[1,2,4]Triazolo[1,5-a]pyridine-Based Host Materials for Green

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[1,2,4]Triazolo[1,5-a]pyridine-based Host Materials for Green Phosphorescent and Delayed-Fluorescence OLEDs with Low Efficiency Roll-off Wenxuan Song, Yi Chen, Qihao Xu, Haichuan Mu, Jing-Jing Cao, Jinhai Huang, and Jianhua Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07462 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

[1,2,4]Triazolo[1,5-a]pyridine-based Host Materials for Green Phosphorescent and Delayed-Fluorescence OLEDs with Low Efficiency Roll-off Wenxuan Song,a Yi Chen,a Qihao Xu,a Haichuan Mu,b Jingjing Cao,*c Jinhai Huang *d and Jianhua Su*a a

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of

Chemistry & Molecular Engineering, East China University of Science & Technology, Shanghai 200237, PR China. b

Department of Physics, School of Science, East China University of Science and

Technology, Shanghai 200237, PR China. c

State Key Laboratory of Applied Organic Chemistry (SKLAOC); College of

Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China. d

Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, PR China.

Keywords: [1,2,4]triazolo[1,5-a]pyridine, host materials, delayed-fluorescence organic light-emitting diode, phosphorescent organic light-emitting diodes (PhOLEDs), high efficiency, low efficiency roll-off.

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ABSTRACT

Herein,

a

series

of

universal

bipolar

host

materials,

9,9'-([1,2,4]triazolo[1,5-a]pyridine-2,6-diylbis(4,1-phenylene))bis(9H-carbazole) (TP26Cz1),

3-(2-(4-(9H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5-a]

pyridine-6-yl)-9-phenyl-9H-carbazole

(TP26Cz2),

9,9'-([1,2,4]triazolo[1,5-a]pyridine-2,7-diylbis(4,1-phenylene))bis(9H-carbazole) (TP27Cz1)

and

3-(2-(4-(9H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5-a]pyridin-7-yl)-9-phenyl-9H-ca rbazole

(TP27Cz2),

using

[1,2,4]triazolo[1,5-a]pyridine

(TP)

moiety

as

electron-transporting moiety and carbzole as hole-transporting moiety, were designed and synthesized. All four compounds possess remarkable carriers transporting properties and excellent thermal stability with high glass-transition temperature (Tg) in the range of 136-144 oC. The hole and electron transporting abilities could be regulated by adjusting the linkage mode between the carbazole and TP units, and balanced charge-transport property were realized in TP26Cz2 and TP27Cz2. The phosphorescent and thermally activated delayed fluorescence (TADF) OLEDs based on these host materials exhibit superior performance with high efficiency and low roll-off. For example, TP26Cz2 hosted PhOLED exhibits the maximum external quantum efficiency (ηext) of 25.6%, and at the high luminance of 5000 cd m−2, the ηext still remained at 25.2%. TP27Cz1 hosted TADF device exhibits the maximum ηext of 15.5%, and only dropped to 15.4% at the luminance of 1000 cd m−2. Moreover, the influence of linking mode of carbazole unit and TP units in these hosts on their


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photophysical

and

carrier

transporting

properties

as

well

as

devices

electroluminescence (EL) performances was discussed.

INTRODUCTION

After years of development, OLEDs have played an important role in display, such as smartphones and televisions, and started their application in solid state lighting.1-6 Up to now, phosphorescent and TADF emitters have attracted a lot of attention due to their nearly 100% internal quantum efficiency (IQE) compared to 25% IQE of the traditional fluorescent emitters. Phosphorescent emitters in OLEDs are normally heavy metal complexes, which can utilize singlet and triplet excitons to achieve 100% internal quantum efficiency (IQE) by heavy atom effect.7-10 Besides, the TADF materials with narrow singlet−triplet energy difference (∆EST) also can achieve 100% IQE by virtue of the up-conversion of the triplet exciton from the lowest triplet excited energy (T1) to the lowest singlet excited energy (S1)11-13. To achieve excellent performance, both PhOLEDs and TADF based OLEDs usually adopt the doping structures to reduce competitive factors such as triplet-triplet annihilation (TTA)14-15. Therefore, host materials, as important as the emitters, impose significant effects on the devices EL performance.

Conventional

unipolar

hosts

for

OLEDs

are

electron-transport

or

hole-transport materials, such as bis[2-((oxo)diphenylphosphino)phenyl] ether (DPEPO),

4,4'-Bis(9H-carbazol-9-yl)biphenyl


(CBP)

and


1,3-bis(carbazolyl)benzene (mCP). However, due to the unipolar charge transporting property, the recombination zone in emitting layer (EML) tends to be narrow and closed to the EML/electron transport layer (ETL) or EML/ hole transport layer (HTL) interface, leading to serious efficiency roll-off at high luminance. The brightness level of OLEDs required for the application in high-resolution displays and illumination is usually above 5000 cd m-2.15 And this problem is more serious in TADF based OLEDs as TADF emitters generally have long exciton lifetime16. The efficiency roll-off problem could be addressed by using co-host systems incorporating both electron and hole transporting materials or bipolar materials. However, co-host systems complicate the OLEDs fabrication processes and limit their mass production.17 Bipolar host materials consisted by hole-transporting and electron-transporting units can transport hole and electron evenly in EML and thus be conducive to the improvement of OLEDs EL performance.17-21 However, the balanced charge-transporting property is hard to realize, since the hole mobilities of hole-transporting moieties are orders of magnitude higher than the electron mobilities of many electron-transporting moieties.22-24 Recently, our group reported an electron-deficient unit TP to build bipolar host materials for PhOLED.25 In our previous work, two TP-based host materials with high triplet energy level T1 were obtained by introducing TP moieties to mCP framework, and the structures of these two hosts are highly twisted, which lead to relatively poor carrier transporting behavior. In this report, we modified the


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electron-deficient unit to obtain bipolar host materials for both phosphorescent and TADF based OLEDs. Furthermore, the influence of linking mode of carbazole unit and TP units in these hosts on their photophysical and carrier transporting

properties

as

well

as

devices

electroluminescence

(EL)

performances was discussed.

Herein, four TP/carbazole based bipolar hosts, namely TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2, were designed and synthesized by adjusting the linkage mode between the carbazole and TP moieties. These four compounds exhibit relatively high ETs, appropriate highest occupied molecular orbital (HOMO)/ lowest unoccupied molecular orbital (LUMO) energy levels, superior thermal stability with high Tg. Interestingly, The carrier-transporting abilities of these compounds can be regulated through adjusting the linkage mode between the carbazole and TP moieties and balanced charge-transporting property were realized in TP26Cz2 and TP27Cz2, which could broad recombination zones and thus improve the device EL performance.26-29 Green PhOLEDs and TADF based OLEDs using these four compounds as hosts were fabricated, and excellent device EL performances were achieved. For example, green PhOLEDs using TP26Cz2 as host exhibits the most optimal current efficiency (ηc) and external quantum efficiency (ηext) of 90.3 cd A-1 and 25.6%, respectively, with the extremely low efficiency roll-off (reduced by 1.5% for ηext at the luminance of 5000 cd m−2). A striking ηext of 24.4% could be achieved at fairly high luminance of 10000 cd m−2. As for TADF based



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OLEDs, TP26Cz1 hosted device exhibits the maximum ηext of 15.5% (ηc of cd A-1), and at the luminance of 1 000 cd m−2 the ηext still remain at 15.4%. It is worth mentioning that the efficiency of our devices (ηexts of 25.2% and 24.9%) at 5000 cd m−2 are among the best values ever reported for single-host green OLEDs (See in Table S1).15,16,30-33

EXPERIMENTAL SECTION

Synthesis of 4-(9H-carbazol-9-yl)benzonitrile (1) 4-Bromobenzonitrile (5 g, 27.5 mmol), 9H-carbazole (4.2 g, 25 mmol), sodium tert-butoxide (4.8 g, 50 mmol),

palladium

acetate

(20

mg,

0.09

mmol)

and

2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl (60 mg, 0.13 mmol) were added to 50mL of dried toluene. The mixture was heated to reflux for 24h under N2. After cooling, the solvent was removed by rotary evaporation, and the residuum was dissolved by chloroform and washed with distilled water three times. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation and the crude product purified by silica gel column chromatography using a petroleum and ethyl acetate mixture (v : v = 2 : 1) to give white solid 1 (3.9 g, 58.2%). 1H NMR (400 MHz, CDCl3) δ = 8.13 (d, J = 7.6 Hz, 1 H), 7.88 (d, J = 8.4 Hz, 1 H), 7.71 (d, J = 8.4, 1 H), 7.43 (d, J = 5.6, 2 H), 7.33 (ddd, J = 7.6 Hz, 6.0 Hz, 2.1 Hz, 1 H).

Synthesis

of

9-(4-(6-bromo-[1,2,4]triazolo[1,5-a]pyridin-2-yl)phenyl)-9H-carbazole


(2)

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5-Bromopyridin-2-amine (0.78g, 4.5 mmol), compound 1 (1 g, 3.7 mmol), cuprous bromide (27 mg, 0.18 mmol), 1,10-phenanthroline (33 mg, 0.18 mmol) and zinc iodide (118 mg, 0.37 mmol) were added to 5 mL of 1,2-dichlorobenzene. The mixture was heated to 130 °C for 24 h under atmospheric air. After cooling, the solvent was removed by rotary evaporation, and then the crude product purified by silica gel column chromatography using a petroleum and ethyl acetate mixture (v : v = 3 : 1) to give compound 2 as white solid (0.5 g, 30.8 %). 1H NMR (400 MHz, CDCl3) δ 8.80 (s, 1 H), 8.50 (d, J = 8.4 Hz, 2 H), 8.16 (d, J = 7.6 Hz, 3 H), 7.76 – 7.68 (m, 4 H), 7.64 (dd, J = 9.6, 1.8 Hz, 1 H), 7.51 (d, J = 8.4 Hz, 2 H), 7.44 (t, J = 7.2 Hz, 3 H), 7.31 (t, J = 7.2 Hz, 3 H).

Synthesis

of

9,9'-([1,2,4]triazolo[1,5-a]pyridine-2,6-diylbis(4,1phenylene))bis(9H-carbaz ole)

Compound

(TP26Cz1)

2

(0.5

g,

1.15

mmol),

(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl) boronic acid (0.39g, 1.35 mmol),

potassium

carbonate

(0.34

g,

2.5

mmol),

[1,1'-bis(diphenylphosphino)ferrocene]dichloro palladium(II) (36mg, 0.056 mmol) were added to the mixed solvent of water (2 mL) and tetrahydrofuran (6 mL). The mixture was heated to reflux for 6 h under nitrogen. After reaction finished, the solution was poured into water and extracted with CH2Cl2. After drying with anhydrous sodium sulfate, the solvent was removed by rotary evaporation

and

the

crude

product

purified


by

silica

gel

column


chromatography using a petroleum and ethyl acetate mixture ( v : v = 10 : 1) to give TP26Cz1 as white solid(0.41 g, 60%).1H NMR (400 MHz, CDCl3) δ = 8.86 (s, 1 H), 8.49 (d, J = 8.4 Hz, 2 H), 8.09 (dd, J = 7.6, 5.2 Hz, 4 H), 7.89 – 7.73 (m, 4 H), 7.67 (dd, J = 8.4, 2.4 Hz, 4 H), 7.50 – 7.33 (m, 8 H), 7.30 – 7.20 (m, 4 H).

13

C NMR (101 MHz, CDCl3) δ 164.09, 151.06, 140.66, 140.63,

139.51, 138.14, 135.17, 130.08, 129.64, 128.93, 128.49, 127.86, 127.77, 127.17, 126.13, 126.10, 125.75, 123.62, 120.48, 120.39, 120.32, 120.23, 116.48, 109.90, 109.73. HRMS (ESI, m/z): [M+H]+ calcd for: C42H28N5, 602.2345, found, 602.2337.

Synthesis

of

3-(2-(4-(9H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5-a]

pyridine-6-yl)-9-phenyl-9H-carbazole (TP26Cz2) Compound TP26Cz2 was synthesized by the same procedure as TP26Cz1 using compound 2 and (9-phenyl-9H-carbazol-3-yl)boronic acid. White solid. Yield 65.1%. 1H NMR (400 MHz, CDCl3) δ = 8.85 (s, 1 H), 8.48 (d, J=8.5 Hz, 2 H), 8.30 (d, J=1.5 Hz, 1 H), 8.14 (d, J=7.7 Hz, 1 H), 8.08 (d, J=7.7 Hz, 2 H), 7.89 (dd, J=9.2, 1.6 Hz, 1 H), 7.82 (d, J=9.2 Hz, 1 H), 7.66 (d, J=8.5 Hz, 2 H), 7.60 – 7.49 (m, 5 H), 7.47 – 7.33 (m, 8 H), 7.30 – 7.20 (m, 3 H). 13C NMR (101 MHz, CDCl3) δ 163.55, 150.60, 141.54, 140.89, 140.64, 139.37, 137.34, 130.97, 130.07, 129.67, 128.86, 128.10, 127.86, 127.15, 127.12, 126.68, 126.08, 125.35, 125.01, 124.25, 123.60, 123.06, 120.47, 120.37, 120.19, 118.92, 116.00, 110.70, 110.18, 109.92, 99.99. HRMS (ESI, m/z): [M+H]+ calcd for: C42H28N5, 602.2345, found, 602.2341.


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Synthesis

of

9-(4-(7-bromo-[1,2,4]triazolo[1,5-a]pyridin-2-yl)phenyl)-9H-carbazole

(3)

Compound 3 was synthesized by the same procedure as compound 2 using compound 1 and 4-bromopyridin-2-amine. White solid. Yield 48.2%. 1H NMR (400 MHz, CDCl3) δ = 8.50 (d, J = 8.0 Hz, 1 H), 8.16 (d, J = 7.6 Hz, 1 H), 7.98 (d, J = 1.6 Hz, 1 H), 7.73 (d, J=8.4Hz, 1 H), 7.51 (d, J = 8.4 Hz, 1 H), 7.44 (t, J = 7.6 Hz, 1 H), 7.31 (t, J = 7.6 Hz, 1 H), 7.17 (dd, J = 7.2 Hz, 1 H).

Synthesis

of

9,9'-([1,2,4]triazolo[1,5-a]pyridine-2,7-diylbis(4,1-phenylene))bis(9H-carba zole) (TP27Cz1) Compound TP27Cz1 was synthesized by the same procedure as

TP26Cz1

using

compound

3

and

(4-(1-phenyl-1H-benzo[d]

imidazole-2-yl)phenyl) boronic acid. White solid. Yield 77.7%.1H NMR (400 MHz, CDCl3) δ = 8.75 (d, J = 7.1 Hz, 1 H), 8.60 (d, J = 8.5 Hz, 2 H), 8.20 (dd, J = 7.7 Hz, 4.2, 4 H), 8.10 (d, J = 1.0 Hz, 1 H), 7.96 (d, J = 8.5 Hz, 2 H), 7.79 (dd, J = 8.5, 2.1 Hz, 4 H), 7.59 – 7.45 (m, 8 H), 7.43 – 7.32 (m, 5 H). 13C NMR (101 MHz, CDCl3) δ 164.31, 152.25, 142.11, 140.63, 140.59, 139.48, 138.72, 136.70, 129.70, 128.93, 128.61, 128.34, 127.68, 127.16, 126.16, 126.10, 123.67, 123.62, 120.48, 120.37, 120.23, 113.55, 113.23, 109.91, 109.77. HRMS (ESI, m/z): [M+H]+ calcd for: C42H28N5, 602.2345, found, 602.2334.

Synthesis

of

3-(2-(4-(9H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5-a]pyridin-7-yl)-9-phen



Page 10 of 39

yl-9H-carbazole (TP27Cz2) Compound TP27Cz2 was synthesized by the same

procedure

as

TP26Cz1

using

compound

3

and

(9-phenyl-9H-carbazol-3-yl)boronic acid. White solid. Yield 79.4%. 1H NMR (400 MHz, CDCl3) δ = 8.71 (d, J = 7.1 Hz, 1 H), 8.59 (d, J = 8.5 Hz, 2 H), 8.51 (d, J = 1.5 Hz, 1 H), 8.26 (d, J = 7.7 Hz, 1H), 8.19 (d, J = 7.7 Hz, 2H), 8.10 (d, J = 0.9 Hz, 1 H), 7.81 – 7.75 (m, 3 H), 7.71 – 7.59 (m, 4 H), 7.58 – 7.44 (m, 9 H), 7.42 – 7.32 (m, 3 H).

13

C NMR (101 MHz, CDCl3) δ 163.91, 152.37,

144.07, 141.59, 141.33, 140.66, 139.34, 137.27, 130.07, 129.84, 129.74, 128.88, 127.94, 127.90, 127.11, 126.70, 126.08, 125.07, 124.17, 123.60, 123.15, 120.56, 120.47, 120.37, 120.18, 119.06, 114.16, 112.45, 110.62, 110.21, 109.93. HRMS (ESI, m/z): [M+H]+ calcd for: C42H28N5, 602.2345, found, 602.2339.

RESULTS AND DISCUSSION

Scheme 1. The structure and synthetic routes of compounds TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2.


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Synthesis and characterization. As shown in Scheme 1, the key intermediates 2 and 3 were synthesized by reaction of 5-bromopyridin-2-amine or 4-bromopyridin-2-amine and 4-(9H-carbazol-9-yl) benzonitrile involving transition-metal-catalyzed N-C bond formation and oxidative N-N coupling in 38% and 50% yields.34 Four target compound were synthesized by Suzuki coupling

reaction

in

good

yields

(60%-77%).

All

compounds were

characterized by 1H nuclear magnetic resonance spectroscopy (NMR). And the structural formulas of four target compounds were fully verified by additional 13

C NMR and high-resolution mass spectrometry (HRMS). The details of

synthetic procedures were described in experimental section and the characterization was shown in electronic supplementary information (ESI).



Figure 1. (a) Absorption, fluorescence spectra in tetrahydrofuran solution and (b) phosphorescent spectra in 2-methyl-tetrahydrofuran at 77K of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2.

Photophysical properties. The UV–vis absorption, photoluminescence (PL) spectra (at room temperature) in tetrahydrofuran solution (1×10−5 M) and phosphorescence spectra (at 77K) in 2-methyl-tetrahydrofuran of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 are shown in Figure 1 and the data were collected in Table 1. In UV-vis spectra, all four compounds exhibit two characteristic absorption bands, the bands below 300 nm could be ascribed to the π−π* transition of the carbazole unit, while the maximums band peak around 340nm could be assigned to n-π* transition of carbazole-centered.35 The optical energy gaps calculated from absorption edge of the absorption spectra of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 are 3.31 eV 3.30 eV, 3.26 eV and 3.28 eV, respectively.

All of the compounds emit purple-blue light in dilute tetrahydrofuran (THF) solution. And it was noted that TP26Cz1, TP26Cz2, and TP27Cz2 exhibit similar fluorescence emission (FL) with the peaks at 385 nm, while TP27Cz1 shows a more red shift FL with the peak at 413 nm. To understand this phenomenon, the emission spectra of these four compounds in various solutions were tested and the results was shown in Figure S5. The FL of these four compounds show different levels of solvatochromic shifts with the increase of


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solvent polarity, indicating that their excited state employed various degrees of intramolecular charge transfer (ICT). According to Figure S5, from hexane to dimethyl formamide (DMF), the fluorescence peak were red shifted up to 60 nm, 31nm, 80nm and 43 nm, respectively for TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2. Furthermore, Lippert-Mataga equation was used to analyze stokes shifts of the hosts in different solvents. From the plots of stokes shifts (va - vf) against the solvent polarity (f) (see in Figure S5)., the slopes of the linear fitting for TP26Cz1, TP26Cz2, TP27Cz1, and TP27Cz2 are 11368, 5472, 13980 and 6195, respectively. The well fitting linearity of these plots could also offer a clear description of the solvatochromism of these four compounds. It is certain that the degree of ICT is depend on the molecular structure. The solvato red shifts of TP26Cz1 and TP27Cz1 are more significantly than those of TP26Cz2 and TP27Cz2, which could be attribute to steric hindrance. For TP26Cz2 and TP27Cz2, the carbazole is attached to the TP unit through a phenyl, which enhanced steric hindrance and degrees of twisted intramolecular charge transfer (TICT) in excited state. Moreover, 2,7-substituted TP derivatives exhibit large solvato red shifts, which also could be explained by the similar steric hindrance. Based on the above two reasons, more significant redshift from TP27Cz1 could be observed. Density functional theory (DFT) calculation

(shown

in following

section)

was

confirmation.


employed

for

further


To obtain the lowest triplet excited state energy ET (T1), the PL spectra in the 2-methyltetrahydrofuran at 77 K were measured. By calculated form the highest-energy vibronic sub-band in phosphorescence spectra at 77K, ET of TP26Cz1, TP26Cz2, P27Cz1 and TP27Cz2 were determined to be 2.60 eV, 2.60 eV, 2.59 eV and 2.56 eV, respectively. The ET energy levels of these compounds are higher than that of commonly used green emitter, indicating that they could act as host materials for green OLEDs.

Figure 2. (a) TGA and (b) DSC curves of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2


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Thermal properties. Thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements (Figure 2) were carried out to evaluate the thermal stabilities of these four compounds and the results were listed in Table 1. From the DSC curve, all compounds exhibit high glass transition temperature (Tg) in the range of 136-144 oC, which are significantly higher than those of classical host materials CBP (62 oC)36 and mCP (60 oC)37. In addition, the thermal decomposition temperatures (Tds) of these four compounds range from 478 to 504 oC. All of these indicated that these four compounds have superior thermal properties to bear high temperature during materials evaporation and device operation.

Electrochemical properties. To evaluate the electrochemical properties of these four compounds, cyclic voltammetric (CV) was performed in nitrogen-saturated dimethyl formamide and dichloromethane solvent. As shown in Figure3, all the compounds had a oxidation wave at ~ 0.8 V, which could be attributed to the oxidation of carbazole moiety.38 The HOMO energy levels of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 could be obtained from the onset potential of the first oxidation wave(Eonset ox ) according to the equation of EHOMO = -( Eonset + 4.4) eV as -5.25, -5.25, -5.25 and -5.23 eV.39-41 Accordingly, the ox reduction waves of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 are -1.97 V, -2.06 V, -1.83 V and -1.99 V, respectively. And LUMO energy levels can be determined from the reduction waves according to the equation ELUMO = -( Eonset rex + 4.4) eV, which are -2.43 eV, -2.34 eV,-2.57 eV and -2.41 eV, respectively.



The little differences between the HOMO energy levels of host materials and adjacent tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (-5.3 eV) indicate that holes could be injected into the EML very effectively

42,43

Besides, the

HOMO/LUMO energy levels of these compounds are somewhat similar, implying that the change of the linkage mode between the carbazole and TP units did not impose much influence on the electrochemical properties of these compounds.

TP26Cz1 TP26Cz2 TP27Cz1 TP27Cz2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

-2.5

-2.0

-1.5

0.5

1.0

1.5

2.0

E(V vs SEC)

Figure 3. CV curves of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 measured in CH2Cl2 (anodic) and DMF (cathodic) solutions.

Table 1 Physical properties of compounds TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 Compounds

λabs (nm)

λem (nm)

HOMOa (eV)

LUMO (eV)

Eg b (eV)


HOMOc (eV)

LUMOc (eV)

ETd (eV)

Td e o

( C)

Tge (oC)

Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TP26Cz1

292, 331, 341

385

-5.27

-2.43

2.84

-5.28

-1.66

2.60

478

144

TP26Cz2

293, 332, 340

389

-5.24

-2.34

2.90

-5.20

-1.38

2.60

502

141

TP27Cz1

293, 331, 342

413

-5.25

-2.57

2.68

-5.27

-1.75

2.59

504

143

TP27Cz2

293, 333, 341

389

-5.23

-2.41

2.82

-5.19

-1.42

2.56

487

136

a

Deduced from cyclic voltammetry; Electrochemical band gap calculated from the difference between oxidation onset and reduction onset; c Values from DFT calculation; d Measured in 2-MeTHF at 77 K.; e Measured by TGA and DSC b

DFT calculation. Density functional theory (DFT) calculation was performed to further research the structures and molecular orbitals (MOs) of these four compounds. As shown in Figure 4, in TP26Cz1 and TP27Cz1, the two donors carbazole and accepter TP are connected through phenylene spacer. This kind of connection lead to larger dihedral angles of 52.6o and 53.6o. In contrast, TP26Cz2 and TP27Cz2 carbazole directly linked to TP at its 3-site C atom and exhibit planar molecular conformations with the respective dihedral angles of 36.1o and 38.6o. As for MOs, the HOMO orbitals distribution of all four compounds are located on the electron-donating phenyl-carbazole linked to the C2 of [1,2,4]triazolo[1,5-a]pyridine, while the LUMO orbital energy level are mainly distributed on the [1,2,4]triazolo[1,5-a]pyridine and two phenyl linked to the [1,2,4]triazolo[1,5-a]pyridine. The adequate separation of HOMO and LUMO for these four compounds ensure their bipolar carrier-transport feature, which could improve the devices performance.



Figure 4. The MOs of TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2.

Carrier transport properties. To evaluate the carrier transport properties of the four hosts, single carriers devices with the structures of indium tin oxid (ITO) / poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (40nm) / 4,4’-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) (5 nm) / host materials (60 nm)/ TAPC (5 nm)/ Al (80 nm) and ITO / 1,3,5-tri[(3-pyridyl)phen-3-yl]-benzene (TmPyPB) (5 nm) / host (60 nm) / TmPyPB (5 nm) / LiF (0.6 nm) / Al (80 nm) were fabricated.42 From Figure 5, all single carriers devices exhibit high current densities, indicating these compounds possess bipolar carrier-transporting feature.


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Figure 5. J-V curves of single carrier devices for TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2

It is noticeable that all these four compounds showed comparable electron-current densities, manifesting their comparable electron-transporting abilities. This proved that [1,2,4]triazolo[1,5-a]pyridine has excellent electron mobility. Moreover, differences in the hole mobility among these four compounds are more significant than those in the electron mobility. And interestingly, the order of the hole current density of the hole-only devices as TP26Cz2 > TP27Cz2 > TP26Cz1> TP27Cz1, which as exactly the same as the order of solvato red shifts. The differences in hole-transporting abilities of these host materials could also be explained by the linked mode of carbazole unit and [1,2,4]triazolo[1,5-a]pyridine. Carbazole linked at its 3-site C atom is more favorable for planar molecular conformation and ordered packing in the neat film, thus improved hole-transporting abilities, which had been proved by DTF



calculation and reported elsewhere.40, 44 Besides, 2,6-substituted TP derivatives exhibiting higher hole hole-transporting abilities could also be explained by more planar molecular conformation.

Electroluminescent properties. To investigate the devices performance of these bipolar hosts，we fabricated green PhOLEDs first with the structure of ITO / Pedot:PSS / TAPC / TCTA / EML / TmPyPB / LiF / Al. The EMLs consisted of TP26Cz1 (device P1, P stood for phosphorescence), TP26Cz2 (device P2), TP27Cz1 (device P3) or TP27Cz2 (device P4) doped with Ir(ppy)3. The doping concentration of 8% was applied without further study. Pedot:PSS and LiF were served as the hole injection layer (HIL) and electron injection layer (EIL), TAPC was used as hole transporting layer (HTL), TCTA was used as second HTL and electron-blocking layer (EBL),42 TmPyPB was used as electron-transporting layer (ETL) and hole-blocking layer (HBL). The energy levels and the molecular structures of each layer in the device are shown in Figure S7. To find out the optimized device performance, a series of OLEDs with different thickness of HTL and ETL were fabricated and measured as shown in Figures S8 and S9. Finally, the configuration of ITO / Pedot:PSS (40nm) / TAPC (45nm) / TCTA (5nm) / EML (20nm) / TmPyPB (55nm) / LiF (0.6nm) / Al (80nm) was selected.


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Table 2 Electroluminescence characteristics of green OLEDs with TP26Cz1, TP26Cz2, TP27Cz1 and TP27Cz2 host materials ηext(%) Devices

Hosts

Von(V)a

ηc (cd A-1)b

ηp (lm W-1)b

CIE(x, y)c -2

ηext(%)d

-2

maximum

@1000 cd m

@5000 cd m

P1

TP26Cz1

4.2

77.0(±1.5%)

43.2(±1.5%)

22.4(±1.9%)

22.3(±1.8%)

21.2(±1.2%)

0.28, 0.64

19.1 – 28.6

P2

TP26Cz2

3.1

90.3(±2.1%)

69.6(±2.3%)

25.6(±2.8%)

25.1(±2.6%)

25.2(±1.9%)

0.28, 0.64

19.5 – 29.3

P3

TP27Cz1

4.6

77.9(±1.5%)

40.2(±1.3%)

22.4(±1.1%)

22.0(±1.9%)

22.0(±1.6%)

0.28, 0.64

19.1 – 28.7

P4

TP27Cz2

3.6

89.4(±1.5%)

61.2(±0.5%)

25.4(±0.4%)

24.9(±1.0%)

24.2(±0.6%)

0.27, 0.64

19.3 – 28.9

G1e

o-CzTP

3.4

83.4

62.1

25.0

24.5

19.9

0.25, 0.64

NA

G4e

m-CzTP

4.6

76.5

42.8

22.3

20.5

17.4

0.26, 0.64

NA

F1

TP26Cz1

4.2

52.8(±2.2%)

26.7(±1.7%)

15.5(±1.6%)

15.4(±1.6%)

11.9(±2.5%)

0.29, 0.60

14.7 – 22.1

F2

TP26Cz2

3.2

49.7(±1.5%)

36.2(±1.1%)

15.3(±0.7%)

13.8(±1.1%)

11.3(±1.3%)

0.30, 0.60

13.5 – 23.0

F3

TP27Cz1

4.5

48.4(±1.5%)

24.9(±2.2%)

14.2(±2.5%)

13.0(±2.9%)

7.9(±3.8%)

0.31, 0.60

14.3 – 21.4

F4

TP27Cz2

3.8

52.2(±1.5%)

33.1(±1.5%)

15.4(±2.0%)

12.4(±1.6%)

6.7(±4.4%)

0.31, 0.60

14.5 – 21.8

a

Von, turn-on voltage, at 1 cd m-2;

b

Maximum value.

c

Recorded at 6 V.

d

Calculated from the equation: ηext = ηop× ηPLQY× ηr× γ

e

Cited from ref 25



Figure 6. (a) EL spectra at 6 V; (b) Current density-voltage-luminance (J-V-L) characteristics; (c) ηc and ηext versus current density of PhOLEDs..


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The TP26Cz1 hosted device P1 and TP26Cz2 hosted device P3 demonstrate maximum efficiencies ηc of 77.0 cd A-1 (power efficiency (ηp) of 43.2 lm W-1 and ηext of 22.4%) and 77.9 cd A-1 (40.2 lm W-1 and 22.4%), respectively. Moreover, the performance of device P2 and device P4 are superior compared to those of device P1 and device P3. Device P2 based on TP26Cz2 displays the most optimal efficiencies with a maximum ηc of 90.3 cd A-1 (ηext of 25.6%, ηp of 69.6 lm W-1) and a low turn on voltage (Von) of 3.1 V. Meanwhile, device P4 based on TP27Cz2 exhibited similar performance with a turn-on voltage (Von) of 3.6 V and a maximum ηc of 89.4 cd A-1 (ηext of 25.4%, ηp of 61.2 lm W-1). The obvious lower turn-on voltage and better efficiency of P2 and P4 might be attributed to the factor that the hole mobility and electron mobility of TP26Cz2 and TP27Cz2 are more balanced than TP26Cz1 and TP27Cz1. In particular, we noticed that the device P2 exhibits more superior performance than device P4, which could be ascribed to the factor that both hole and electron transport abilities of TP26Cz2 are higher than that of TP27Cz2. These explanations could be confirmed by the J-V curves of single carrier devices of four compounds (Figure 5). To confirm the maximum ηext of our devices are reasonable, photoluminescence quantum yields (PLQYs) of doped thin-films were measured by intergrating sphere. The PLQYs of 8 wt% Ir(ppy)3-doped host films are 95.5% (TP26Cz1), 97.5% (TP26Cz2), 95.7% (TP27Cz1) and 96.4% (TP27Cz2). According to the formula: ηext = ηop× ηPLQY× ηr× γ, whrer ηop stands for optical out-coupling factor, ηPLQY stands for photoluminescence



quantum yield, ηr stands for the ratio of radiative excitons and γ stands for the carrier balance factor.44,45 Refering to perious literature, assuming the γ is 1 and ηop is in the range of 20%−30%, the theoretical maximum ηext agree well with the experimental data.11,46 Besides, as showed in Figure 6(a), the slight differences among the EL spectra of these four devices can be explain by the recombination zone shifting.

25

And from Figure S10, all devices showed

stable green emission at different electric fields.

It is particularly worth mentioning that these four devices exhibit extremely low efficiency roll-off. At the luminance of 1 000 cd m−2, device P2 exhibits ηext of 25.1%, while P4 exhibitedηext of 24.9%, which could be comparable to their maximum values. Even at the fairly high luminance of 10 000 cd m−2, P2 and P4 still showed excellent efficiency stability (ηext of 24.2% for P2, and ηext of 23.1% for P4) with the efficiency roll-off of 5% – 8%. Remarkably, the external quantum efficiencies of device P2 and P4 could be achieved to 22% when the luminance up to 20 000 cd m-2. Compared with our previously reported green devices hosted by o-CzTP and m-CzTP, these four devices exhibit lower roll-off efficiencies, which can be attributed to the higher and more balanced carrier mobilities. The excellent performance of these four devices, especially devices P2 and P4, could be attributed to the greatly balanced hole and electron transporting property of TP26Cz2 and TP27Cz2, which is favorable to balanced charge fluxes and broad recombination regions in EMLs.


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Figure 7. (a) EL spectra at 6 V (b) J-V-L characteristics; (c) ηc and ηext versus current density of TADF - OLEDs.

Inspired by the excellent performance of PhOLEDs, TADF OLEDs (devices F1 – F4, F stand for delayed fluorescence) using 4CzIPN as emitter were



Page 26 of 39

fabricated. The device structure was similar to PhOLEDs: ITO / Pedot:PSS (40nm) / TAPC (45nm) / TCTA (5nm) / hosts : 4CzIPN

(8 wt%, 20nm) /

TmPyPB (55nm) / LiF (0.6nm) / Al (80nm). And the doping concentration was 8% without optimization. The performance of TADF OLEDs are shown in Figure 7 and the data was collected in Table 2. As shown in Figure 7(a), all devices exhibit pure green light emissions at ~525nm, suggesting effective energy transfer from the host to TADF emitter. As the PhOLEDs, there are slight differences among the EL spectra of these four delayed fluorescent devices. One reason could be recombination zone shift resulting from different carrier mobilities of four host materials. In addition, the emission of TADF emitters is intrinsically originated from their charge transfer (CT) states, which can be influenced by the polar environment.47 From the Lippert-Mataga equation, the dipoles of these four hosts have much difference, which could be another reason for the differences among the EL spectra of these four delayed fluorescent devices. The turn on voltages of devices F2, F4, F1 and F3 increased in sequence, which is consistent with the behavior of phosphorescent devices. All TADF based devices demonstrat maximum ηext and ηc in the range of 14.2 – 15.5% and 48.4 – 52.8 cd A-1, respectively. The PLQYs of 8 wt% 4CzIPN- doped hosts films are in the ranges of 71.5 - 76.8%, and the theoretical maximum ηext agree well with the experimental data. More importantly, all these TADF based devices exhibited low efficiency roll-off, for example, the device F1 hosted by TP26Cz1 demonstrate ηext of 15.4% at


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luminance of 1000 cd m−2, almost same as the maximum value (15.5%). Even at the high brightness of 5000 cd m−2, the ηext could still reach 11.9% with 22.7% efficiency roll off.

Figure 8. J-V curves of hole-only device and electron-only device based on the light-emitting layers for (a) 26TPCz1; (b) 26TPCz2; (c) 27TPCz1 and (d) 27TPCz2.

However, the maximum ηexts of TADF-OLEDs (14.2 – 15.5%) cannot be compared to that of PhOLEDs (22.4 - 25.6%). The relatively lower PLQYs of



4CzIPN- doped hosts films could be one reason. Besides, since the current densities of delay fluorescent devices F1 – F4 are much lower than that of phosphorescent devices P1 – P4 at the same driving voltage, carrier trapping could be another reason.35 To further study the performance difference of our devices, single carrier devices based on the EML of the developed devices were fabricated. The hole-only devices structures are ITO / PEDOT:PSS (40nm) / TAPC (5 nm) / hosts: emitter(Ir(ppy)3 or 4CzIPN) (8 wt%, 60 nm)/ TAPC (5 nm)/ Al (80 nm) and the electron-only devices structures are ITO / TmPyPB (5 nm) / hosts: emitter(Ir(ppy)3 or 4CzIPN) (8 wt%, 60 nm) / TmPyPB (5 nm) / LiF (0.6 nm) / Al (80 nm). As shown in Figure 8, when doped with Ir(ppy)3, all hole current densities and electron current densities drop slightly by a similar order of magnitude in comparison to that of the no-doped device. It seems that Ir(ppy)3 did not affect the carrier balance of EMLs significantly. But when doped with 4CzIPN, electron current densities of all four devices drop significantly. Since the LUMO energy levels of these four hosts are in the range of 2.34 – 2.57 eV, which have large difference with the LUMO energy level of 4CzIPN (ELUMO= -3.4 eV), 4CzIPN could act as a electron-trapping center in these host materials.

CONCLUSIONS

In summary, a series of bipolar hosts using [1,2,4]triazolo[1,5-a]pyridine as electron-transporting unit and carbazole as hole-transporting unit were


Page 28 of 39

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synthesized.

All

four

hosts

exhibited

high

T g,

Td and

appropriate

HOMO/LUMO energy levels compared to adjacent HTL and ETL. Notably, the linkage mode between [1,2,4]triazolo[1,5-a]pyridine and carbazole imposed remarkably influence on the molecular structures and thus hole-transporting abilities. Compared with carbazole linked at 9-site N atom, carbazole linked at its 3-site C atom is more beneficial to hole transportation. And specifically balanced charge-transportation are realized in TP26Cz2 and TP27Cz2. High efficiencies (ηext of 22.4 %, 25.6%, 22.4% and 25.4% for respective four hosts) and excellent efficiency roll-off were achieved for these compounds hosted green PhOLEDs, particularly for the devices based on TP26Cz2 and TP27Cz2. Even at the luminance of 10000 cd m−2, the ηext of TP26Cz2 hosted devices still remained at 24.2%, which is only reduced by 5% relative to the maximum. Besides, TADF based OLEDs also exhibit superior EL performance with the ηext of 14.2 – 15.5% and low efficiency roll-off. For device F1 hosted by TP26Cz, the ηext still remain at 15.4% (maximum ηext of 15.5%) at a luminance of 1000 cd m−2, and fairly slightly decreased to 11.9% at the high brightness of 5000 cd m−2. Those results indicated that these [1,2,4]triazolo[1,5-a]pyridine based host materials employed promising potentials for the OLEDs application in high-resolution displays and solid state illumination.

ASSOCIATED CONTENT

Supporting Information.



Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000.

General information; synthesis and characterization; device fabrication and performance measurements; PL spectra in various solutions; energy diagram of device; device performance with different thickness of HTL and ETL; EL spectra at different voltages; device performance measured many times under the same conditions.

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected] (J.C.)

* E-mail: [email protected] (J.H.)

* E-mail: [email protected] (J.S.)

ORCID

Jianhua Su: 0000-0002-4746-6022

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the fnancial support from National Natural Science Foundation of China (Nos. 21790361 and 21602093).


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[1,2,4]Triazolo[1,5-a]pyridine-Based Host Materials for Green

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