Efficient Orange-Red Thermally Activated Delayed Fluorescence

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Efficient Orange-Red Thermally Activated Delayed Fluorescence Emitters Feasible for Both Thermal Evaporation and Solution Process Jia-Xiong Chen, Wen-Wen Tao, Ya-Fang Xiao, Kai Wang, Ming Zhang, Xiao-Chun Fan, WenCheng Chen, Jia Yu, Shengliang Li, Feng-Xia Geng, Xiaohong Zhang, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08729 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Efficient Orange-Red Thermally Activated Delayed Fluorescence Emitters Feasible for Both Thermal Evaporation and Solution Process

Jia-Xiong Chen,†,‡,§ Wen-Wen Tao,† Ya-Fang Xiao,‡ Kai Wang,*,† Ming Zhang,† Xiao-Chun Fan,† Wen-Cheng Chen,*,‡ Jia Yu,† Shengliang Li,‡ Feng-Xia Geng,§ Xiao-Hong Zhang,*,† and Chun-Sing Lee*,‡

†Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory

for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P.R. China ‡Center

of Super-Diamond and Advanced Films (COSDAF) and Department of

Chemistry, City University of Hong Kong, Hong Kong SAR, P.R. China §College

of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou 215123, P.R. China

KEYWORDS: thermally activated delayed fluorescence, orange-red OLED, solution process, thermal evaporation, rigid segment

ABSTRACT: Development of red thermally activated delayed fluorescence (TADF) emitters has been falling behind comparing with those of blue and green fluorophores, especially for solution-processable ones. In this work, two novel orange-red TADF emitters 3,6-di(10H-phenoxazin-10-yl)dibenzo[a,c]phenazine (DBPZ-DPXZ) and

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10,10'-(11,12-bis(3,5-di-tert-butylphenyl)dibenzo[a,c]phenazine-3,6-diyl)bis(10Hphenoxazine) (tDBBPZ-DPXZ) are developed. A high-performance orange-red TADF emitter, DBPZ-DPXZ, is first prepared by connecting a rigid acceptor and two rigid donor segments. While this design strategy endows DBPZ-DPXZ with excellent TADF performance leading to a vacuum-processed organic light-emitting diode (OLED) with a high external quantum efficiency (EQE) of 17.8%, the rigid segments limit its solubility and applications in solution-processed devices. Based on this prototype, tDBBPZ-DPXZ is designed with the addition of 3,5-di-tert-butylphenyl groups to boost its solubility with barely influence on the photophysical properties. In particular, tDBBPZ-DPXZ maintains nearly identical photoluminescence quantum yield of 83% and singlet-triplet energy splitting of 0.03 eV with EQE of 17.0% in a vacuumprocessed orange-red OLED. Furthermore, it can be applied on the orange-red solutionprocessed OLED realizing an EQE as high as 10.1%, representing one of the state-ofthe-art results of the reported orange-red solution-processed TADF-OLEDs. This work provides an effective strategy to address the conflicting requirements between high efficiency and good solubility and develop efficient soluble orange-red TADF emitters.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) mechanism have drawn much attention for their potential in achieving 100% internal quantum efficiency (IQE) with only low-cost pure organic emitters.1–6 Up till now, most of the TADF OLEDs are fabricated via thermal evaporation.7–11 As the importance and advantages of solution processing in organic optoelectronic devices are increasingly recognized, much recent efforts have been


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devoted to development of solution-processable high-performance TADF emitters.12–15 Solubility of molecules are typically enhanced by adding flexible moieties, such as alkyl groups etc., to the main molecules.16,17 Considerable success has been achieved by Kaji et al. and Lee et al. with this approach for developing solution-processable blue and green emitters.18,19 Unfortunately, red-emitting TADF emitters developed using this approach show much lower efficiencies comparing to those of the blue and the green TADF counterparts.20 The most important reason is that red TADF emitters typically exhibit serious nonradiative process governed by the energy gap law,21–23 which is sensitive to flexible segments in the molecules. In fact, this is affecting both evaporation- and solutionprocessed red TADF emitters. Thus, red TADF emitters generally suffer from the dilemma between realizing efficient reverse intersystem crossing (RISC) and fluorescence processes.24 Yang et al. and our group have recently addressed this issue by introducing rigid and aromatic frameworks to suppress the non-radiative channel and achieved ~20% external quantum efficiencies (EQEs) in vacuum-processed red OLEDs.24,25 However, due to the rigid structures in these high-performance red TADF emitters, they have very limited solubility. So far, there are few reports on solutionprocessible red TADF emitters in literature,20,26-28 and the highest reported EQE in solution-processed device is less than half of those in vacuum-processed devices.26 It is thus highly desirable to develop a new approach for designing solution-processable red TADF emitters with performances match to their blue and green counterparts. In this work, we address this issue by employing rigid molecule as main components for suppressing non-radiative processes and strategically introducing flexible groups with minimal influences on the photophysical properties to improve the solubility.

A

novel

orange-red

TADF

emitter

3,6-di(10H-phenoxazin-10-

yl)dibenzo[a,c]phenazine (DBPZ-DPXZ) with rigid structure is firstly developed (Scheme 1), which realizes an extremely small singlet-triplet energy splitting (ΔEST) of 0.03 eV and high photoluminescence quantum yield (ΦPL) of 84%, simultaneously. Evaporation-processed OLED based on DBPZ-DPXZ achieves orange-red emission with high EQE of 17.8%. However, due to its rigid components, DBPZ-DPXZ has poor



solubility and cannot be solution-processed. We then introduce two 3,5-di-tertbutylphenyl (tBPh) groups to DBPZ-DPXZ to design 10,10'-(11,12-bis(3,5-di-tertbutylphenyl)dibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (tDBBPZ-DPXZ). While tDBBPZ-DPXZ is found to have nearly the same photophysical properties as DBBPZ-DPXZ, it shows much improved solubility and solution-processability. In evaporation-processed OLED, tDBBPZ-DPXZ achieves similar orange-red emission with an EQE of 17.0%. Notably, red emission with the maximum EQE as high as 10.1% is obtained for solution-processed OLED employing tDBBPZ-DPXZ as the emitter, which is among the best results of the reported solution-processed red OLEDs based on TADF mechanism. This work proposes an effective strategy to address the conflicting requirements between high efficiency and high solubility and develop soluble red TADF emitters with high efficiency.

2. RESULTS AND DISCUSSION 2.1 Design, Synthesis and Characterization The molecular structures of DBPZ-DPXZ and tDBBPZ-DPXZ are shown in Scheme 1. Rigid phenoxazine (PXZ) and aromatic dibenzo[a,c]phenazine (DBPZ) are employed as main components for suppressing the non-radiative processes to construct the highperformance orange-red TADF emitter DBPZ-DPXZ. Solubilizing groups are then added to endow the molecule with better solubility. The selection of solubilizing group has to be carefully considered such that it will have minimal influences on the molecule’s photophysical properties. In tDBBPZ-DPXZ, two tBPh groups, with weak electron donating strength, are incorporated into the DBPZ segment with ortho position. This can introduce significant steric hindrance for twisting the two tBPh groups to severely limit their conjugation with the main molecular skeleton. Therefore, the use of tBPh groups is not expected to influence the original highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions and photophysical properties. Their synthetic procedures are illustrated in the EXPERIMENTAL SECTION.


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The intermediate benzene-1,2-diamine for DBPZ-DPXZ can be acquired via commercial sources. While for tDBBPZ-DPXZ, the intermediate 3,3'',5,5''-tetra-tertbutyl-[1,1':2',1''-terphenyl]-4',5'-diamine (tBuPDA) is first synthesized using (3,5-ditert-butylphenyl)boronic acid and 4,5-dibromobenzene-1,2-diamine via a Suzuki crosscoupling reaction. Finally the two TADF emitters are obtained through cyclization reactions between the intermediates and 3,6-di(10H-phenoxazin-10-yl)phenanthrene9,10-dione (DPXZ-PhO).24 Their chemical structures are characterized and confirmed via mass spectrometry technology (MS) and nuclear magnetic resonance (NMR) spectroscopy, and the target molecules are further purified by sublimation before use.

HO

B

OH

+ Br

K2CO3 Pd(PPh3)4 1,4-Dioxane/H2O

H2N

Br

NH2

tBuPDA

N

N

1-Butanol H2N

NH2

O

O

N

N

O N

N

O

O

tDBBPZ-DPXZ O

DPXZ-PhO N

1-Butanol

N H2N

NH2

N

N

O

O

DBPZ-DPXZ

Scheme 1. Chemical structures and synthetic routes of tDBBPZ-DPXZ and DBPZ-DPXZ.

2.2 Theoretical Calculations Density functional theory (DFT) calculations are performed at the B3LYP/6-31G level to compare influences of modifications on the frontier molecular orbital (FMO) distributions. As shown in Figure 1, due to evident steric hindrance between donor (D) and acceptor (A) segments, both DBPZ-DPXZ and tDBBPZ-DPXZ show similar highly twisted geometries with dihedrals of 85° and 82°, respectively, which are essential for efficient HOMO and LUMO separations.29-31 For tDBBPZ-DPXZ, two solubilizing tBPh groups with weak electron-donating abilities are attached on the A



segment with relatively high twisted angles of 52°, suppressing its potential electron coupling with the main framework. As expected, both tBPh moieties barely participate in the FMO distributions. This is the main difference from the previous reports which typically cannot completely suppress conjugation between the added solubilizing groups and the main light-emitting cores.19,20 DBPZ-DPXZ and tDBBPZ-DPXZ show nearly identical FMO distributions with the HOMO mainly localized on the two PXZ units and the LUMO mostly restricted on the DBPZ core. Thus, small ΔESTs and efficient RISC can be expected for both emitters. Natural transition orbitals (NTOs) of their optimized S1 states are further investigated using time-dependent DFT (TD-DFT) calculation. As depicted in Figure S1, the excited states of both compounds show obvious intramolecular charge transfer (ICT) feature. “Hole” and “particle” are well separated in the PXZ segments and the DBPZ cores, respectively. In particular, the extra tBPh segments show little contribution in the NTO distributions of tDBBPZ-DPXZ. These results are well consistent with those of their S0 states. Furthermore, comparing connections between tBPh groups and DBPZ core in S1 and S0 states, the dihedrals and bonds both remain nearly identical, further confirming that the introduction of tBPh groups has negligible effect on photophysical properties and energy loss from ICT induced molecular relaxion.

Figure 1. Energy-minimized geometries and HOMO/LUMO distributions of tDBBPZ-DPXZ and DBPZ-DPXZ.


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2.3 Electrochemical Properties Cyclic voltammetry (CV) measurements are then carried out to investigate the influences of the tBPh groups. As shown in Figure S2, from the onset of the oxidation curve (vs a ferrocene/ferrocenium reference), HOMO energy level of tDBBPZ-DPXZ is estimated to be -5.29 eV. While its LUMO energy level is determined as -3.38 eV based on the onset of the reduction curve. The electrochemical band gap (Eg) of tDBBPZ-DPXZ is therefore calculated to be 1.91 eV. All these energy levels are very close to those of DBPZ-DPXZ as summarized in Table 1. These results further confirm that the introduction of the tBPh groups on the DBPZ cores can barely influence the HOMO and the LUMO levels.

Table 1. Summary of physical properties of tDBBPZ-DPXZ and DBPZ-DPXZ. HOM Compound

Oa [eV]

LUMO b

[eV]

λFLc [nm ]

S1d

T1e

ΔEST

ΦPLg

Tg/Td

Si

[eV]

[eV]

f [eV]

[%]

[oC]

[wt%]

tDBBPZ-DPXZ

-5.29

-3.38

617

2.22

2.19

0.03

83

n.a.h/508

>30

DBPZ-DPXZ

-5.29

-3.35

614

2.38

2.35

0.03

84

n.a.h/445

2.7

Determined from the onset of the oxidation potential in 10-3 M DCM solution; b Determined from the onset of the reduction potential in 10-3 M DCM solution; c Determined from the emission peak in 10-5 M toluene solution at room temperature (r. t.); d Determined from the onset of fluorescence spectrum in 10 wt% doped CBP film at r. t.; e Determined from the onset of phosphorescence spectrum in 10 wt% doped CBP film at 77 K; f ΔE = S - T ; ST 1 1 g Measured in 10 wt% doped CBP film under oxygen-free condition, excited at 330 nm; h n.a. represents as not available; i S represents as solubility.

a

2.4 Photophysical Properties Ultraviolet-visible absorption and photoluminescence (PL) spectra in dilute toluene solution of tDBBPZ-DPXZ and DBPZ-DPXZ are then measured. As presented in Figure 2a and 2b, for both compounds, weak and broad absorption bands can be observed in similar regions from 550 nm to 430 nm, suggesting their nearly identical ICT properties from PXZ units to the A core. While at around 390 nm, strong and sharp absorption bands can be observed with obviously distinct profiles suggesting the



evident influence of addition of tBPh groups on high-lying excited states. When excited at 475 nm, a structureless emission with peak at 617 nm is obtained for tDBBPZ-DPXZ, which is similar with that of DBPZ-DPXZ (614 nm). Furthermore, similar emission properties are observed in solvents with various polarities. As shown in Figure S3, from low polarity hexane to high polarity toluene and ethyl ether, both emitters show regular and similar red-shifts in their fluorescence spectra, with peaks at 536, 617 and 657 nm for tDBBPZ-DPXZ and at 531, 614 and 646 nm for DBPZ-DPXZ, respectively. To further evaluate energy levels of the two compounds in solid states, tDBBPZDPXZ and DBPZ-DPXZ 10 wt% doped 4,4'-di(9H-carbazol-9-yl)-1,1'-biphenyl (CBP) films are prepared by thermal evaporation. Figure S4 showed their fluorescence spectra at room temperature and phosphorescence spectra at 77 K. From the onsets, the S1 and T1 levels are calculated to be 2.22 and 2.19 eV for tDBBPZ-DPXZ, and 2.38 and 2.35 eV for DBPZ-DPXZ, respectively. The ΔESTs of tDBBPZ-DPXZ and DBPZ-DPXZ are both determined to be 0.03 eV. These small ΔESTs would favor efficient RISC process for both emitters. To further confirm their TADF properties, time-resolve PL spectra and transient PL decay of 10 wt% tDBBPZ-DPXZ and DBPZ-DPXZ in CBP doped films are measured. As shown in Figure S5, both tDBBPZ-DPXZ and DBPZ-DPXZ exhibit similar fluorescence emissions for the prompt and delayed fluorescence spectra. As shown in Figure 2c and 2d, both tDBBPZ-DPXZ and DBPZ-DPXZ showed the prompt decays with lifetimes of 33.8 and 39.4 ns in the range of 100 ns and the delayed profiles with lifetimes of 9.78 and 6.47 μs in the range of 50 μs at room temperature, which are comparable to most of the highly efficient TADF emitters.32-35 To further compare exciton utilizations, ΦPLs are measured under oxygen-free condition and estimated to be 83% for tDBBPZ-DPXZ and 84% for DBPZ-DPXZ, respectively. According to the ΦPL and transient PL spectra, their prompt and delayed components of ΦPL are respectively estimated to be 40% and 44% for DBPZ-DPXZ and 38% and 45% for tDBBPZ-DPXZ. Their rate constants of RISC process (kRISC) are further estimated to be 2.7 × 105 and 1.8 × 105 s-1, respectively, (Seen in Table S1). These results further


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confirm both emitters as highly efficientTADF emitters with similar exciton utilizations and they have potential to acquire high efficiency in OLEDs. Assuming triplet exciton loss can be ignored due to their small ΔEST values and internal conversion process of singlet excitons is the main loss channel for both emitters, the theoretically EQEs of devices based on both emitters can be estimated as:36,37 EQE = γ × χ ×ΦPL × ηout

(1)

where γ is the charge balance factor (assuming as 1 for ideal balanced carrier injection), ηout is the optical out-coupling efficiency (typically around 0.2), χ is the overall utilization efficiency (assuming both singlet and triplet excitons are totally harvested, the maximum of χ is 1). Therefore, DBPZ-DPXZ and tDBBPZ-DPXZ are expected to realize similar maximum EQE of 16.8% and 16.6%, respectively.

Figure 2. Normalized UV-vis absorption and fluorescence spectra of tDBBPZ-DPXZ (a) and DBPZ-DPXZ (b) in toluene at room temperature; Prompt (inset) and delayed transient PL decay curves of 10 wt% tDBBPZ-DPXZ (c) and DBPZ-DPXZ (d) doped CBP thin film at room temperature.

2.5 Properties for Solution Processing



To explore solution-processabilities of the two TADF emitters, their solubilities are evaluated in chlorobenzene. Table S2 summarized their solubilities in various common solvents at room temperature., Due to the rigid components, the maximum solubilities of DBPZ-DPXZ is only 5.2 wt% (in chloroform) at room temperature. With such low solubility, DBPZ-DPXZ can hardly applied in solution processing. On the other hand, thanks to the solubilizing tBPh groups, tDBBPZ-DPXZ exhibits much superior solubility with an excellent value beyond 30 wt% in all these solvents. This result confirms that tDBBPZ-DPXZ fulfills the prerequisite for solution process. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements under nitrogen atmosphere are then carried out on tDBBPZ-DPXZ to further investigate its thermal stability. As listed in Table 1 and shown in Figure S6, due to the rigid components, DBPZ-DPXZ shows excellent thermal stability with high decomposition temperature of 445 oC (Td, defined to 5% weight loss in TGA) and no obvious glass transition temperature (Tg) in the range from 30 to 300 oC. Meanwhile with the additional tBPh groups, the excellent thermal stabilities are well maintained. Similarly, no obvious glass transition can be found for tDBBPZ-DPXZ in the range from 30 to 300 oC; and an even higher Td of 508 oC is obtained. These excellent thermal properties further guarantee the thermal stability of tDBBPZ-DPXZ for solution process. Moreover, when doping 10 wt% tDBBPZ-DPXZ into CBP as host matrix, there is only an additional exothermic process observed at 263 °C comparing neat CBP, which is very close to the melting point of CBP and can be ascribed to crystallization process, suggesting a good dispersion of tDBBPZ-DPXZ. (Figure S7a) Furthermore, XRD measurement and atomic force microscopy (AFM) study are carried out on a CBP film doped with 10 wt% tDBBPZ-DPXZ prepared by spin-coating. From the XRD results, no obvious crystallization signals can be found suggesting good amorphous features of our prepared film. (Figure S7b) Moreover as shown in Figure 3, similar with other reports,38,39 the tDBBPZ-DPXZ doped film also has a smooth surface morphology with a root-mean-square (RMS) roughness of only 0.267 nm. The excellent morphology of the thin film further confirms that tDBBPZ-DPXZ is suitable for solution process.


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Figure 3. AFM topographic image of the solution-processed CBP: 10 wt% tDBBPZ-DPXZ film (5 × 5 μm).

2.6 Electroluminescence Properties To compare electroluminescence (EL) performances of the two emitters, vacuumdeposited OLEDs are first fabricated with a configuration of ITO/TAPC (35 nm)/TCTA (10 nm)/mCP (10 nm)/CBP:10 wt% emitters (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al, where tDBBPZ-DPXZ or DBPZ-DPXZ are used as the emitters; ITO (indium tin oxide) and LiF/Al are used as the anode and the cathode, respectively; TAPC (1,1-bis[4-[N,Ndi(p-tolyl)-amino]phenyl]cyclohexane)

and

TmPyPB

(1,3,5-tri(m-pyrid-3-yl-

phenyl)benzene) are employed as the hole-transporting layer and electron-transporting layer, respectively; TCTA (4,4’,4’’-tris(carbazol-9-yi)triphenylamine) and mCP (1,3bis(N-carbazoly)benzene) both acted as exciton blocking layers. As shown in Figure 4, both vacuum-deposited devices showed nearly identical orange-red EL spectra with peaks at 608 nm, corresponding to Commission Internationale de l’Eclairage (CIE) coordinates/half-bandwidth of (0.58,0.42)/113 nm for tDBBPZ-DPXZ and (0.57,0.43)/107 nm for DBPZ-DPXZ, respectively, further confirming their similar emission properties. In addition, the device based on tDBBPZDPXZ and DBPZ-DPXZ respectively achieved maximum EQE of 17.0% and 17.8%, well consistent with our earlier estimations. These similar results reveal that the introduction of tBPh groups showing little influence on exciton utilization in OLEDs, and both emitters are favorable for vacuum-deposited OLEDs.



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Figure 4. (a) Luminance - EQE curves (inset: EL spectra at 100 cd m-2) and (b) current densityvoltage-luminance curves of the vacuum-processed devices based on tDBBPZ-DPXZ and DBPZDPXZ.

Due to the excellent solubility, thermal stability and film morphology, tDBBPZDPXZ is further expected to be an ideal candidate for red solution-processed OLEDs. Herein, we fabricated a series of solution-processed OLEDs by employing tDBBPZDPXZ as the emitter as shown in Figure 5 and Figure S8, S9, and the optimized structure

is:

ITO/PEDOT:PSS

(40

nm)/CBP:10

wt%

tDBBPZ-DPXZ

(40

nm)/TmPyPB (5 nm)/TPBi (35 nm)/LiF (1 nm)/Al, in which, PEDOT:PSS (poly(3,4ethylenedioxythiophene)poly(styrene

sulfonate))

and

TPBi

(1,3,5-tris(N-

phenylbenzimidazol-2-yl)benzene) are used as the hole-injection layer and the electrontransporting layer, respectively. TmPyPB is used as the hole-blocking layer to obtain the optimized device performance. (seen in Figure S8) As shown in Figure 5 and summarized in Table 2, due to the different device structures and stronger intermolecular interactions and increasing aggregation by spincoating than evaporation,40 tDBBPZ-DPXZ-based OLED prepared by solution process exhibited an slight red-shift EL emission peak at 620 nm with a half bandwidth of 102 nm and a CIE coordinate of (0.62, 0.37) comparing the evaporated one. Meanwhile, the onset voltage of the device is 4.5 V, which is evidently higher than that of thermally evaporated ones. But still, the tDBBPZ-DPXZ-based OLED realized remarkably high efficiencies with maximum EQE, current efficiency (CE) and power efficiency (PE) of 10.1%, 12.8 cd A-1 and 7.3 lm W-1, respectively. Table S3 summarizes the performances of the reported solution-processed yellow to red TADF OLEDs. This device shows one


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of state-of-the-art performance for solution-processed orange-red TADF OLED, indicating the superiority of tDBBPZ-DPXZ.

Figure 5. (a) PE - luminance - EQE curves of tDBBPZ-DPXZ-based device (inset: EL spectra at 100 cd m-2). (b) Current density-voltage-luminance curves of the solution-processed tDBBPZDPXZ-based device. Table 2. Summary of device performances of tDBBPZ-DPXZ and DBPZ-DPXZ. Vona

ELmaxb

EQEc

CIEb

(V)

(nm)

(%)

(x, y)

DBPZ-DPXZ

3.5

608

17.8/17.1/13.0

(0.57, 0.43)

tDBBPZ-DPXZ

3.5

608

17.0/13.3/8.9

(0.58, 0.42)

tDBBPZ-DPXZ

4.5

620

10.1/8.2/4.4

(0.62, 0.37)

Process

Emitter

vacuumdeposited solutionprocessed a

At a luminance of 100 cd m-2; luminance of 100 and 500 cd m-2. Turn-on voltage;

b

c

EQE values determine at maximum, and at

3. CONCLUSION In conclusion, we designed and synthesized a novel efficient rigid orange-red TADF emitter DBPZ-DPXZ and further modified it into a soluble orange-red TADF emitter tDBBPZ-DPXZ by introducing two tBPh groups. Due to the rigid donor-acceptor structure, DBPZ-DPXZ exhibits effective TADF characteristics with extremely small ΔEST of 0.03 eV and high ΦPL of 84%. The evaporation-processed OLED using DBPZDPXZ as emitter obtained orange-red emission with high EQE of 17.8%. However, its poor solubility prohibits the possibility of applying it into solution process. For tDBBPZ-DPXZ, good photophysical properties are inherited from the prototype.



tDBBPZ-DPXZ retained the excellent properties with ΦPL of 83% and ΔEST of 0.03 eV, and its evaporation-processed OLED can achieve high EQE of 17.0%. More importantly, the introduction of tBPh groups improvs its solubility. The solutionprocessed OLED realizes red emission with CIE coordinate of (0.62, 0.37) and high EQE of 10.1%, which is the best performance in the reported red solution-processed TADF-OLEDs. This work proposes an effective approach to solve the conflicting requirements between high efficiency and high solubility and develop soluble red TADF emitters with high efficiency.

4. EXPERIMENTAL SECTION 4.1 Synthesis. 3,3'',5,5''-tetra-tert-butyl-[1,1':2',1''-terphenyl]-4',5'-diamine (tBuPDA). (3,5-di-tertbutylphenyl)boronic acid (1.03 g, 4.4 mmol), 4,5-dibromobenzene-1,2-diamine (0.53g, 2 mmol), K2CO3 (1.11 g, 8 mmol), and Pd(PPh3)4 (115.5 mg, 0.1 mmol) is added into a 100 mL round-bottom flask at nitrogen atmosphere, followed by adding 40 mL of THF/H2O (1:1). Then, the mixture is stirred at 80 °C for one day. After completion of the reaction, DCM and water are added to the cooled mixture. The organic layers are separated, and dried over Na2SO4, and concentrated in vacuo. The residue solid is purified by column chromatography to give the product (0.68g, 70.5%): 1H NMR (600 MHz, DMSO-d6) δ 7.04 (s, 2H), 6.73 (s, 4H), 6.60 (s, 2H), 4.59 (s, 4H), 1.09 (s, 32H). MS (EI) m/z: 484.19 [M]+ calcd for C34H48N2 484.38. 10,10'-(11,12-bis(3,5-di-tert-butylphenyl)dibenzo[a,c]phenazine-3,6-diyl)bis(10Hphenoxazine) (tDBBPZ-DPXZ). DPXZ-PhO (0.28 g, 0.5 mmol) and tBuPDA (0.24g, 0.5 mmol) are added into 5 mL of 1-Butanol and refluxed 12 h under N2. After completion of the reaction, the organic solvent is concentrated in vacuo, and the crude product is purified by column chromatography to give the red product (0.31 g, 62%). 1H

NMR (600 MHz, CDCl3) δ 9.72 (d, J = 8.5 Hz, 2H), 8.49 (d, J = 13.1 Hz, 4H), 7.78

(d, J = 8.5 Hz, 2H), 7.32 (s, 2H), 7.15 (s, 4H), 6.73 (d, J = 7.9 Hz, 4H), 6.67 (t, J = 7.6 Hz, 4H), 6.59 (t, J = 7.7 Hz, 4H), 6.06 (d, J = 8.0 Hz, 4H), 1.23 (s, 36H). 13C NMR (151


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MHz, CDCl3) δ 150.42, 146.03, 145.67, 143.96, 141.89, 141.81, 141.04, 139.57, 134.10, 133.85, 130.93, 130.52, 129.60, 125.71, 124.55, 123.30, 121.65, 120.83, 115.62, 113.31, 34.74, 31.41. MALDI-TOF MS (mass m/z): 1018.83 [M]+; calcd for C72H66N4O2 1018.52. 3,6-di(10H-phenoxazin-10-yl)dibenzo[a,c]phenazine (DBPZ-DPXZ). DBPZ-DPXZ is synthesized by a similar procedure as for tDBBPZ-DPXZ with benzene-1,2-diamine instead of tBuPDA. 1H NMR (600 MHz, CDCl3) δ 9.68 (dd, J = 8.4, 1.5 Hz, 1H), 8.48 (t, J = 1.8 Hz, 1H), 8.41 - 8.36 (m, 1H), 7.95 - 7.91 (m, 1H), 7.77 (dt, J = 8.4, 1.6 Hz, 1H), 6.74 - 6.70 (m, 1H), 6.68 - 6.63 (m, 2H), 6.59 - 6.54 (m, 3H), 6.03 (dd, J = 8.1, 3.0 Hz, 2H).

13C

NMR (151 MHz, CDCl3) δ 142.54, 141.84, 138.01, 134.05, 134.02,

130.52, 130.43, 129.52, 129.48, 124.85, 123.38, 123.33, 123.27, 113.50. MALDI-TOF MS (mass m/z): 642.54 [M]+; calcd for C44H26N4O2 642.21.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Descriptions of general information and device fabrication, natural transition orbital of S1 state for tDBBPZ-DPXZ and DBPZ-DPXZ, cyclic voltammetry, solvent-dependent fluorescence spectra, fluorescence and phosphorescence spectra in doped CBP thin films, TGA and DSC curves of tDBBPZ-DPXZ and DBPZ-DPXZ, summary of solution-processed OLEDs based on red TADF emitters.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (K.W.). *E-mail: [email protected] (W.-C.C.). *E-mail: [email protected] (X.-H.Z.). *E-mail: [email protected] (C.-S.L.). ORCID



Jia-Xiong Chen: 0000-0002-4379-490X Ya-Fang Xiao: 0000-0002-3664-1526 Wen-Cheng Chen: 0000-0003-3788-3516 Chun-Sing Lee: 0000-0001-6557-453X Author Contributions These authors J.-X.C. and W.-W.T. contributed equally to the work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Key Research & Development Program of China (Grant No. 2016YFB0401002), the National Natural Science Foundation of China (Grant No. 51533005, 51821002, 51773029), the China Postdoctoral Science Foundation (Grant No. 2018M640517, 2018M642307), the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices.

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Abstract Graphic


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Efficient Orange-Red Thermally Activated Delayed Fluorescence

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