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4-Diphenylaminocarbazole: Switching Substituent Position for Voltage Reduction and Efficiency Enhancement of OLEDs Shahid Ameen, Seul Bee Lee, Yong Guk Lee, Sung Cheol Yoon, Jaemin Lee, and Changjin Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18044 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Materials & Interfaces

4-Diphenylaminocarbazole: Switching Substituent Position for Voltage Reduction and Efficiency Enhancement of OLEDs Shahid Ameen,†,‡,∇ Seul Bee Lee,†,‡ Yong Guk Lee † Sung Cheol Yoon,†,‡ Jaemin Lee*,†,‡ and Changjin Lee*,†,‡

†Advanced

Materials Division, Korea Research Institute of Chemical Technology, 141 Gajeong-

ro, Yuseong-gu, Daejeon 34114, Republic of Korea

‡Department

of Advanced Materials and Chemical Engineering, UST-KRICT School, 217,

Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea

∇Department

of Chemistry, Kohat University of Science and Technology (KUST) Kohat 26000,

Pakistan Corresponding Author *E-mail: [email protected] (Jaemin Lee). *E-mail: [email protected] (Changjin Lee).

ACS Paragon Plus Environment

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KEYWORDS: carbazole, electroluminescence, host, phosphorescence, OLED

ABSTRACT: Simple but exceptionally efficient 4-diphenylaminocarbazole host material, 4DPACz, is presented and compared with its positional isomer, 1-DPACz. The shift of diphenylamino substituent from 1-position to 4-position of carbazole resulted into increase of HOMO energy level as well as increase of triplet energy level. Having high triplet energy level (2.76 eV) and well-matched HOMO energy level (-5.61 eV), 4-DPACz showed reduced driving voltage and higher efficiencies for solution-processed green PhOLEDs than PVK as well as 1DPACz. Maximum luminous, power and external quantum efficiencies reaching to 47.9 cd A-1, 25.2 lm W-1 and 14.3%, respectively, were achieved with a device configuration of [ITO / PEDOT:PSS / 4-DPACz:Ir(mppy)3 / TPBi / CsF / Al]. Additional enhancement of efficiencies of 4-DPACz was verified when incorporating another dopant, Ir(Si-bppy)2(acac), resulting in 59.1 cd A-1, 29.5 lm W-1 and 15.8%. Furthermore, reduced efficiency roll-off was clearly observed for 4-DPACz compared with PVK. Such improved device characteristics of 4-DPACz were attributed to its high hole mobility and charge balance inside the emitting layer therof. The excellent results using such a simple-structured 4-DPACz could promote various applications of this 4-DPACz unit as a building block structure for further possible oligomeric, dendritic and polymeric materials.


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INTRODUCTION Organic light-emitting diodes (OLEDs) represent one of the hot topics in advanced materials field, both in academia and industry, based on the warm welcomed entry of the OLED displays and lighting panels into the markets.1 Phosphorescent OLEDs (PhOLEDs) where the emitting materials are dispersed into a host matrix to avoid the exciton loss caused by the mutual interaction of the emitters, are the best devices when considering the efficiencies. These devices can be manufactured either by the expensive vacuum deposition technique or by the economical methods like spin-coating or ink-jet printing.2 For both processes, careful selection of the host materials is one important factor to affect the efficiencies of the PhOLEDs.3,4 Generally, for fabricating PhOLEDs by solution-process, polymeric materials like poly(9-vinylcarbazole) (PVK) and others5,6 are used as hosts because of their high triplet energy levels, good solubilities (in organic solvents) and excellent film forming properties, but the inherent problems with the polymeric materials like polydispersity affect the device performance to a large extent making it very difficult to maintain the efficiencies of the devices using materials from different batches of syntheses. On the other hand, use of small molecular host materials for high efficiency PhOLEDs and thermally activated delayed fluorescence (TADF) OLEDs are advantageous because of the specific molecular weights of the small molecular materials and easy purification. Carbazole derivatives are the most and best studied materials as hosts in PhOLEDs7-10 and as both hosts and emitters in TADF OLEDs.11-15 Functionalization of the carbazole nucleus (the functional part of PVK) to improve its electronic properties has been a topic of research in OLEDs for a long time. 3-, 6- and 9-positions of carbazole nucleus are easily accessible, so, the respective derivatives have been widely studied.16-20 Furthermore, host materials based on 3- or 6-substituted carbazole


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result into an unbalanced charge injection into the emission layer (EML) resulting into decreased device performance.21 However, host materials having substitutions at 1- and 4-position of carbazole are still much rare and only a little work has been reported mainly as bipolar hosts during the past couple of years.22 Diphenylamino (DPA)-group is an electron rich functionality that can impart excellent electronic properties to the carbazole system when coupled to it as reported previously for 2- and 3-positions of carbazole.16,23 Model compounds play an important role while exploring the use of new functional moieties in organic electronics as they provide an easy and fast way to check the potential of new functionalities or the new combinations of the well-known functionalities. In order to explore the potential of the DPA functionality at 4-position of carbazole and comparing it with the recently described work on 1-position of carbazole,24 we prepared two new carbazole derivatives, 1-DPACz and 4-DPACz. 1-DPACz is an analogue of the previously reported host material differing only in the absence of the tertiary butyl groups at the 3- and 6positions of carbazole, and was synthesized as a reference material. Both the compounds were highly soluble in common organic solvents, mainly because of the free rotation of the DPA functionality, and high quality thin films were produced when spin-coated from their solutions. These two properties encouraged us to using these materials for solution-processed devices albeit the molecular dimensions of the materials were very short that, generally, is not ideal for solution-process. Based upon high triplet energy and well-matched HOMO energy level, 4DPACz showed reduced driving voltage and higher efficiencies for solution-processed green PhOLEDs than PVK as well as 1-DPACz. Furthermore, reduced efficiency roll-off was clearly observed for 4-DPACz compared with PVK. Such improved device characteristics of 4-DPACz were attributed to its high hole mobility and charge balance inside the emitting layer therof.


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RESULTS AND DISCUSSION Syntheses of Materials. The structures and syntheses routes to the two isomeric diphenylaminocarbazoles are presented in Scheme 1. 1-Bromo-N-ethylcarbazole (3) was synthesized in a three step procedure involving cyclization25, dehydrogenation26 and N-alkylation24, whereas 4-Bromo-N-ethylcarbazole (6) was synthesized in a three step procedure involving Suzuki-Miyaura coupling, cyclization and Nalkylation following a reported procedure27. The bromocarbazoles (3 and 6) were converted to the final host materials, 1-DPACz and 4-DPACz, by coupling with diphenylamine using Buchwald-Hartwig cross-coupling.24 The NMR spectra of the intermediates and the final compounds are displayed in Figure S1 to S8. Basic thermal properties of the final compounds have been investigated by differential scanning calorimety (Figure S9), and the melting points of the compounds were 196 oC for 1-DPACz and 132 oC for 4-DPACz, which are higher than previously reported melting point of 3-substituted analogue, 129 oC.16

Scheme 1. Syntheses schemes for 1- and 4-DPACz.


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Physical Properties. Photophysical properties of the diphenylaminocarbazoles determined by ultraviolet-visible (UVvis) absorption and photoluminescence (PL) spectroscopy in dichloromethane (DCM) solution are shown in Figure 1, and the photophysical data is presented in Table 1. The basic absorption pattern of both compounds is almost similar with three distinct bands at 270 to 280 nm, 294 to 317 and 352 to 368 nm. However, the positions and intensities of these bands are highly dependent upon the position of the strongly electron donating diphenylamino group. The lowestenergy absorption bands located at 352–368 nm, assigned to π →π* 28 are shifted to longer wavelengths (red shifting) as compared to the absorption of single carbazole moiety (345 nm) due to the extension of conjugation. The positions of the bands remain almost same in both 1DPACz and 4-DPACz but the intensities for the second and the third bands are highly different indicating a different extension of the electron wavefunctions from the carbazole to diphenylamino groups due to the attachment positions of the diphenylamino groups over the carbazole nucleus. Fluorescence emission spectral peaks of 1-DPACz and 4-DPACz appeared around 420 nm. If we compare this value with the fluorescence maximum of carbazole moiety (365 nm), these are considerably red-shifted because of extension of electron wavefunctions from the carbazole to diphenylamino groups. The band gaps of the materials were calculated from the absorption edges of the respective spectra and are summarized in Table 1. Triplet energy levels of the host materials (1-DPACz and 4-DPACz) were determined from the highest energy emission peaks of the phosphorescent spectra of the solutions in methyltetrahydrofuran at 77K (Figure 1). The triplet energy levels of the host materials are also summarized in Table 1. As shown in the table, the triplet energy levels of the host materials are dependent upon the attachment positions, 1- or 4-, of the diphenylamino functionality over the carbazole nucleus, the


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value being higher for 4-DPACz. Both the values being far higher than the green phosphorescent emitter, Ir(mppy)3 (Tris[2-(p-tolyl)pyridine]iridium(III)) (2.4 eV) making these materials suitable as hosts for green PhOLEDs.

Figure 1. UV-vis absorption, fluorescence and phosphorescence spectra of 1- and 4-DPACz.

Table 1. Physical properties of the host materials, 1-DPACz and 4-DPACz. λmax,abs

λmax,em

∆Eg

ET

Eox,1/2

HOMO

LUMO

[nm][a]

[nm][a]

[eV]

[eV][b]

[mV][c]

[eV][d]

[eV][e]

1-DPACz

270, 295, 353

423

3.31

2.69

724

-5.68

-2.37

4-DPACz

269, 294, 357

418

3.29

2.76

655

-5.61

-2.32

Material

[a]

Measurements from dichloromethane solution.

[b]

Values determined from the highest energy

emission peaks of the phosphorescent spectra of the solutions in methyltetrahydrofuran at 77K. [c]

Potential values versus 1st oxidation potential of ferrocene.

[d]

Values calculated by regarding

HOMO of TPD as -5.4 eV (Ref 29). HOMO (eV) = -(5.4 + (Eox,1/2,DPACz (V) – Eox,1/2, TPD (V))). [e]

Values calculated by adding up the respective optical band gaps and the HOMO values.


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The distribution of electron densities over the molecules and hence the theoretical electronic properties of the materials were also computed by DFT calculations at B3LYP and 631G (d) level of theory. The calculation results are shown in Figure 2. It shows the confinement of the HOMO levels of these materials mainly on the more electron rich diphenylamino moieties where as LUMO are located on the carbazole nucleus.

Figure 2. Optimized geometrical structures and HOMO/LUMO distributions of 1- and 4-DPACz computed at the B3LYP/6-31G(d) level of theory.

Electrochemical Properties. Electrochemical properties of the host materials were studied by cyclic voltammetry (CV) using DCM solutions of the materials and tetrabutylammonium hexafluorophosphate as a supporting electrolyte. Electrochemical data of TPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine) was used for comparison and estimation of the HOMO energy levels of the new materials. As shown by the CV graphs of the compounds (Figure 3), the first oxidation potentials of 1-DPACz, 4-DPACz and TPD were observed at 724, 655 and 445 mV, respectively, versus ferrocene/


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ferrocene+. Based on the HOMO energy level of TPD (-5.4 eV),29 the HOMO energy levels of 1DPACz and 4-DPACz were in the order, 4-DPACz (-5.61 eV) > 1-DPACz (-5.68 eV). For calculating the LUMO energy levels, the optical band gaps were added to the HOMO energy levels. The electrochemical data is shown in Table 1.

Figure 3. Cyclic voltammogram of 1-DPACz, 4-DPACz and TPD.

Device Fabrication and Measurement. The potential of our diphenylaminocarbazoles (1-DPACz and 4-DPACz) as model phosphorescent host materials was investigated by fabricating simple solution-processed green phosphorescent OLEDs with the device configuration [ITO / PEDOT:PSS (35 nm) / Emitting layer (40 nm) / TPBi (57 nm) / CsF (1 nm) / Al (80 nm)], where the emitting layer was composed of 93wt% of each of the host materials and 7wt% of green emitting dopant, Ir(mppy)3 with PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)), TPBi (2,2′,2"(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) and CsF serving as hole-injecting, electron transporting/hole blocking and electron-injecting layers, respectively. The energy level


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diagram for the devices is shown in Figure 4. The OLED devices containing each of the host materials, 1-DPACz and 4-DPACz are designated with the same abbreviations, respectively. PVK, whose repeating unit is a non-substituted N-ethylcarbazole, was also introduced as a host for comparison purpose, and its OLED device characteristics have also been compared with 1DPACz and 4-DPACz.

Figure 4. Energy level diagram of the devices using 1-DPACz and 4-DPACz hosts.

Figures 5 and 6 show the resultant device characteristics which are summarized in Table 2. All of the three OLEDs showed typical green electroluminescence emission (Inset of Figure 5). As is clearly seen from the luminance-voltage curves (Figure 5) and current density-voltage curves (Figure S10), the operating voltage of the OLED was found to be lower for 4-DPACz than 1-DPACz, in accordance with their respective HOMO levels i.e. the reduction of the holeinjection barrier from the ITO/PEDOT:PSS anode to the emitting layer. On the other hand, PVKbased reference device showed far higher driving voltage characteristics than any of the two


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diphenylaminocarbazoles, which may be due to the deeper HOMO energy level of PVK. Order of the maximum luminance for these devices was, 1-DPACz < PVK < 4-DPACz. Whether in terms of driving voltage or luminance, it is certain that our new 4-substituted diphenylcarbazole, 4-DPACz, outperforms one of the most popular solution-processable host materials, PVK, as well as its 1-substituted analog, 1-DPACz.

Figure 5. Luminance-voltage curves of the OLED devices (Inset: EL spectra at 1,000 nits).

Figure 6a shows the luminous efficiency-luminance curves of the devices. 4-DPACz showed higher maximum luminous efficiency (47.9 cd A-1 @ 168 cd m-2) than 1-DPACz (29.5 cd A-1 @ 148 cd m-2); PVK showed an intermediate value (39.5 cd A-1 @ 8 cd m-2). The maximum external quantum efficiency values of each of the devices were 8.8, 14.3, and 11.5%, respectively for 1-DPACz, 4-DPACz, and PVK. Another important feature of the devices employing 1-DPACz and 4-DPACz hosts is much lower efficiency roll-off showing the relative higher stability of these devices than the reference device employing PVK. As shown in Table 2, at a practical luminance of 1,000 cd m-2, 4-DPACz shows much higher luminous efficiency than


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the PVK-based reference device in order of more than two. Furthermore, even 1-DPACz also showed the higher luminous efficiency at 1,000 cd m-2 than PVK which shows maximum efficiency only at a low luminance regime. The high values of the efficiencies are even maintained at a very high luminance of 5,000 cd m-2 (25.7 cd A-1) for 4-DPACz. The roll-off characteristics between diphenylaminocarbazoles and PVK become more prominent for power efficiency (Figure 6b). Because power efficiency is a kind of figure of merit concerning the operating voltage of the devices, the higher the operating voltages the lower the power efficiencies. Power efficiencies of 1-DPACz (13.4 lm W-1) and 4-DPACz (17.5 lm W-1) at 1,000 cd m-2 were much higher than the reference device (5.9 lm W-1).


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(a)

(b)

Figure 6. (a) Luminous efficiency-luminance curves and (b) power efficiency-luminance curves of the OLED devices.

To the best of our knowledge, the luminous efficiency values achieved using 4-DPACz host are the highest ever achieved for solution-processed bilayer green PhOLEDs based on common green phosphorescent dyes Ir(ppy)3 or Ir(mppy)3 employing a hole-transport type simple small molecular30 or a dendritic host material31, and are comparable to the best values


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reported in the scientific literature for bipolar32-34 or mixed type35 host materials (Table S1). Furthermore, the maximum power efficiency of 4-DPACz also showed one of the highest values among these kinds of devices.

Table 2. Device characteristics of 1-DPACz, 4-DPACz and PVK with Ir(mppy)3 dopant. Maximum values

At 1,000 nits Voltage

LE

PE

EQE

[V]

[cd A-1]

[lm W-1]

[%]

1-DPACz

7.6

27.1

13.4

8.2

4-DPACz

6.9

38.0

17.5

PVK

9.2

17.3

5.9

Lmax

CEmax

PEmax

EQEmax

[cd m-2]

[cd A-1]

[lm W-1]

[%]

(0.29, 0.62)

8 865

29.5

14.8

8.8

11.4

(0.29, 0.62)

14 750

47.9

25.2

14.3

5.0

(0.32, 0.61)

12 070

39.5

25.4

11.5

CIE (x,y)

Based on the positive aspects of 4-DPACz discussed so far, we then conducted another experiment to further investigate the potential of this material. Because the emitting layer of phosphorescent OLEDs is a mixture of host and dopant, the overall OLED device performance is determined from dopant as well as host. We have previously reported a modified green phosphorescent dopant, Ir(Si-bppy)2(acac), which is useful to solution-processing.36 We therefore decided to incorporate Ir(Si-bppy)2(acac) (Iridium(III) bis(2-(biphenyl-3-yl)-5(trimethylsilyl)pyridinato-N, C4‘) (acetylacetonate)) instead of Ir(mppy)3 to investigate the potential of 4-DPACz as a host. The device configuration was the same as those of previously-


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mentioned devices, except that the dopant was Ir(Si-bppy)2(acac) instead of Ir(mppy)3. PVK was again applied and compared with 4-DPACz. Figure 7 shows the resultant device characteristics. Similar to the previous results with Ir(mppy)3, 4-DPACz again showed far reduced driving voltage than PVK as is shown in Figure 7a, and what was more, remarkable increase of luminance of 4-DPACz was also observed. The maximum luminance of 4-DPACz increased up to 34,200 cd m-2 as a result of Ir(Si-bppy)2(acac). This luminance value is almost 3 times higher than PVK (11,700 cd m-2). In addition to this luminance increase, 4-DPACz host again showed far reduced efficiency roll-off than PVK with increased efficiency at the same time, as is shown in Figure 7b. The achieved maximum luminous efficiency of 4-DPACz:Ir(Si-bppy)2(acac) was 59.1 cd A-1, which corresponds to 15.8% of external quantum efficiency. The device characteristics are summarized in Table 3. These results repeatedly demonstrate the outstanding potential of 4-DPACz as a phosphorescent host material.

Table 3. Device characteristics of 4-DPACz and PVK with Ir(Si-bppy)2(acac) dopant. Maximum values

At 1,000 nits Voltage

LE

PE

EQE

[V]

[cd A-1]

[lm W-1]

[%]

4-DPACz

6.9

58.7

26.6

15.7

PVK

8.1

35.7

14.0

9.7

Lmax

CEmax

PEmax

EQEmax

[cd m-2]

[cd A-1]

[lm W-1]

[%]

(0.39, 0.59)

34 222

59.1

29.5

15.8

(0.41, 0.58)

11 702

56.4

27.3

15.3

CIE (x,y)


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(a)

(b)

Figure 7. (a) Current density-Voltage-Luminance curves and (b) luminous efficiency-luminance curves (Inset: EL spectra at 1,000 nits) of the OLED devices with Ir(Si-bppy)2(acac) dopant.

The above results about suppressed efficiency roll-off of 4-DPACz led us to further investigate the electrical properties of 4-DPACz. We therefore fabricated and characterized holeonly devices (HODs) using 4-DPACz or PVK. The structure of HOD was [ITO / PEDOT:PSS / 4-DPACz or PVK / MoOx / Au], and we followed the proposed characterization procedure.37 As


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is shown in Figure 8, there is almost three order of magnitude difference in hole mobility between 4-DPACz and PVK. The calculated hole mobility values at 2.5×105 V cm-1 were 5.6×10-5 and 8.3×10-8 cm2 V-1 s-1, respectively for 4-DPACz and PVK. Considering that charge balance inside the organic layer is one of the parameters that determines the efficiency and also its roll-off, the negative influence of low hole mobility of PVK to the overall charge balance can be more and more severe as the current increases. The high hole mobility of 4-DPACz therefore resulted in suppression of efficiency roll-off when it is adopted as a phosphorescent host. Because PVK is a kind of side-chain polymer where each of the N-ethylcarbazole moieties are merely linked, our current result implies that additional functionalization of 4-diphenylamino substituent to the carbazole ring can drastically increase the hole mobility of the carbazole derivatives, which resulted in increased efficiency and reduced roll-off.

Figure 8. Hole mobility of 4-DPACz and PVK.


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CONCLUSIONS In conclusion, two new diphenylaminocarbazoles with unusually placed electron rich diphenylamino moieties were prepared as model compounds in order to investigate the effect of the diphenylamino substituent position over the relatively less explored 1- and 4-positions on carbazole nucleus as well as their effects on the physical properties of the modified materials, and finally to investigate their role as host materials in solution-processed green PhOLEDs. It was found that substitution of electron donating diphenylamino substituent at 4-position (4DPACz) resulted in a higher triplet energy material as compared to 1-position (1-DPACz). The two materials, in particular 4-DPACz, when used as hosts for green PhOLEDs, resulted in very high efficiencies of the devices with low efficiency roll-offs. The efficiency roll-off was particularly low in case of 4-DPACz. Maximum luminous, power and external quantum efficiencies reaching to 47.9 cd A-1, 25.2 lm W-1 and 14.3%, respectively, were achieved with 4DPACz as a host and Ir(mppy)3 as a dopant. Additional enhancement of efficiencies of 4-DPACz was verified when incorporating another dopant, Ir(Si-bppy)2(acac), resulting in 59.1 cd A-1, 29.5 lm W-1 and 15.8%. The results of 4-DPACz are the best for solution-processed bilayer green PhOLEDs using small molecular hole-transport type host materials. Considering the excellent results using these model small molecules, work is in progress in utilizing these systems as units to synthesize oligomeric, dendritic and polymeric host materials.


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EXPERIMENTAL SECTION Materials. Carbazole, tetrabutylammonium bromide (TBAB), bromoethane, anhydrous zinc chloride (ZnCl2), N-bromosuccinimide (NBS), diphenylamine, sodium tert-butoxide (tBuONa), and tris(dibenzylideneacetone) dipalladium (0) [Pd2(dba)3] were purchased from Sigma-Aldrich Co. tert-Butylchloride (tBuCl) was purchased from Wako Pure Chemical Industries Ltd. Tri-tertbutylphosphine (P(tBu)3) was purchased from Kanto Chemical Co., Inc. All the reagents were used as received without further purification. Instrumentation. 1

H and 13C NMR spectra were measured using Bruker DPX-300 or DPX-400 NMR

spectrometers. Differential scanning calorimetry (DSC) was conducted using a Mettler Toledo DSC 1. UV-Vis. absorption spectra were obtained using SHIMADZU UV-2550 spectrophotometer and room temperature & low-temperature photoluminescence spectra were obtained using Perkin Elmer LS55 luminescence spectrometer. Cyclic voltammetry was performed with BAS 100B electrochemical analyzer. Syntheses. Synthesis of 8-bromo-2,3,4,9-tetrahydro-1H-carbazole (1) Cyclohexanone (6.18 g, 63.0 mmol) was dissolved in glacial acetic acid (30 mL), and to this solution was added 2-bromophenylhydrazine hydrochloride (14.08 g, 63.0 mmol) over a period of one hour, under N2. The reaction mixture was stirred for one hour at 80 oC followed by


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cooling to room temperature and pouring into 100 mL of vigorously stirring cold water. The precipitated product was filtered and washed several times with 5% aqueous sodium hydrogen carbonate solution. The solid was dissolved in 15 mL of dichloromethane and dried over anhydrous sodium sulfate. Evaporation of the solvent provided us with the title compound as a white solid (3.5 g, 83%). The product was pure enough to be used in the next step. 1

H NMR (300 MHz, CDCl3, ppm): δ 7.86 (bs, 1H), 7.39 (d, J= 7.8 Hz, 1H), 7.24 (d, J= 6.6 Hz,

1H), 6.94 (t, J= 8.1 Hz, 1H), 2.76 (t, J= 5.7 Hz, 2H), 2.69 (t, J= 5.4 Hz, 2H), 1.89 (m, 4H). Synthesis of 8-bromo-9-ethyl-2,3,4,9-tetrahydro-1H-carbazole (2) Compound (1) (2.40 g, 9.6 mmol), powdered NaOH (0.60 g, 15.0 mmol) and tetrabutylammonium bromide (TBAB) (0.32 g, 1.0 mmol) were dissolved in acetone (30 mL) and stirred at room temperature for ten minutes. Bromoethane (1.5 g, 13.7 mmol) was added, drop-wise, to the resulting solution and the reaction mixture was heated under reflux for one hour. The reaction mixture was cooled and dropped into water (50 mL). The product was extracted with ethyl acetate (50 mL), washed with water, dried over MgSO4 and concentrated. Pure product (2.50 g, 93.6%) as colorless viscous oil was obtained by column chromatography (nHexane). 1

H NMR (300 MHz, CDCl3, ppm): δ 7.37 (d, J= 7.8 Hz, 1H), 7.27 (d, J= 8.1 Hz, 1H), 6.87 (t, J=

7.8 Hz, 1H), 4.47 (q, J= 7.2 Hz, 2H), 2.68 (m, 4H), 1.94 (m, 2H), 1.86 (m, 2H). Synthesis of 1-bromo-9-ethyl-9H-carbazole (3) 2.3 g (8.27 mmol) of (2) was dissolved in 20 mL of sulfur-free xylene to form a clear solution. An excess of chloranil (15.0 g, 61.0 mmol) was added to the vigorously stirred solution and the


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reaction mixture refluxed overnight. The dark colored reaction mixture was cooled, filtered and diluted with ether, shaken first with 10% sodium hydroxide and then with water. The organic phase was separated, dried over anhydrous sodium sulfate and the solvent evaporated in vacuo. Purification by column chromatography (hexane/DCM, 10/1) afforded the title compound as a colorless oil (2.00 g, 88.2%). 1

H NMR (400 MHz, CDCl3, ppm): δ 8.04 (d, J= 7.6 Hz, 1H), 8.00 (dd, J=8.0, 1.2 Hz, 1H), 7.58

(dd, J=8.0, 1.2 Hz, 1H), 7.47 (m, 1H), 7.40 (d, J= 8.4 Hz, 1H), 7.23 (m, 1H), 7.03 (t, J= 7.6 Hz, 1H), 4.78 (q, J= 7.2 Hz, 2H), 1.45 (t, J= 7.2 Hz, 3H). Synthesis of 9-ethyl-N,N-diphenyl-9H-carbazol-1-amine (1-DPACz) The title compound was prepared using the general Buchwald-Hartwig coupling procedure using 1-bromo-9-ethyl-9H-carbazole (0.548 g, 2 mmol.), diphenylamine (0.423 g, 2.5 mmol.), Pd2(dba)3 (0.116 g, 0.127 mmol), tBu3P (0.05 g, 0.247 mmol), and tBuONa (0.72 g, 7.5 mmol) in toluene (10 mL). The reaction mixture was was evacuated and back filled with dry nitrogen three times before heating under N2 atmosphere at reflux temperature for 10 h. The reaction mixture was then filtered through a celite plug, the filtrate evaporated under vacuum and subjected to column chromatography on silica gel (n-hexane/DCM, 10/1) to get a white solid (0.55 g, 76.4%). 1

H NMR (400 MHz, CDCl3, ppm): δ 8.11 (d, J= 7.6 Hz, 1H), 8.04 (d, J= 6.8 Hz, 1H), 7.45 (t, J=

7.2 Hz, 1H), 7.34 (d, J= 8.0 Hz, 1H), 7.35-7.20 (m, 8H), 7.09 (d, J= 7.6 Hz, 4H), 6.93 (t, J= 7.2 Hz, 2H), 4.46 (q, J= 6.8 Hz, 2H), 0.92 (t, J= 6.8 Hz, 3H).


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13

C NMR (100 MHz, CDCl3, ppm): δ 142.14, 142.08, 140.89, 137.49, 131.29, 129.23, 128.97,

128.79, 127.87, 125.98, 124.53, 123.11, 121.90, 121.13, 120.17, 119.68, 119.47, 119.24, 113.99, 109.17, 38.55, 14.10. HRMS (EI): calcd. for C26H22N2 362.1783, Found 362.1777. Synthesis of 2-bromo-2'-nitro-1,1'-biphenyl (4) To a solution of 1-bromo-2-nitrobenzene (2.3 g, 11.4 mmol), 2-bromophenylboronic acid (2.5 g, 12.5 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.66 g, 0.57 mmol) in tetrahydrofuran (40 mL), was added aqueous K2CO3 (2 M) solution (15 mL). The solution was refluxed for 12h, cooled to room temperature and was extracted with methylene chloride. The combined organic phase was dried over MgSO4 and concentrated. Upon purification by silica gel chromatography (n-hexane/DCM, 4/1) a pale crystalline solid was obtained (2.84 g, 89.6%). 1

H NMR (400 MHz, CDCl3, ppm): δ 8.10 (dd, J= 8.4, 1.2 Hz, 1H), 7.70-7.64 (m, 2H), 7.57 (m,

1H), 7.40-7.34 (m, 2H), 7.29-7.24 (m, 2H). Synthesis of 4-bromo-9H-carbazole (5) A solution of (4) (2.8 g, 1.01 mmol) and triphenylphosphine (0.68 g, 2.6 mmol) in 1,2dichlorobenzene (10 mL) was refluxed for 12h, cooled to room temperature and was extracted with methylene chloride. The combined organic phase was dried over MgSO4 and concentrated. Purification by silica gel chromatography (n-hexane/ethyl acetate, 4/1) gave a white solid (0.75 g, 83.9%).


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1

H NMR (400 MHz, CDCl3, ppm): δ 8.74 (d, J= 8.0 Hz, 1H), 8.10 (bs, 1H), 7.49-7.45 (m, 1H),

7.41-7.38 (m, 2H), 7.34 (d, J= 7.6 Hz, 1H), 7.31-7.27 (m, 1H), 7.25-7.21 (m, 1H). Synthesis of 4-bromo-9-ethyl-9H-carbazole (6) Compound (6) was prepared by using the procedure for compound (2) using compound (5) (0.5 g, 2.0 mmol), tetrabutylammonium bromide (TBAB) (0.03 g, 0.01 mmol), NaOH (0.11g, 2.75 mmol) and ethyl bromide (0.30 g, 2.75 mmol) in acetone (10 mL). Column chromatography (nhexane) gave colorless oil that solidified to a white crystalline solid upon standing (0.49 g, 87.97%). 1

H NMR (400 MHz, CDCl3, ppm): δ 8.80 (d, J= 8.4 Hz, 1H), 7.55-7.51 (m, 1H), 7.42 (d, J= 8.4

Hz, 1H), 7.40-7.35 (m, 2H), 7.33-7.27 (m, 2H), 4.36 (q, J= 7.2 Hz, 2H), 1.42 (t, J= 7.2 Hz, 3H). Synthesis of 9-ethyl-N,N-diphenyl-9H-carbazol-4-amine (4-DPACz) The title compound was prepared using the Buchwald-Hartwig coupling procedure for 1-DPACz using 4-bromo-9-ethyl-9H-carbazole (0.27 g, 1 mmol), diphenylamine (0.38 g, 4.5 mmol), Pd2(dba)3 (0.065 g, 0.064 mmol), tBu3P (0.025 g, 0.124 mmol), and tBuONa (0.36 g, 3.7 mmol) in toluene (5 mL). Column chromatography (n-hexane : DCM, 10:1) on silica gel gave a white solid (0.26 g, 71.8%). 1

H NMR (400 MHz, CDCl3, ppm): δ 7.74 (d, J= 7.6 Hz, 1H), 7.43 (t, J= 8.0, 1H), 7.36-7.35 (m,

2H), 7.29 (d, J= 8.0 Hz, 1H), 7.18 (d, J= 7.4, 2H), 7.16 (d, J= 7.2, 2H), 7.94 (d, J= 8.4, 4H), 6.97-6.93 (m, 2H), 6.90 (t, J= 7.2 Hz, 2H), 4.38 (q, J= 7.2 Hz, 2H), 1.47 (t, J= 7.2 Hz, 3H).


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C NMR (100 MHz, CDCl3, ppm): δ 147.81, 141.92, 141.68, 139.99, 129.29, 126.71, 125.64,

123.38, 122.01, 121.86, 121.45, 120.26, 120.21, 119.16, 108.04, 106.06, 37.95, 14.08. HRMS (EI): calcd. for C26H22N2 362.1783, Found 362.1774. Device Fabrication and Characterization. Commercial indium tin oxide (ITO) coated glass with sheet resistance of 10 Ω per square was used as the starting substrates. The substrates were precleaned carefully and treated by UV-ozone for 20 min before device fabrication followed by spin-coating PEDOT:PSS (35 nm) which was dried then at 120 °C for 30 min under vacuum. Then, the emissive layer (40 nm) of host materials doped with 7 wt% green emitting dopant, Ir(mppy)3 was spin-coated from chlorobenzene solution onto the PEDOT:PSS layer and dried at 80 °C for 30 min to remove residual solvent followed by thermal deposition of electron transport layer (ETL) of TPBi (57 nm), and a cathode composed of cesium fluoride (CsF, 1 nm) and aluminum (Al, 80 nm) successively at 10−6 Torr. The devices were then encapsulated by glass lids with epoxy-type adhesives in a nitrogen-filled glove box. The electrical characteristics of the devices were characterized by Keithley 2400 source measurement unit. The luminance and emission spectrum of OLED devices were characterized by Konica-Minolta CS-100A luminance meter and CS1000 spectroradiometer, respectively. All measurements of the fabricated devices were carried out at room temperature under ambient conditions.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. NMR spectra of the intermediates and the final compounds, current-voltage curves of the OLEDs, summary of device characteristics of recent solution-processed green PHOLEDs (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Jaemin Lee). *E-mail: [email protected] (Changjin Lee). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIP) (No. CMP-16-05-ETRI) and also supported by the Global Leading Technology Program funded by the Korean government (MOTIE) (No. 10042477).


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Table of Contents/Abstract Graphic


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Syntheses schemes for 1- and 4-DPACz. 177x56mm (300 x 300 DPI)


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Figure 1. UV-vis absorption, fluorescence and phosphorescence spectra of 1- and 4-DPACz. 84x57mm (300 x 300 DPI)



Figure 2. Optimized geometrical structures and HOMO/LUMO distributions of 1- and 4-DPACz computed at the B3LYP/6-31G(d) level of theory. 84x43mm (300 x 300 DPI)


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Cyclic voltammogram of 1-DPACz, 4-DPACz and TPD. 84x61mm (300 x 300 DPI)



Figure 4. Energy level diagram of the devices using 1-DPACz and 4-DPACz hosts. 84x108mm (300 x 300 DPI)


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Figure 5. Luminance-voltage curves of the OLED devices (Inset: EL spectra at 1,000 nits). 84x61mm (300 x 300 DPI)



Figure 6. (a) Luminous efficiency-luminance curve of the OLED devices. 84x61mm (300 x 300 DPI)


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Figure 6. (b) Power efficiency-luminance curve of the OLED devices. 84x61mm (300 x 300 DPI)



Figure 7. (a) Current density-Voltage-Luminance curve of the OLED devices with Ir(Si-bppy)2(acac) dopant. 84x55mm (300 x 300 DPI)


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Luminous efficiency-luminance curves (Inset: EL spectra at 1,000 nits) of the OLED devices with Ir(Sibppy)2(acac) dopant. 264x186mm (300 x 300 DPI)



Figure 8. Hole mobility of 4-DPACz and PVK. 84x60mm (300 x 300 DPI)


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Table of Contents/Abstract Graphic 81x35mm (300 x 300 DPI)


Switching Substituent Position for Voltage ... - ACS Publications

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