Novel Hole-Transporting Materials with High Triplet Energy for Highly

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Novel Hole-Transporting Materials with High Triplet Energy for Highly Efficient and Stable Organic Light-Emitting Diodes Hirohiko Fukagawa, Takahisa Shimizu, Hiroyuki Kawano, Shota Yui, Toshinobu Shinnai, Arata Iwai, Kazuhiko Tsuchiya, and Toshihiro Yamamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05099 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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The Journal of Physical Chemistry

Novel Hole-Transporting Materials with High Triplet Energy for Highly Efficient and Stable Organic Light-Emitting Diodes

Hirohiko Fukagawa*,†, Takahisa Shimizu†, Hiroyuki Kawano#, Shota Yui#, Toshinobu Shinnai‡, Arata Iwai‡, Kazuhiko Tsuchiya‡ and Toshihiro Yamamoto†

†

Japan Broadcasting Corporation (NHK), Science and Technology Research Laboratories,

1-10-11, Kinuta, Setagaya-ku, Tokyo 157-8510, Japan

#

Tokyo University of Science, 1–3 Kagurazaka, Tokyo 162-8610, Japan

‡

Kanto Chemical Co., Inc., Central Research Laboratory Technology and Development Division,

1-7-1 Inari, Soka-city, Saitama 340-0003, (Japan)

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ABSTRACT Demonstration of highly efficient organic light-emitting diodes (OLEDs) is becoming commonplace; however, there have been few reports on hole-transporting materials (HTMs) designed for highly efficient and stable green OLEDs. Here, operationally stable HTMs with high triplet energy were synthesized by incorporating dibenzothiophene and dibenzofuran into hole-transporting amino groups. The triplet energy of the amine derivative with dibenzothiophene was increased from 2.35 eV to 2.56 eV by introducing o,o'-quaterphenyl without impairing the stability. Since the largest triplet energy of the synthesized HTMs is 2.59 eV, the triplet excitons of green phosphorescent emitters and thermally activated delayed fluorescence (TADF) emitters are confined effectively. The operational stability of the phosphorescent OLED (PHOLED) using the synthesized HTM was about 15 times longer than that of the PHOLED using 2,2'-bis(3-ditolylaminophenyl)-1,1'-biphenyl. The optimized green PHOLED exhibits EQE of over 20% for a luminance of 10 to 10,000 cd m–2 and an expected half lifetime of over 10,000 h with an initial luminance of 1,000 cd m–2. The synthesized HTM is effective for improving the efficiency of OLEDs incorporating a green TADF emitter, as well as green phosphorescent OLEDs.

INTRODUCTION Organic light-emitting diodes (OLEDs) have been widely studied in recent decades as a key technology for next-generation displays and lighting.1 The OLEDs utilizing phosphorescent emitters and thermally activated delayed fluorescence (TADF) emitters have attracted much attention owing to their exceptionally high theoretical internal quantum efficiency of 100%, and their application to commercialized devices is expected.2-5 To demonstrate highly efficient and operationally stable OLEDs, the materials used in emitting layers (EMLs), such as emitter dopants and surrounding host materials, have been intensively studied.6-11 On the other hand, many carrier-transporting materials (CTMs), such as hole-transporting materials (HTMs) and 2 ACS Paragon Plus Environment

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electron-transporting materials (ETMs), suitable for highly efficient OLEDs have also been developed.6,7 The CTMs that can confine charges and excitons inside the EML are essential for obtaining maximum efficiency from the emitter. In particular, triplet excitons should be confined to obtain high efficiency not only in phosphorescent OLEDs (PHOLEDs) but also in the OLEDs incorporating TADF emitters.12,13 However, there are few reports on CTMs with high triplet energy (ET) suitable for an operationally stable OLED, which is essential for the practical use of PHOLEDs and OLEDs incorporating TADF emitters. The lack of stable CTMs has also stymied the development of emitting materials with high operational stability since the operational stability of emitting layer materials cannot be examined properly if the stability of CTMs is poor. Actually, little has been discussed about the operational lifetime of green/blue OLEDs, although there have been many reports on emitting layer materials suitable for highly efficient OLEDs.6,7,11 The typical HTMs with high ET reported so far are phenylamine derivatives. On the other hand, the reported ETMs with high ET are pyridine derivatives. In recent years, ETMs with high ET have been intensively developed compared with HTMs.6,7,14-17 As typified by 1,1-bis[(4-ditolylamino)

phenyl]cyclohexane

(TAPC)

and

2,2'-bis(3-ditolylaminophenyl)-1,1'-biphenyl (3DTAPBP),18-20 the HTMs that were developed many years ago are often used even in recent highly efficient OLEDs.6,7,21-23 Since the stability of such phenylamine derivatives with high ET is poor,24,25 the stability of the host/emitting materials

has

had

to

be

evaluated

in

low-efficiency

OLEDs

using

4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD).26-29 As has been noted, novel HTMs suitable for highly efficient and stable OLEDs had rarely been discussed until recently. In 2013, on the other hand, we first reported that the amine derivative with dibenzothiophene acts as an effective HTM suitable for an efficient and stable green PHOLED.30 However, the ET of the

reported

amine

derivative,

N4,N4'-bis(dibenzo[b,d]thiophen-4-yl)-N4,N4'-diphenylbiphenyl-4,4'-diamine (DBTPB), is 2.35 3 ACS Paragon Plus Environment


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eV, which is insufficient for confining completely the triplet exciton of green emitters. Operationally stable HTMs with higher ET are essential for demonstrating highly efficient and stable green/blue OLEDs. In

the

work

reported

here,

novel

amine

derivatives

based

on

DBTPB,

N4,N4'-bis(dibenzo[b,d]thiophen-4-yl)-2,2'-dimethyl-N4,N4'-diphenyl-[1,1'-biphenyl]-4,4'-diami ne

(4DBTPBD),

N3,N3'''-bis(dibenzo[b,d]thiophen-4-yl)-N3,N3'''-diphenyl-[1,1':2',1'':2'',1'''-quaterphenyl]-3,3'''-d iamine

(4DBTP3Q)

and

N3,N3'''-bis(dibenzo[b,d]furan-4-yl)-N3,N3'''-diphenyl-[1,1':2',1'':2'',1'''-quaterphenyl]-3,3'''-diam ine (4DBFP3Q) were synthesized as high-triplet-energy HTMs for efficient and stable OLEDs. The PHOLED using 4DBFP3Q as an HTM showed a high external quantum efficiency (EQE) of 22%, whereas that of the PHOLED using DBTPB was 12% in a similar device configuration. The operational lifetime of the PHOLED using 4DBFP3Q was about 15 times longer than that of the PHOLED using 3DTAPBP. In addition, the synthesized amine derivative is confirmed to be effective for confining the excitons of the green TADF emitter in the OLED. This is the first publication on the HTMs, which enable us to demonstrate highly efficient and stable green OLEDs.

RESULTS AND DISCUSSION Synthesis and Characterization. In a previous study on HTMs, an amine derivative with dibenzothiophene such as DBTPB was found to be one of the promising HTMs for highly efficient and stable PHOLEDs; however, the ET of DBTPB is insufficient to confine the triplet exciton of green emitters.30 Thus, 4DBTPBD, 4DBTP3Q and 4DBFP3Q were designed to enlarge the bandgap (Eg) and ET compared with DBTPB. The HTMs were synthesized by Suzuki–Miyaura cross-coupling and Buchwald–Hartwig amination31,32 as shown in Scheme 1. To divide the π-conjugated system of DBTPB, dimethylbiphenyl and o,o'-quaterphenyl were 4 ACS Paragon Plus Environment

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introduced into 4DBTPBD and 4DBTP3Q, respectively. In addition, we evaluated the applicability of dibenzofuran instead of dibenzothiophene by synthesizing 4DBFP3Q. Photophysical Properties. The photophysical properties of the synthesized HTMs were analyzed by UV–vis and photoluminescence (PL) measurements. Figure 1 shows the UV–vis, fluorescence and phosphorescence spectra of the synthesized HTMs. The Eg values of 4DBTPBD, 4DBTP3Q and 4DBFP3Q were calculated from the absorption edge of the UV–vis spectra to be 3.2, 3.2 and 3.3 eV, respectively. Compared with the DBTPB having a Eg of 3.0 eV, Eg was increased in the developed HTMs because of the divided π-conjugated system in the molecular structure (Figure S1 of the Supporting Information). As a result, not only the fluorescent peaks but also the phosphorescent peaks of 4DBTPBD and 4DBTP3Q are shifted to the short-wavelength side compared with those of DBTPB. The ET values of 4DBTPBD, 4DBTP3Q and 4DBFP3Q were calculated from the emission peaks of the phosphorescent spectra to be 2.47, 2.56 and 2.59 eV, respectively. Thus, the triplet excitons of the green emitters are expected to be confined effectively by using 4DBTP3Q and 4DBFP3Q in OLEDs. Since the ET values of 4DBTP3Q and 4DBFP3Q are relatively large, the introduction of an o,o'-quaterphenyl is particularly suitable for increasing the ET values of amine derivatives. We see from the PL spectra of 4DBTP3Q and 4DBFP3Q that the dibenzofuran substituent is effective in demonstrating HTM with higher Eg and ET than the dibenzothiophene substituent. HTM-dependent device characteristics of the green PHOLEDs. We first investigated the HTM-dependent device characteristics of the green PHOLEDs to examine the effects of the novel HTMs on the efficiency and operational lifetime. The device configuration of the fabricated PHOLEDs is ITO/PEDOT:PSS (35 nm)/α-NPD

(20 nm)/HTM (10

nm)/PIC-TRZ:Ir(mppy)3 (1 wt%, 30 nm)/TPBi (35 nm)/LiF (1 nm)/Al (100 nm), where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid), PIC-TRZ Ir(mppy)3

is is

2-biphenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine,33 fac-tris(3-methyl-2-phenylpridinato-N,C2'-)iridium(III) 5 ACS Paragon Plus Environment

and

TPBi

is


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1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene. A hole-transporting layer (HTL) consists of α-NPD and an HTM to eliminate the effect of the difference in hole injection from PEDOT:PSS on the device characteristics (Figure S2 of the Supporting Information). In addition to the bipolar host

PIC-TRZ,

we

sometimes

used

the

electron-transporting

host

bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2).34 Since the ETs of both PIC-TRZ and Bepp2 are larger than that of Ir(mppy)3, the effects of HTM on the device characteristics can be evaluated properly.33,35 The reason why we mainly used 1 wt%-Ir(mppy)3-doped PIC-TRZ as the emitting layer in this study will be discussed below. We used TPBi as the electron-transporting layer since its operational stability is relatively high among the commercially available ETMs (Figure S3 of the Supporting Information). Figure 2(a) shows examples of current density (J)–voltage (V)–luminance (L) characteristics of the PHOLEDs, where α-NPD, 3DTAPBP, DBTPB, 4DBTPBD, 4DBTP3Q and 4DBFP3Q are used as HTMs. The chemical structure of 3DTAPBP is similar to those of 4DBTP3Q and 4DBFP3Q, and the ET for 3DTAPBP is larger than that for 4DBFP3Q. In most of these studies, the HTLs are formed by combining α-NPD and HTMs, as shown in Fig. 2(a); however, the hole injection efficiency and the hole mobility of the synthesized HTMs seem to be similar to those of α-NPD because the J–V–L characteristics of the PHOLED consisting of only 4DBTP3Q are superior to those of the PHOLED consisting of α-NPD and 4DBTP3Q (Figure S4 of the Supporting Information). The increase in the driving voltage in the PHOLED consisting of α-NPD and 4DBTP3Q may be caused by the existence of an energy barrier at the α-NPD/4DBTP3Q interface. The EQE of the PHOLEDs strongly depends on the HTM, as shown in Fig. 2(b). The tendency of the HTM-dependent EQE is almost the same even in the PHOLEDs, whose emitting layer consists of 6 wt%-Ir(mppy)3-doped Bepp2 (Figure S5 of the Supporting Information). On the other hand, we can observe the host dependence of the EQE value particularly clearly in the PHOLEDs using an HTM with a lower ET. For instance, the EQE of the DBTPB-based PHOLED employing 6 wt%-Ir(mppy)3-doped Bepp2 is about 5% 6 ACS Paragon Plus Environment

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higher than that of the DBTPB-based PHOLED employing 1 wt%-Ir(mppy)3-doped PIC-TRZ. This difference in EQE may be caused by the difference in the dopant concentration. The distance of energy transfer from PIC-TRZ to 1 wt% Ir(mppy)3 is longer than that from Bepp2 to 6 wt% Ir(mppy)3. In the case of long-range energy transfer, it is reasonable to assume that the excitons tend to be quenched by the lower ET of HTM. Thus, 1 wt%-Ir(mppy)3-doped PIC-TRZ is an ideal emitting layer for evaluating the effect of HTM on the EQE of the PHOLED even though PIC-TRZ is a bipolar host.36 Actually, we succeeded in observing the difference in EQE between the PHOLED using 4DBTP3Q and that using 4DBFP3Q, although the difference in ET between 4DBTP3Q and 4DBFP3Q is only 0.03 eV. The

key

data

for

the

six

PHOLEDs

and

N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)-benzidine

the

PHOLEDs

using

(TPD)

and

N4,N4,N4',N4'-tetra(biphenyl-4-yl)biphenyl-4,4'-diamine (TPD15) as the HTM are listed in Table 1 (Figure S6 of the Supporting Information). The device configuration of the PHOLED using TPD and TPD15 is the same as that shown in the inset of Fig. 2(a). We can see from Table 1 and Fig. 3 that the EQE is almost proportional to ET for the HTMs, and the increase in EQE is saturated at approximately ET for 4DBFP3Q. Since the LUMO levels of DBTPB, 4DBTPBD, 4DBTP3Q and 4DBFP3Q are almost the same (Figure S2 of the Supporting Information), we concluded that the EQE has a strong correlation with ET for the HTMs rather than the LUMO level in this study. The triplet excitons of Ir(mppy)3 are expected to be confined completely by using 4DBFP3Q as the HTM. The fact that the EQEs of the PHOLEDs using 4DBTPBD and 4DBTP3Q are lower than those of the PHOLEDs using 4DBFP3Q and 3DTAPBP suggests that the ideal ET of the CTM for confining the triplet excitons in the phosphorescent emitter is 0.2 eV larger than that of the emitter.37 Although a highly efficient and stable orange PHOLED has recently been reported, the HTM used in the previous study can sometimes insufficient to confine the triplet excitons of green phosphorescent emitters since its ET was only 2.49 eV.38 7 ACS Paragon Plus Environment


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The operational lifetimes (LT50s) of PHOLEDs, defined as the time for the luminance to decay to 50% of the initial luminance of 1,000 cd m−2, were estimated using the well-known stretched exponential decay function (Figure S7 of the Supporting Information).39 For the PHOLEDs using DBTPB, 4DBTPBD, 4DBTP3Q and 4DBFP3Q, their LT50 values are also proportional to ET for the HTM as summarized in Table 1. A similar tendency of LT50 is observed in the PHOLED employing 6 wt%-Ir(mppy)3-doped Bepp2 (Figure S8 of the Supporting Information). Although the operational lifetime of the PHOLED using DBTPB is longer than those of the other OLEDs using several other HTMs,30 the LT50s of the PHOLEDs using 4DBTPBD, 4DBTP3Q and 4DBFP3Q are longer than that of the PHOLED using DBTPB. The PHOLED using 4DBFP3Q exhibits the longest LT50, which is about 15 times longer than that of the PHOLED using 3DTAPBP. A similar short LT50 for a PHOLED using 3DTAPBP, where 6 wt%-Ir(mppy)3-doped Bepp2 was used as the emitting layer, has already been reported.30 We compared the resistance of 4DBFP3Q and 3DTAPBP to electrons by fabricating electron-only devices (EODs) (Figure S9 of the Supporting Information). The role of HTMs in the PHOLEDs is not only to transport holes but also to block electrons. It can be seen from Fig. S9 that the resistance of 4DBFP3Q to electrons is higher than that of 3DTAPBP since the driving voltage of the EOD using 3DTAPBP increases more rapidly under a constant dc current of 10 mA/cm2 Thus, we can conclude that the resistance of HTMs to electrons is increased by utilizing a dibenzofuran substituent, resulting in the longer LT50 of the PHOLED using 4DBFP3Q than that of the PHOLED using 3DTAPBP. In addition, the fact that the LT50s of the PHOLEDs using TPD and 3DTAPBP are shorter than those of the other PHOLEDs suggests that the stability of HTMs with a larger π-conjugated system outward is higher than that of HTMs with only the phenyl substituent outward. On the other hand, dimethylbiphenyl and o,o'-quaterphenyl have been widely used to increase ET for CTMs and hosts; however, their effect on the operational stability has never been discussed.6 As can be seen from the experimental results of 4DBTPBD and 4DBTP3Q, the ET of the amine derivatives with dibenzothiophene can be 8 ACS Paragon Plus Environment

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increased by introducing dimethylbiphenyl and o,o'-quaterphenyl without impairing the stability. Moreover, we succeeded in observing the difference in the operational stability between a dibenzothiophene derivative and a dibenzofuran derivative. Although many phosphorescent host materials employing dibenzothiophene and dibenzofuran have been reported, little has been discussed on their operational stability.40-43 Our experimental results clearly demonstrate that the operational stability of the amine derivatives with dibenzofuran is similar to or greater than that of the amine derivatives with dibenzothiophene. Thus, the amine derivatives with dibenzofuran are demonstrated to be a basic skeleton for synthesizing an operationally stable HTM with high ET. Device characteristics of the optimized green PHOLED. A highly efficient and operationally stable green PHOLED with a greatly reduced amount of phosphorescent emitter was demonstrated by optimizing the device configuration using 4DBFP3Q. The device configuration of the optimized PHOLEDs was ITO/Clevios HIL 1.5 (30 nm)/α-NPD (20 nm)/4DBFP3Q (10 nm)/PIC-TRZ: Ir(mppy)3 (1 wt%, 25 nm)/TPBi (35 nm)/LiF (1 nm)/Al (100 nm), where Clevios HIL 1.5 (supplied by Heraeus Holding GmbH, 30 nm) is used as the HIL to demonstrate an operationally stable PHOLED. The EQE-L characteristics are shown in Figure 4 (Figure S10 of the Supporting Information). As can be seen in Fig. 4 the EQE of the optimized PHOLED exceeds 20% over a wide range of luminances from 10 to 10,000 cd m−2. The effect of the triplet-triplet annihilation is quite small owing to the rapid energy transfer from PIC-TRZ to Ir(mppy)3.36 The maximum current efficiency of 81 cd A−1 is obtained at a luminance of approximately 1,000 cd m−2. The luminance versus time characteristics of the PHOLEDs under a constant dc current with an initial luminance of 1,000 cd m−2 are shown in the inset. The LT50 of the optimized device was estimated to be over 10,000 h.39 Device characteristics of the green OLED incorporating a TADF emitter. The applicability of the synthesized HTM to the OLED incorporating a TADF emitter is also examined. The device configuration of the fabricated OLEDs is ITO/PEDOT:PSS (35 nm)/HTM 9 ACS Paragon Plus Environment


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(30 nm)/Ad-CzPd:4CzIPN (6 wt%, 25 nm)/TPBi (45 nm)/LiF (1 nm)/Al (100 nm), where Ad-CzPd

is

2-[(4-carbazolyl-9-yl)phenyl]-2-[(4-pyridoindolyl-9-yl)phenyl]adamantane

and

4CzIPN is (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile.4,44 We selected Ad-CzPd as a host since Ad-CzPd has bipolar carrier transportability and high ET.44 The EQE–J characteristics of the PHOLEDs are shown in Figure 5. We see from Fig. 5 that the OLED using 4DBFP3Q exhibits a much higher EQE than that using α-NPD. The maximum EQE of about 19% is achieved, which is almost the same as the EQE of the previously reported OLED incorporating 4CzIPN.4 In addition, the OLED using 4DBFP3Q exhibits greater operational stability than that of the OLED using α-NPD (Figure S11 of the Supporting Information). Since CTMs with high ET are essential for improving the efficiency of OLEDs incorporating a TADF emitter,12 the HTMs synthesized in this study can also contribute to demonstrating highly efficient and operationally stable OLEDs incorporating TADF emitters.

CONCLUSIONS In summary, we proposed novel HTMs, namely, 4DBTPBD, 4DBTP3Q and 4DBFP3Q, whose good hole transportability, high ET and high operational stability were demonstrated. Our results show that the ET of the amine derivatives with dibenzothiophene can be increased by introducing dimethylbiphenyl and o,o'-quaterphenyl without impairing the stability. The amine derivatives with dibenzofuran were found to have a more favorable basic skeleton for synthesizing a stable HTM with high ET than the amine derivatives with dibenzothiophene. The optimized green PHOLED using 4DBFP3Q exhibited high efficiency and high stability. EQE of over 20% was obtained from a luminance of 10 to 10,000 cd m−2 with the maximum current efficiency of 81 cd A–1. The LT50 of the PHOLED using 4DBFP3Q was about 15 times longer than that of the PHOLED using 3DTAPBP, and the estimated LT50 of the optimized PHOLED was over 10,000 h with an initial luminance of 1,000 cd m–2. The synthesized HTM was demonstrated to be effective for improving the efficiency of OLEDs incorporating a green TADF 10 ACS Paragon Plus Environment

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emitter, as well as green PHOLEDs. The proposed HTMs must be a standard material not only for realizing highly efficient and stable OLEDs but also for evaluating the efficiency and stability of novel materials for emitting layers such as host materials and phosphorescent/TADF emitters.

EXPERIMENTAL SECTION Synthesis: Synthesis of N-phenyldibenzo[b,d]thiophen-4-amine (1). A mixture of 4-bromodibenzo[b,d]thiophen, aniline (50 mmol), palladium (II) acetate (1.0 mmol), tri-t-butylphosphine (1.0 mmol), dry toluene (150 ml) and potassium t-butoxide (50 mmol) was added to a flask. The mixture was degassed and then stirred at 100 °C for 7 h under an argon atmosphere. The crude product was purified by column chromatography on silica gel with a 1/3 dichloromethane –n-hexane mixture, yielding a white powder. Synthesis

of

N4,N4'-bis(dibenzo[b,d]thiophen-4-yl)-2,2'-dimethyl-N4,N4'-diphenyl-[1,1'-biphenyl ]-4,4'-diamine

(4DBTPDB).

A

mixture

of

(8.0

1

mmol),

4,4’-diido-2,2’-dimethyl-1,1’-biphenyl (4.0 mmol), palladium (II) acetate (0.16 mmol), tri-t-butylphosphine (1.0 mmol), dry toluene (50 ml) and potassium t-butoxide (8.0 mmol) was added to a flask. The mixture was degassed and then stirred at 100 °C for 10 h under an argon atmosphere. Purification by column chromatography on silica gel with a 1/3 dichloromethane – n-hexane mixture yielded a white powder. Synthesis

of

3,3’’’-dibromo-1,1’:2’,1’’:2’’,1’’’-quaterphenyl

(3).

2,2’-dibromo-1,1’-biphenyl (42 mmol) and diethyl ether (100 mL), and hexane (1.6 mol

A

mixture

of

n-butyllithum in

L-1,48 mmol) was then added and stirred at -15 °C for 1 h. Trimethyl borate

(48 mmol) was added to the mixture and then allowed to warm up to room temperature, followed 11 ACS Paragon Plus Environment


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Page 12 of 27

by stirring for 18 h. The solvent was removed by distillation to obtain a precursor of boronic acid 2.

A mixture of 2, m-dibromobenzene (72 mmol), tetrakis (triphenylphosphine) palladium(0)

(0.48 mmol), toluene (100 mL) and aqueous K2CO3 (1.0 M, 50 mL) was added to a flask. The mixture was degassed and heated under reflux for 5 h under a nitrogen atmosphere. The crude product was purified by column chromatography on silica gel with a 1/3 dichloromethane – n-hexane mixture, yielding a white powder. Synthesis

of

N3,N3'''-bis(dibenzo[b,d]thiophen-4-yl)-N3,N3'''-diphenyl-[1,1':2',1'':2'',1'''-quaterph enyl]-3,3'''-diamine (4DBTP3Q). A mixture of 3 (6.0 mmol), 1 (12.0 mmol), palladium (II) acetate (0.12 mmol), tri-t-butylphosphine (0.12 mmol), dry toluene (50 ml) and potassium t-butoxide(12.0 mmol) was added to a flask. The mixture was degassed and then stirred at 100 °C for 7 h under an argon atmosphere. The crude product was purified by column chromatography on silica gel with a 1/3 dichloromethane –n-hexane mixture, yielding a white powder 4DBTPBD: 1H-NMR (400 MHz, CD2Cl2) δ: 1.99(s, 6H), 6.91(dd, J=2.3Hz, 8.2Hz, 2H), 7.00(t, J=8.0Hz, 6H), 7.07(d, J=8.7Hz, 4H), 7.25(t, J=8.0Hz, 4H), 7.33(d, J=6.4Hz, 2H), 7.40-7.51(m, 6H), 7.70(dd, J=1.6Hz, 6.6Hz, 2H), 8.04(d, J=7.8Hz, 2H), 8.18(dd, J=1.6Hz, 6.6Hz,

2H)

13

C-NMR (100 MHz, CD2Cl2) δ: 118.4, 120.6, 122.0, 122.7, 123.0, 123.1, 124.6, 124.7, 125.9,

126.1, 127.2, 129.4, 130.7, 135.9, 136.4, 137.2, 137.6, 138.1, 140.2, 142.3, 145.8, 147.0 Ms(EI-Q):m/z 728 [M]+・. 4DBTP3Q: 1H-NMR (400 MHz, CD2Cl2) δ: 6.40(t, J=2.0Hz, 2H), 6.52(d, J=7.8Hz, 6H), 6.73(d, J=7.7Hz, 4H), 6.84-6.92(m, 4H), 6.92-7.05(m, 12H), 7.10(t, J=8.0Hz, 2H), 7.32(t, J=7.8Hz, 2H), 7.38-7.46(m, 4H), 7.68(d, J=7.3Hz, 2H), 7.95(d, J=7.3Hz, 2H), 8.14(d, J=7.3Hz, 2H) 13

C-NMR (100 MHz, CD2Cl2) δ: 118.5, 121.4, 122.0, 122.4, 122.6, 123.0, 124.2, 124.6, 124.7,

126.0, 126.1, 127.0, 127.1, 127.3, 128.9, 129.4, 130.1, 131.8, 136.0, 137.4, 138.0, 139.7, 140.2, 140.8, 141.8, 142.8, 145.8, 146.8 12 ACS Paragon Plus Environment

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Ms(EI-Q):m/z 852 [M]+・ 4DBFP3Q: 1H-NMR (400 MHz, CD2Cl2) δ:6.41(s, 2H), 6.50(d, J=7.8Hz, 2H), 6.73(d, J=7.8Hz, 4H), 6.82(d, J=9.6Hz, 2H), 6.89(t, J=7.3Hz, 4H), 6.99-7.12(m, 14H), 7.19(t, J=7.8Hz 2H), 7.31-7.40(m, 6H), 7.74(dd, J=0.92Hz, 7.8Hz, 2H), 7.95(d, J=7.3Hz 2H) 13

C-NMR (100 MHz, CD2Cl2) δ:112.0, 117.4, 120.9, 121.1, 122.1, 122.2, 123.1, 124.1, 124.2,

124.5, 124.7, 126.3, 126.4, 127.0, 127.3, 127.5, 128.9, 129.3, 130.1, 131.7, 131.8, 139.9, 141.0, 142.7, 146.4, 147.4, 151.6, 156.3 Ms(EI-Q):m/z 820[M]+・. Device Fabrication and Measurements The OLEDs were developed on a glass substrate coated with a 150-nm-thick indium tin oxide (ITO) layer. Prior to the fabrication of the organic layers, the substrate was cleaned with ultrapurified water and organic solvents, and treated with a UV-ozone ambient. To reduce the possibility of electrical shorts within the device, PEDOT:PSS or Clevios HIL 1.5 was spun onto the substrate. The other organic layers were sequentially deposited onto the substrate without breaking the vacuum at a pressure of about 10-5 Pa. The devices were encapsulated using a UV-epoxy resin and a glass cover within a nitrogen atmosphere after cathode formation. The EL spectra and luminance were measured with a spectroradiometer (Minolta CS-1000). A digital source meter (Keithley 2400) and a desktop computer were used to operate the devices. We assumed that the emission from the OLED was isotropic, such that the luminance was Lambertian, and calculated the EQE from the luminance, current density and EL spectra. The effective area of the OLED was 0.3 × 0.3 cm2.

ASSOCIATED CONTENT Supporting Information Computational details, energy diagrams, operational stability of the OLEDs using several electron transporting layers, device characteristics using an electron-transporting host, and the 13 ACS Paragon Plus Environment


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detailed device performances. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENTS The authors thank Heraeus, Holding GmbH for supplying Clevios HIL 1.5. Part of this work was supported by the Strategic Information and Communications R&D Promotion Programme (SCOPE) of the Ministry of Internal Affairs and Communications of Japan.

REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151-154. (3) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-Efficiency Fluorescent Organic Light-Emitting Devices Using a Phosphorescent Sensitizer. Nature 1998, 403, 750-753. (4) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes From Delayed Fluorescence. Nature 2012, 492, 234-238. (5) Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic Light-Emitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nature Photon. 2012, 6, 253-258. (6) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent Progresses on 14 ACS Paragon Plus Environment

Page 14 of 27

Page 15 of 27

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


Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926-952. (7) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 21, 3169-3190. (8) Moon, C.-K; Kim, K.-H.; Lee, J. W.; Kim, J.-J. Influence of Host Molecules on Emitting Dipole Orientation of Phosphorescent Iridium Complexes. Chem. Mater. 2015, 27, 2767-2769. (9) Fleetham, T.; Li, G.; Wen, L.; Li, J. Efficient “Pure” Blue OLEDs Employing Tetradentate Pt Complexes with a Narrow Spectral Bandwidth. Adv. Mater. 2014, 26, 7116-7121. (10) Hashimoto, S.; Ikuta, T.; Shiren, K.; Nakatsuka, S.; Ni, J.; Nakamura, M.; Hatakeyama, T. Triplet-Energy Control of Polycyclic Aromatic Hydrocarbons by BN Replacement: Development of Ambipolar Host Materials for Phosphorescent Organic Light-Emitting Diodes. Chem. Mater. 2014, 26, 6265-6271. (11) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W. Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Adv. Mater. 2014, 26, 7931-7958. (12) Nakanotani, H.; Masui, K.; Nishide, J.; Shibata, T.; Adachi, C. Promising Operational Stability of High-Efficiency Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence. Sci. Rep. 2013, 3, 2127. (13) Swensen, J. S.; Polikarpov, E.; Ruden, A. V.; Wang, L.; Sapochak, L. S.; Padmaperuma, A. B. Improved Efficiency in Blue Phosphorescent Organic Light-Emitting Devices Using Host Materials of Lower Triplet Energy than the Phosphorescent Blue Emitter. Adv. Funct. Mater. 2011, 21, 3250-3258. (14) Su, S.-J.; Sasabe, H.; Pu, Y.-J.; Nakayama, K.; Kido, J. Tuning Energy Levels of Electron-Transport Materials by Nitrogen Orientation for Electrophosphorescent Devices with an ‘Ideal’ Operating Voltage. Adv. Mater. 2010, 22, 3311-3316. 15 ACS Paragon Plus Environment


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

(15) Ahmed, E.; Earmme, T.; Jenekhe, S. A. New Solution-Processable Electron Transport Materials for Highly Efficient Blue Phosphorescent OLEDs. Adv. Funct. Mater. 2011, 21, 3889-3899. (16) Lee, J.-H.; Cheng, S.-H.; Yoo, S.-J.; Shin, H.; Chang, J.-H.; Wu, C.-I.; Wong, K.-T.; Kim, J.-J. An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage. Adv. Funct. Mater. 2015, 25, 361-366. (17) Cho, Y. J.; Lee, J. Y. Low Driving Voltage, High Quantum Efficiency, High Power Efficiency, and Little Efficiency Roll-Off in Red, Green, and Deep-Blue Phosphorescent Organic Light-Emitting Diodes Using a High-Triplet-Energy Hole Transport Material. Adv. Mater. 2011, 23, 4568-4572. (18) Borsenberger, P. M.; Pautmeier, L.; Richert, R.; Bässler, H. Hole transport in 1,1-bis(di-4-tolylaminophenyl)cyclohexane. J. Chem. Phys. 1991, 94, 8276-8281. (19) H. Bässler, Charge Transport in Random Organic Photoconductors. Adv. Mater. 1993, 5, 662-665. (20) Agata, Y.; Shimizu, H.; Kido, J. Syntheses and Properties of Novel Quarterphenylene-Based Materials for Blue Organic Light-Emitting Devices. Chem. Lett. 2007, 36, 316. (21) Sasabe, H.; Nakanishi, H.; Watanabe, Y.; Yano, S.; Hirasawa, M.; Pu, Y.-J.; Kido, J. Extremely Low Operating Voltage Green Phosphorescent Organic Light-Emitting Devices., Adv. Funct. Mater. 2013, 23, 5550-5555. (22) Seino, Y.; Sasabe, H.; Pu, Y.-J.; Kido, J.; High-Performance Blue Phosphorescent OLEDs Using Energy Transfer from Exciplex. Adv. Mater. 2014, 26, 1612-1616. (23) Liu, X.-K.; Chen, Z.; Zheng, C.-J.; Liu, C.-L.; Lee, C.-S.; Li, F.; Ou, X.-M.; Zhang, X.-H. Prediction and Design of Efficient Exciplex Emitters for High-Efficiency, Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 2378-2383. (24) Kondakov, D. Y.. Role of Chemical Reactions of Arylamine Hole Transport Materials in 16 ACS Paragon Plus Environment

Page 16 of 27

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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


Operational Degradation of Organic Light-Emitting Diodes. J. Appl. Phys. 2008, 104, 084520. (25) Gaspar, D. J. and Polikarpov, E. OLED Fundamentals: Materials, Devices, and Processing of Organic Light-Emitting Diodes; CRC Press: Boca Raton, FL, 2015; Section 8. (26) Cho, Y. J.; Yook, K. S.; Lee, J. Y. A Universal Host Material for High External Quantum Efficiency Close to 25% and Long Lifetime in Green Fluorescent and Phosphorescent OLEDs. Adv. Mater. 2014, 26, 4050-4055. (27) Zhu, Z.-Q.; Fleetham, T.; Turner, E.; Li, J.; Harvesting All Electrogenerated Excitons through Metal Assisted Delayed Fluorescent Materials. Adv. Mater. 2015, 27, 2533-2537. (28) Fleetham, T.; Huang, L.; Li, J. Tetradentate Platinum Complexes for Efficient and Stable Excimer-Based White OLEDs. Adv. Funct. Mater. 2014, 24, 6066-6073. (29) Li, G.; Fleetham, T.; Turner, E.; Hang, X.-C.; Li, J.; Highly Efficient and Stable Narrow-Band Phosphorescent Emitters for OLED Applications. Adv. Opt. Mater. 2015, 3, 390-397. (30) Fukagawa, H.; Shimizu, T.; Kiribayashi, Y.; Osada, Y.; Kamada, T.; Yamamoto, T.; Shimidzu, N.; Kurita, T. Molecular Design of Hole-Transporting Material for Efficient and Stable Green Phosphorescent Organic Light-Emitting Diodes. Appl. Phys. Lett. 2013, 103, 143306. (31) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev., 1995, 95, 2457. (32) Wolfe, J. P.; Buchwald, S. L. Palladium-Catalyzed Amination of Aryl Halides and Aryl Triflates: N-Hexyl-2-Methyl-4-Methoxyaniline and N-Methyl-N-(4-Chlorophenyl)Aniline Org. Synth., 2002, 78, 23. (33) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-Conversion of Triplet Excitons into a Singlet State and Its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 083302. 17 ACS Paragon Plus Environment


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Page 18 of 27

(34) Jeon, W. S.; Park, T. J.; Park, J. J.; Kim, S. Y.; Jang, J.; Kwon, J. H.; Pode, R. Highly Efficient Bilayer Green Phosphorescent Organic Light Emitting Devices. Appl. Phys. Lett. 2008, 92, 113311. (35) Fukagawa, H.; Shimizu, T.; Kamada, T.; Kiribayashi, Y.; Osada, Y. Hasegawa, M.; Morii, K.; Yamamoto, T. Highly Effi cient and Stable Phosphorescent Organic Light- Emitting Diodes Utilizing Reverse Intersystem Crossing of the Host Material Adv. Opt. Mater. 2014, 2, 1070-1075. (36) Fukagawa, H.; Shimizu, T.; Kamada, T.; Yui, S.; Hasegawa, M.; Morii, K.; Yamamoto, T. Highly Efficient and Stable Organic Light-Emitting Diodes with a Greatly Reduced Amount of Phosphorescent Emitter Sci. Rep. 2015, 5, 9855. (37) Tanaka, I.; Tabata, Y.; Tokito, S. Förster and Dexter Energy-Transfer Processes in Fluorescent BAlq Thin Films Doped with Phosphorescent Ir(ppy)3 Molecules. J. Appl. Phys. 2006, 99, 073501. (38) Seo, S.; Shitagaki, S.; Ohsawa, N.; Inoue, H.; Suzuki, K.; Nowatari, H.; Yamazaki, S. Exciplex-Triplet Energy Transfer: A New Method to Achieve Extremely Efficient Organic Light-Emitting Diode with External Quantum Efficiency Over 30% and Drive Voltage Below 3V. Jpn. J. Appl. Phys. 2014, 53, 042102. (39) Féry, C.; Racine, B.; Vaufrey, D.; Doyeux, H.; Cinà, S. Physical Mechanism Responsible for the Stretched Exponential Decay Behavior of Aging Organic Light-Emitting Diodes. Appl. Phys. Lett. 2005, 87, 213502. (40) Cai, X.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.; Burrows, P. E. Electron and Hole

Transport

in

a

Wide

Bandgap

Organic

Phosphine

Oxide

for

Blue

Electrophosphorescence. Appl. Phys. Lett. 2008, 92, 083308. (41) Kim, D.; Salman, S.; Coropceanu, V.; Salomon, E.; Padmaperuma, A. B.; Sapochak, L. S.; Kahn, A.; Brédas, J.-L. Phosphine Oxide Derivatives as Hosts for Blue Phosphors: A Joint Theoretical and Experimental Study of Their Electronic Structure. Chem. Mater. 2010, 22, 18 ACS Paragon Plus Environment

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247-254. (42) Jeong, S. H.; Seo, C. W.; Lee, J. Y.; Cho, N. S.; Kim, J. K.; Yang, J. H. Comparison of Bipolar Hosts and Mixed-Hosts as Host Structures for Deep-Blue Phosphorescent Organic Light Emitting Diodes. Chem. Asia J. 2011, 6, 2895-2898. (43) Dong, S.-C;. Zhang, L.; Liang, J.; Cui, L.-S.; Li, Q.; Jiang, Z.-Q.; Liao, L.-S. Rational Design of Dibenzothiophene-Based Host Materials for PHOLEDs. J. Phys. Chem. C 2014, 118, 2375-2384. (44) Fukagawa, H.; Irisa, S.; Hanashima, H.; Shimizu, T.; Tokito, S.; Yokoyama, N.; Fujikake, H. Simply Structured, Deep-Blue Phosphorescent Organic Light-Emitting Diode with Bipolar Host Material. Org. Electro. 2011, 12, 1638-1643.



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Scheme 1. Synthesis and structures of 4DBTPBD and 4DBTP3Q.


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Figure 1. Absorption spectra of hole-transporting material films measured at room temperature, fluorescence spectra of the hole-transporting material films measured at room temperature and phosphorescence spectra of the hole-transporting material films measured at 77 K.



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Figure 2. (a) Current density (left, open symbols)– and luminance (right, filled symbols)–voltage characteristics of PHOLEDs. Inset: Device configuration of PHOLEDs. (b) External quantum efficiency–current density curves of PHOLEDs.


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Figure 3. External quantum efficiencies of the fabricated PHOLEDs with various hole-transporting materials (HTMs) as a function of HTM triplet energy. The triplet energy level of Ir(mppy)3 is also shown.



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Figure 4. External quantum efficiency–luminance curve of optimized PHOLED. Inset: Luminance–time characteristics of optimized device under a constant dc current with an initial luminance of 1,000 cd m–2.


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Figure 5. External quantum efficiency–current density curves of OLEDs incorporating 4CzIPN. Inset: Molecular structures of host and emitting materials used in the OLEDs.



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Table 1. Electroluminescence data for the devices. The device configuration is PEDOT:PSS (35 nm)/α-NPD (20 nm)/HTM (10 nm)/PIC-TRZ:Ir(mppy)3 (1 wt%, 30 nm)/TPBi (35 nm)/LiF (1 nm)/Al (100 nm).

a)

HTM

Eg [eV]

ET [eV]

Tg [°C]

Voltage [V] (at 1 mA/cm2)

EQE [%] (at 1 mA/cm2)

CE [cd/A] (at 1 mA/cm2)

Lifetime [h] (LT50)

α-NPD

2.9

2.28

96

4.5

5.6

20.5

1200a)

TPD15

2.9

2.30

132

4.9

7.6

27.6

2000a)

TPD

3.1

2.35

58

4.9

8.2

30.0

400a)

DBTPB

3.0

2.35

127

4.6

9.8

35.9

2100a)

4DBTPBD

3.2

2.47

125

4.7

18.5

68.0

2500a)

4DBTP3Q

3.2

2.56

117

4.8

20.0

73.1

5300a)

4DBFP3Q

3.3

2.59

103

4.9

21.0

77.1

6700a)

3DTAPBP

3.6

2.70

87

5.2

20.6

75.5

430

Estimated value using the well-known stretched exponential decay function.39


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Table of Contents Graphic


Novel Hole-Transporting Materials with High Triplet Energy for Highly

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