doi: https://doi.org/10.1016/j.orgel.2019.105610. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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A Novel Design Strategy for Deeper Blue and More Stable Thermally Activated Delayed Fluorescent Emitters Hua Sun,2‡ Xiao Tan,3‡ Shenglong Sang,3 Qian Liu,3 Po Sun,4 Jing Zhang,2 Xiao-Chun Hang,3 Fei Chen,1 Zhi-Kuan Chen1 H.S and X.T contributed equally to this work. 1

Department of Mechanical, Materials and Manufacturing Engineering and Department of

Chemical Engineering, The University of Nottingham, Ningbo China, 199 Taikang East Road, Ningbo 315100, P. R. China 2

Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced

Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, P. R. China 3

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, P. R. China 4

Analysis and Testing Central Facility, Engineering Research Institute, Anhui University of

Technology, Maanshan 243002, P. R. China

ABSTRACT Thermally activated delayed fluorescent (TADF) materials are considered as the competitive alternative to traditional fluorescent and noble-metal-based phosphorescent materials due to their high electroluminescence (EL) efficiency and low cost. Solution-processable TADF OLEDs have drawn great research attention and can be potentially applied in both flat-panel displays and lighting sources in the near future. However, the blue light emitting TADF materials with high luminance, satisfying color purity and excellent stability are still rare. Herein, two blue light emitting TADF molecules (CzAC-TRZ, DFAC-TRZ) are designed and synthesized. Due to the modified acridine units by introducing 9-phenylcarbazole and dibenzofuran as σ-groups into the

1

molecular skeletons, CzAC-TRZ, DFAC-TRZ possess high energy states and rigid molecular configuration. Solution-processed OLEDs based on CzAC-TRZ and DFAC-TRZ achieved high EL efficiency with the external quantum efficiency (EQE) up to 11.8% and 11.5%, respectively. Moreover, compared with the analogue emitter (DMAC-TRZ) fabricated control devices, CzACTRZ and DFAC-TRZ OLEDs presented lower roll-offs, blue-shift and narrower EL spectra. These results have shown that introducing high energy state building blocks into the donor units via σ bonds offers a promising approach of developing stable deeper blue TADF emitters. Key words: blue-light emission, OLEDs, TADF, σ-link, roll-off

1. INTRODUCTION Benefiting from the unprecedented visual experience of the flat panel display composed of organic light-emitting diodes (OLEDs), i.e., high contrast ratio, fast response time and wide viewing angle, various OLEDs based high-end flat panel display products such as smart phones, large-size HD TV and smart watches have been widely developed.1-3 Red, green and blue light emitting OLEDs have all undergone significant advances and have being successfully employed in displays and lighting. Presently, red and green phosphorescent emitters in conjunction with blue fluorescent emitters are the optimal solution for constructing commercial flat panel displays, due to their high efficiency and long operation lifetime.4-6 Suffering from the ineffective utilization of triplet excitons restricted by the spin-forbidden principle, the conventional fluorescent OLEDs fundamentally exhibit limited internal quantum efficiency (IQE). Thus, investigation of the materials and their mechanism which can convert spin-forbidden triplet excitons to singlets is very attractive and demanding to achieve more emitting photons.7,8 Fortunately, in some fluorescent emitting materials, triplet excitons can be converted to singlet excitons through several physical processes, such as triplet-triplet annihilation (TTA),9,10 hybridized local and intramolecular charge-transfer (HLCT)11,12 or thermally activated delayed fluorescence (TADF).13-15 Great research efforts have been devoted to develop high efficiency blue electroluminescence fluorescent molecules using TTA, HLCT or TADF.16-22 Among of them, TADF, which can utilize both triplet and singlet excitons via reverse

2

intersystem crossing (RISC) has been the current research focus because of its potential of 100% triplet excitons utilization.23-25 Although the TADF mechanisms have been rationalized for a long time,26-29 pure organic TADF emitters for high efficiency OLEDs was reported till 2011. With an ingenious molecular design principle to achieve a very small ∆EST between the lowest singlet excited state (S1) and the lowest triplet excited state (T1) thus enhanced T1→S1 RISC, Adachi and co-workers synthesized a series of small aromatic molecular light emitters which exhibit efficient TADF with high photoluminescence efficiency.29 The OLEDs based on these emitters attained incredible electroluminescence (EL) efficiency. Their work provides a fire-new design principle for organic electroluminescent materials, especially for blue light emitting materials which normally suffer from low IQE. As stated above, to realize highly efficient RISC up-conversion process, the energy gaps (∆EST) between S1 and T1 of TADF molecules must be quite small to achieve high rate of RISC thus triplet-to-singlet conversion efficiency.30 Therefore, in order to spatially separate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the TADF molecules, thus to decrease the HOMO and LUMO overlap and finally achieve small ∆EST, donor (D)-acceptor (A) molecular structures incorporated with high intramolecular steric repulsions are employed in most cases to design TADF molecules.31,32 Additionally, D-A type TADF molecules show intramolecular charge transfer (ICT) characteristics which normally lowers the energy states. Hence, for constructing blue light-emitting TADF molecules, limited conjugation lengths incorporating with certain high energy state building blocks are often adopted.24 By far, developing blue TADF emitters with satisfying color purity and excellent stability is still a formidable research challenge. To break the conjugation of the organic semiconductors is a practical way to acquire ultrawide energy bandgap. For the building blocks which contain sp3-hybridized carbon atom in the skeletons, connecting other moieties to the sp3hybridized carbon atom can block the extended π-conjugation and finally achieve improved energy states and thermo-stability. According to the σ-link strategy described above, many host materials with high energy states and excellent stability have been reported because of the weakened π-conjugation as well as twisted and rigid molecular conformations.33-35 However, this design strategy has not been widely recognized in developing TADF materials.36, 37

3

In this work, two blue-emitting TADF molecules (CzAC-TRZ, DFAC-TRZ) based on two donor building blocks of 9-phenyl-9-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine, 9(dibenzo[b,d]furan-4-yl)-9-phenyl-9,10-dihydroacridine

coupled

with

2,4,6-triphenyl-1,3,5-

triazine (TRZ) were designed and synthesized. Two kinds of common high energy building blocks, dibenzofuran and 9-phenylcarbazole were introduced to the molecular skeletons as the σgroups to achieve high energy states of the molecules. CzAC-TRZ and DFAC-TRZ exhibited deeper blue photoluminescence, higher glass transition and decomposition temperatures compared with 9,9-dimethyl-9,10-dihydroacridine and TRZ based TADF molecule (DMACTRZ), which was widely studied in both blue and white OLEDs.38-39 Solution processing, which has a significant potential to rapid and low-cost fabrication in large area for organic electronics,40,41 was adopted to evaluate the electroluminescence (EL) performance of the two newly developed emitters. Solution-processed OLEDs based on CzAC-TRZ and DFAC-TRZ achieved decent device performance with the external quantum efficiencies (EQEs) of 11.8% and 11.5%, respectively. More importantly, CzAC-TRZ and DFAC-TRZ based OLEDs presented distinct blue-shifted EL spectra and lower efficiency roll-off in comparison with DMAC-TRZ based OLEDs. These impressive results verified that σ-link strategy with high energy building blocks provides a new means of developing deeper blue and more stable TADF emitters. 2. RESULTS AND DISCUSSION The synthetic routes to CzAC-TRZ and DFAC-TRZ are shown in Scheme 1. The details of the synthesis procedures and characterizations are provided in Supporting Information (SI). CzAC-TRZ and DFAC-TRZ were fully characterized through 1H and

13

C NMR spectroscopy

(Figures S7-S10). The chemical structures of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ are shown in Figure 1a. To gain an insight into the influence of the σ-link strategy on the performance of the TADF molecules, density functional theory (DFT) calculations were performed on the three compounds. As shown in Figure 1b, all of the three compounds exhibited twisted configurations because of the large steric repulsions between acridine fragments and TRZ. The dihedral angles between acridine and the phenyl spacer were calculated to be 90o, 83o and 87o for DMAC-TRZ, CzAC-TRZ and DFAC-TRZ. Due to the twisty configurations, the conjugation of the three compounds are weakened which leads to the

4

separated LUMO and HOMO orbitals distributions. Apparently, the LUMOs are mainly distributed over the TRZs, while the HOMOs are predominantly localized on the acridine units. Benefiting from the large separation of the LUMO and HOMO, the calculated ∆EST are quite small, with the values of 7 meV for DMAC-TRZ, 10 meV for CzAC-TRZ and 13 meV for DFAC-TRZ, respectively. Hence, TADF light emission resulted from effective RISC are expected to be observed in these three compounds. In comparison with DMAC-TRZ, CzACTRZ and DFAC-TRZ possess increased S1 and T1 and more rigid molecular geometries. To fully observe the increased steric hindrance introduced by 9-phenylcarbazole and dibenzo[b,d]furan, the conformations and relative distributions at different twisted angles between D and A segments were also calculated according to the reported method (Figure S1).42,43 In contrast with the planar acridine unit in DMAC-TRZ, the acridine fragments in CzAC-TRZ and DFAC-TRZ changed into twisted and rigid three dimensional (3D) molecular configurations with the sp3 C as the center. Hence, combining with the higher energy states and more rigid molecular structures as well as larger molecular weights, CzAC-TRZ and DFAC-TRZ are expected to achieve deeper blue light emission and better stability compared with DMAC-TRZ. The molecular structures of CzAC-TRZ and DFAC-TRZ were further confirmed by singlecrystal X-ray diffraction analysis. As depicted in Figure 2a, both CzAC-TRZ and DFAC-TRZ exhibited similar molecular geometries in crystalline state compared with the calculation results. The dihedral angles between the donor and acceptor fragments were determined to be 87.94o and 89.03o. The acridine rings present a bent conformation because of the condensed molecular structures consisting of carbazole, dibenzo[b,d]furan and benzene moieties. The donor units of CzAC-TRZ and DFAC-TRZ present rigid and twisted conformations centering at the sp3 hybridization C with the C-C bond angles of ~110o. The absorption and emission spectra of CzAC-TRZ and DFAC-TRZ are shown in Figure S2 and Figure 2b, the spectral parameters are summarized in Table 1. Both CzAC-TRZ and DFAC-TRZ exhibited comparable absorption profiles in solutions and in films, due to the weak intermolecular interactions resulted from the twisted molecular structures. They showed strong absorption from 250 nm to 300 nm, which is derived from the π-π* transition. Due to the ICT transition between acridine derivatives and TRZ moieties, they also showed weak absorption bands from 350 nm to 450 nm. The photoluminescent (PL) spectra of CzAC-TRZ and DFAC-TRZ were characterized at 77 K and room temperature (RT). In solutions, the emission peaks of the fluorescent spectra were 455 nm

5

and 459 nm at 77 K for CzAC-TRZ and DFAC-TRZ, respectively (Figure S2). In contrast with DMAC-TRZ, both CzAC-TRZ and DFAC-TRZ exhibited notable blue-shift fluorescent spectra at RT, with the emission peaks at 475 nm and 462 nm, respectively. Interestingly, for CzACTRZ, besides of the emission peak at 475 nm, a shoulder emission peak at around 413 nm was also observed. This shoulder fluorescence peak can be ascribed to the emission from different molecular conformations which have been found in other acridine and TRZ based TADF emitters.44 The deeper blue emission properties were consistent with the simulation results. Combining the DFT calculations with the single-crystal structure studies, we summarize the following causes that would result in the blue-shift emission. Firstly, the broadened energy gaps and elevated energy states when 9-phenylcarbazole and dibenzo[b,d]furan served as the σgroups.45 For DMAC-TRZ, the methyl groups on DMAC contribute to the HOMO states of DMAC-TRZ because of their hyperconjugation and electron donating property (Figure 1b). However, the σ-bond combining with the big steric hindrance induced by 9-Phenylcarbazole and dibenzo[b,d]furan weaken the aromaticity of CzAC and DFAC in comparison with DMAC. As shown in Figure 1b, the HOMOs are predominantly localized on the acridine units of CzACTRZ and DFAC-TRZ. Hence, introducing 9-phenylcarbazole and dibenzo[b,d]furan to the skeletons via σ-bond induced broadened energy gaps and elevated energy states. Secondly, the increased steric hindrance introduced by 9-phenylcarbazole and dibenzo[b,d]furan resulted in bent acridine conformations. Finally, the conjugation of the molecules was weakened, thus the energy states were elevated. The phosphorescent spectra were also collected at 77 K in toluene, the emission peaks were at 462 nm and 466 nm. The ∆EST of CzAC-TRZ and DFAC-TRZ were determined to be 23 meV and 19 meV, derived from the differences of the onsets in the fluorescent and phosphorescent spectra. The small ∆EST indicates effective RISC, which finally induce enhanced EQE of the OLEDs. Besides of the improved blue emission, CzAC-TRZ and DFAC-TRZ also showed enhanced thermo-stability with the higher decomposition temperatures (Td, 5% weight loss) of 451 oC and 414 oC, and higher glass transition temperatures (Tg) of 173 o

C and 164 oC respectively, relative to 351 oC (Td) and 90 oC (Tg) of DMAC-TRZ (see Figure

2c, d, Figure S3a). The LUMO and HOMO energy levels of CzAC-TRZ and DFAC-TRZ were determined using cyclic voltammetry (CV) measurement (Figure S3b), from which the LUMO and HOMO energy levels were estimated to be -2.74 eV, -5.34 eV and -2.73 eV, -5.36 eV for CzAC-TRZ and DFAC-TRZ respectively, according to the onset potentials of their reduction and

6

oxidation curves. The bandgaps were deduced to be 2.60 eV and 2.63 eV for CzAC-TRZ and DFAC-TRZ, which were slightly higher than that of DMAC-TRZ (2.55 eV). The value of energy bandgaps coincided with the simulation results. The quasi-reversible oxidation/reduction potentials of CzAC-TRZ and DFAC-TRZ in dichloromethane indicated good electrochemical stability of both the donor and acceptor moieties.38 At room temperature, both CzAC-TRZ and DFAC-TRZ in 2,6-DCzPPy films (20 wt%) exhibited excellent photoluminescence properties with the photoluminescence quantum yield (PLQY) up to 99% and 90%, respectively (Table S1). The TADF properties of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ were investigated by measuring the transient PL decay spectra. As shown in Figure 3a, b, the doped films of DMACTRZ, CzAC-TRZ and DFAC-TRZ exhibited both prompt and delayed emissions. The prompt emissions correspond to the fluorescence from S1 to the ground state (S0), while the delayed emissions are generated from the up-conversion excitons formed by RISC. The prompt fluorescence lifetimes of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ were calculated to be 21.3 ns, 16.7 ns and 14.7 ns, while the delayed fluorescence lifetimes were calculated to be 4.71 µs, 6.12 µs and 5.02 µs, respectively. The lifetimes, PLQYs and estimated rate constants are summarized in Table S1. The TADF properties of the emitters can be further confirmed by the transient PL decay spectra of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ in solution at 77 K and room temperature (Figure S4). The typical TADF characteristics of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ are in line with their small ∆EST very well. In order to evaluate the EL performance of the TADF molecules, solution-processed OLEDs with

a

device

architecture

of

ITO/PEDOT:PSS/PVK/2,6-

DCzPPy:TAPC:Emitters/TmPyPB/LiF/Al were fabricated. The schematic device structure and the molecular structures used in the devices are illustrated in Figure 4a. The details on the device fabrication and characterization are provided in SI. The atomic force microscopy (AFM) were adopted to characterize the morphology of the emission layers (EML). As shown in Figure S5, all of the 2,6-DCzPPy:TAPC:Emitters composites exhibited smooth and homogeneous films with the root-mean-square (RMS) surface roughness of 0.74 nm, 1.11 nm and 0.75 nm for DMAC-TRZ, CzAC-TRZ and DFAC-TRZ, respectively. The EL properties of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ-based OLEDs are shown in Figure 4b, c, d and Figure S6. The

7

characteristics of the devices are summarized in Table 2. In comparison with DMAC-TRZ, both CzAC-TRZ and DFAC-TRZ based OLEDs exhibited blue-shift and narrower EL spectra with the emission peaks at 488 nm and 485 nm, respectively. The deeper blue EL emissions agreed well with the PL emissions discussed above. Whereas, an obvious red shift can be observed in the EL spectra compared with the PL spectra, which is a common phenomenon in solutionprocessed OLEDs, and usually induced by the aggregation effect in spin-coating process.39 The optimal OLEDs based on CzAC-TRZ and DFAC-TRZ achieved high EL efficiencies with the maximum EQEs of 11.5% and 11.8%, respectively, which are comparable with that of the DMAC-TRZ based OLEDs. The maximum current and power efficiency for CzAC-TRZ and DFAC-TRZ based OLEDs are 32.0 cd A-1, 33.0 cd A-1 and 12.5 lm W-1, 14.8 lm W-1, respectively. More importantly, CzAC-TRZ and DFAC-TRZ based OLEDs showed reduced efficiency roll-off at high brightness compared with the DMAC-TRZ OLEDs. The EQE roll-offs of the CzAC-TRZ and DFAC-TRZ based OLEDs were calculated to be 11.3% and 14.4% at 1000 cd A-1, 33.0% and 19.5% at 5000 cd A-1, and 47.8% and 26.3% at 10000 cd A-1, respectively, while the roll-offs were estimated to be 21.9%, 39.0% and 51.6% at the same luminance for DMAC-TRZ based OLEDs. The following reasons could be considered to attribute to the lower roll-offs of CzAC-TRZ and DFAC-TRZ OLEDs. Firstly, the rigid molecular conformations play a role to suppress excited state geometry deformation, which could contribute to both suppressed Stokes shift and non-radiative decay process.44 Hence, compared with DMAC-TRZ, the greater steric hindrance of CzAC-TRZ and DFAC-TRZ derived from the introduction of 9-phenylcarbazole and dibenzofuran units suppress the accumulation of molecules, thus reduced TTA formation.46 Secondly, the small ∆EST of CzAC-TRZ and DFACTRZ can effectively enhance the RISC and suppress the TTA process. Thirdly, the excellent thermo-stability of CzAC-TRZ and DFAC-TRZ ensured stable film morphology for OLEDs at high current densities. In addition, DFAC-TRZ exhibited more rigid molecular structure, as the benzene ring in the carbazole unit of CzAC-TRZ remains rotational freedom, thus DFAC-TRZ OLEDs showed impressive reduced roll-off in contrast with CzAC-TRZ OLEDs. In our experiments, CzAC-TRZ and DFAC-TRZ based OLEDs also achieved higher maximum brightness (Bmax) than that of DMAC-TRZ based OLEDs, which were 53226 cd m-2 and 52314 cd m-2, respectively. 3. CONCLUSION

8

In summary, we have demonstrated a simple molecular design strategy for stable deeper blue TADF emitters by introducing high energy groups into the molecular skeletons via σ-bond. On account of this strategy, two novel blue TADF emitters CzAC-TRZ and DFAC-TRZ based on σgroup modified acridine derivatives and TRZ moieties were successfully synthesized and characterized. The σ-link strategy plays a critical role to tune the blue emission property and improve the thermo-stability of the emitters. The pronounced deeper blue emission spectra were observed in PL measurement with ~ 20 nm and ~33 nm blue-shifts for CzAC-TRZ and DFACTRZ, respectively, compared to the control compound DMAC-TRZ. More than that, the Td and Tg were also improved up to 100 oC and 83 oC for CzAC-TRZ, 63 oC and 74 oC for DFAC-TRZ, respectively. Solution-processed OLEDs based on CzAC-TRZ and DFAC-TRZ demonstrated good EL performance with the EQEs achieved at 11.5% and 11.8%. More importantly, OLEDs based on CzAC-TRZ and DFAC-TRZ exhibited reduced roll-offs in comparison with that of DMAC-TRZ-based OLEDs. Especially for the DFAC-TRZ OLEDs, the EQE roll-off was quite low, which was 14.4%, 19.5%, 26.3% at 1000 cd m-2, 5000 cd m-2, and 10000 cd m-2, respectively. All of the results suggested that the σ-link strategy could be a promising approach to develop deeper blue TADF emitters.

9

Scheme 1. Synthetic routes to CzAC-TRZ and DFAC-TRZ.

Figure 1. (a) Chemical structures of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ. (b) Optimized molecular geometries, frontier molecular orbitals and calculated energy levels of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ.

10

Figure 2. (a) The single-crystal structures of CzAC-TRZ and DFAC-TRZ. (b) Normalized fluorescence spectra of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ in toluene solution (2×10-5 M) at RT. (c) TGA and (d) DSC curves of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ (heating rate: 10 oC min-1, N2 atmosphere).

Figure 3. (a) and (b) Transient PL decay spectra of the TADF emitters doped in 2,6-DCzPPy films (20 wt%) at RT.

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Figure 4. (a) Schematic device structure of the solution-processed OLEDs and the corresponding chemical structures of the materials used in different layers; (b) The EL spectra; (c) Currentvoltage-luminance (J-V-L) characteristics; (d) EQE and current efficiency versus luminance of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ based OLEDs. Table 1. Physical Properties of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ. λmax, abs.

λmax, fl.

λmax, phos.

Tg

[nm]a)

[nm]b)

[nm]c)

390#

464f), 495g)

498

90

CzAC-TRZ

270, 387 455f), 475g)

462

DFAC-TRZ

277, 380 459f), 462g)

466

Td

LUMO HOMO

∆EST

Compound

DMAC-TRZ

a)

[oC] [oC]d)

[eV]e)

[eV]e)

[meV]

351

-2.77

-5.32

45

173

451

-2.74

-5.34

23

164

414

-2.73

-5.36

19

Obtained from the absorption spectra in films; b)Measured in toluene solution (2×10-5 M);

Measured in toluene solution (2×10-5 M) at 77 K; e)

d)

c)

The temperature at 5% weight loss;

ox f) Measured by cyclic voltammetry using LUMO/HOMO=−[Ered At 77 onset/Eonset−EFc/Fc+ + 4.8] eV;

K; g)At RT; #)Data obtained from previous report.38

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Table 2. EL performance of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ. Peak

FWHM

Vona)

CEmax

EQEmax

PEmax

CE1000/5000/10000b)

EQE1000/5000/10000b)

EQE roll-offc)

[nm]

[nm]

[V]

[cdA-1]

[%]

[lmW-1]

[cdA-1]

[%]

[%]

DMAC-TRZ

507

91

5.0

36.4

12.8

15.2

28.5/21.8/17.3

10.0/7.8/6.2

21.9/39.0/51.6

CzAC-TRZ

488

86

4.5

32.0

11.5

12.5

28.3/21.1/16.4

10.2/7.7/6.0

11.3/33.0/47.8

DFAC-TRZ

485

81

4.0

33.0

11.8

14.8

28.2/26.5/24.2

10.1/9.5/8.7

14.4/19.5/26.3

Emitters

a)

At the luminance of 1 cd m-2; b)At 1000, 5000, and 10000 cd m−2; c)The efficiency roll-off from

the peak value to the brightness of 1000, 5000, and 10000 cd m−2, respectively.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, UV-visible absorption spectra, PL spectra, DSC curves, CV curves, power efficiency versus luminance of DMAC-TRZ, CzAC-TRZ and DFAC-TRZ based OLEDs and 1H and 13C NMR spectra. The crystallographic data of CzAC-TRZ and DFAC-TRZ can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures/ with CCDC of 1949644 and 1949641.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions ‡

H.S and X.T contributed equally to this work.

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ACKNOWLEDGMENT The authors acknowledge financial support from National Natural Science Foundation of China (Nos. 21875106, 61605075). We are grateful to the High Performance Computing Center at Nanjing Tech University for providing the computational resources.

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Research Highlights: 1. σ-link strategy was employed to construct two blue TADF emitters: CzAC-TRZ and DFAC-TRZ; 2. In contrast to the control compound DMAC-TRZ, CzAC-TRZ and DFAC-TRZ possess blue-shift photoluminescence properties and improved thermal stability; 3. Solution-processed OLEDs based on CzAC-TRZ and DFAC-TRZ demonstrated deeper blue electroluminescence with improved color purity and reduced efficiency roll-offs.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: