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Highly efficient pyrene blue emitters for OLEDs based on substitution position effect
Dyes and Pigments 158 (2018) 42–49
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Highly efficient pyrene blue emitters for OLEDs based on substitution position effect
T
Mina Junga,1, Jaehyun Leea,c,1, Hyocheol Jungb, Seokwoo Kangb, Atsushi Wakamiyac, Jongwook Parkb,∗ a
Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, South Korea Department of Chemical Engineering, Kyung Hee University, Gyeonggi, 17104, South Korea c Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan b
A B S T R A C T
To investigate the effect of substitution position of the side group on a pyrene core, three derivatives having a triphenylbenzene group as a bulky side group at the 1,6-position, 4,9-position, and 1,8-position were successfully synthesized: 1,6-bis(5′-phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,6-DTBP), 4,9-bis(5′-phenyl-[1,1':3′,1″terphenyl]-4-yl)pyrene (4,9-DTBP), and 1,8-bis(5′-phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,8-DTBP). The three synthesized materials showed excellent thermal stability with a high Tg of > 140 °C and a high Td of > 500 °C. Due to the highly twisted structure of 1,8-DTBP in the film state, the absolute photoluminescence quantum yield value was improved. Of the three synthesized materials used as an emitter in a non-doped organic light-emitting diode device, 1,8-DTBP showed highly efficient electroluminescence performance, with a luminance efficiency of 6.89 cd/A, power efficiency of 3.03 lm/W, and external quantum efficiency of 7.10% at 10 mA/cm2. In addition, 4,9-DTBP showed a deep-blue emission of CIE x, y (0.158, 0.063) suitable for HD-TV.
1. Introduction In the past two decades, interest in π-conjugated compounds in the optoelectronic field, such as organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic photovoltaic cells (OPVCs), has increased dramatically [1–3]. In particular, OLEDs have been intensively studied due to potential applications such as full-color flat-panel displays, next-generation lighting, and flexible displays [4]. The current research trends in OLED technologies have focused on optimizing device structure and developing new emitting materials. Among the various development methods for OLEDs, new blue-emitting materials that satisfy high electroluminescence (EL) efficiencies, good thermal stability, long device lifetime, and pure color coordinates remains a hot button issue [5]. Predicting the properties of emitting materials and finding effective ways to develop high-performance materials depends on the relationship between molecular structure and properties [6,7]. These molecular structures consist of core groups and side groups. As a core group, for example, anthracene and pyrene are typically used as blue light-emitting materials, and a variety of derivatives combined side groups have been reported in the blue light-emitting field [8]. Among the many blue
∗
1
Corresponding author. E-mail address: [email protected] (J. Park). Mina Jung and Jaehyun Lee contributed equally to this work as the first coauthor.
https://doi.org/10.1016/j.dyepig.2018.05.024 Received 19 March 2018; Received in revised form 12 May 2018; Accepted 13 May 2018 Available online 14 May 2018 0143-7208/ © 2018 Published by Elsevier Ltd.
emitters using core and side groups, substitution positions can affect optical and electrical properties including conjugation length, electronic state, photophysical properties, and intramolecular and intermolecular interactions [9]. Therefore, when designing the emitting material, an understanding of the substitution position effect is needed. However, few studies have investigated substitution positions between core and side groups and their effects. Therefore, we report the efficiency and color purity of blue light emitters when using different link positions with the same core and side groups. In our previous study, the m-terphenyl side groups were introduced in the ortho, meta, and para link positions on 6,12-dihydro-diindeno [1,2-b; 1′, 2′-e] pyrazine (indenopyrazine) and the optical and EL properties were investigated according to changes in the substitution position [10]. Larger blue shifts in the UV–visible (UV–vis) absorption and photoluminescence (PL) spectra occur in the synthesized ortho- and para-substituted derivatives compared to the meta-substituted derivative. In particular, the OLED device containing the para-substituted derivative exhibited excellent characteristics, with maximum EL emission at 423 nm, a full width at half maximum of 42 nm, pure violet emission with CIE coordinates (0.173, 0.063), and an external quantum efficiency of 1.88%.
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Compound 4. Compound 3 (3.00 g, 7.79 mmol) was added to 13 mL of anhydrous THF and stirred at −78 °C under a nitrogen atmosphere, then 2 M n-BuLi (5.85 mL, 11.7 mmol) was added to the reaction mixture. Next, isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.18 mL, 15.6 mmol) was added to the reaction mixture after 30 min. After the reaction was finished, the reaction mixture was extracted with ethyl acetate and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was removed by evaporation. The product was purified by silica gel column chromatography using THF: n-hexane (1: 10) eluent to afford a white solid (3.01 g, 89%). 1 H -NMR (300 MHz, CDCl3): δ = 7.94 (d, 2H), 7.79 (m, 3H), 7.69 (m, 6H), 7.46 (t, 4H), 7.37 (t, 2H), 1.24 (d, 12H); EI + -Mass: 432. Compounds 5 and 6. Bromine (10.0 mL, 195 mmol) in CHCl3 (500 mL) was dropped into a solution of pyrene (20.0 g, 98.9 mmol) in CHCl3 (500 mL) over 5 h while stirring. The precipitate was collected after 12 h and recrystallized from xylene to give compound 5 (5.01 g, 14%) and compound 6 (2.17 g, 6%). Compound 5: 1H NMR (300 MHz, CDCl3): δ = 8.46 (d, 2H), 8.26 (d, 2H), 8.11 (d, 2H), 8.05 (d, 2H); EI + -Mass: 360. Compound 6: 1H NMR (300 MHz, CDCl3): δ = 8.56 (s, 2H), 8.27 (d, 2H), 8.05 (m, 4H); EI + -Mass: 360. 1,6-DTBP. Compound 5 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was filtered. The product was isolated using silica gel column chromatography with CHCl3 as the solvent. The eluent was removed by evaporation. Recrystallization of the residue from CHCl3 afforded a beige solid product (0.51 g, 23%). 1H NMR (300 MHz, [D8]THF): δ = 8.33 (m, 4H), 8.16 (d, 2H), 8.07 (d, 2H), 8.05 (m, 8H), 7.93 (s, 2H), 7.86 (m, 12H), 7.52 (t, 8H), 7.41 (t, 4H); 13C NMR data could not be obtained because of the low solubility of the 1,6-DTBP; TOF-MS 810.3 m/z. 1,8-DTBP. Compound 6 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated by silica gel column chromatography using the CHCl3: n-hexane (1: 10) eluent to afford a beige solid (1.51 g, 67%). 1H NMR (300 MHz, [D8]THF): δ = 8.35 (d, 2H), 8.29 (s, 2H), 8.20 (s, 2H), 8.11 (d, 2H), 8.03 (m, 8H), 7.92 (s, 2H), 7.83 (m, 12H), 7.50 (t, 8H), 7.39 (t, 4H); 13C NMR (125 MHz, CDCl3): δ = 142.63, 142.10, 141.31, 140.58, 140.20, 137.30, 131.28, 131.20, 129.04, 128.57, 127.89, 127.75, 127.69, 127.54, 127.48, 125.63, 125.48, 125.33, 125.10; HRMS (EI, m/z): [M+] calcd for C64H42, 810.3287; found, 810.3291; elemental analysis calcd (%) for C64H42: C 94.78, H 5.22; found: C 94.39, H 5.31%. Compound 7. Pyrene (20.0 g, 98.9 mmol) was added to 500 mL of 1-pentanol and stirred while refluxing. 32.0 g of sodium was slowly added for 1 h and additionally stirred for 2 h. Temperature was reduced to room temperature and the solution was extracted using water. After vaporizing the solvent, ethanol was used for recrystallization to obtain a white solid (9.25 g, 45%). 1H NMR (300 MHz, CDCl3): δ = 7.12 (s, 4H), 3.04 (t, 8H), 2.00 (m, 4H); EI + -Mass: 208. Compound 8. Compound 7 (42.0 g, 144 mmol) was added to 100 mL of CH2Cl2 and stirred. 15 mL of Br2 was added to 30 mL of CH2Cl2, and this solution was slowly added by dropping. After 30 min of reaction, the material to be extracted was filtered. A white solid was obtained (16.9 g, 32%). 1H NMR (300 MHz, CDCl3): δ = 7.40 (s, 2H), 3.06 (t, 4H), 2.97 (t, 4H), 1.99 (m, 4H), EI + -Mass: 366. Compound 9. Compound 8 (12.0 g, 32.8 mmol) and 22.3 g of DDQ
The pyrene core group is one of the most efficient core groups among organic compounds, and has been used to replace anthracene recently due to advantages such as high quantum efficiency and thermal and chemical stability [11]. With regard to the link positions of pyrene, the 1, 3, 6, and 8 positions are easily accessible in organic synthesis, as established in previous research [12]. However, because of the synthetic challenge, research on pyrene derivatives in terms of the 4 and 9 positions has been not been investigated. Also, it is very difficult to separate 1,6 and 1,8 disubstituted pyrene derivatives because of chemical similarity of two compounds. Especially, we introduce 1,8 disubstituted pyrene in this study which has twisted chemical structure. Thus, pyrene derivatives which are substituted for various positions, can provide not only numerous new chromophores but also an effective strategy for performance optimization of pyrene-based emitters. In this study, we successfully synthesized Br-substituted pyrene compounds at the 1,6, 4,9, and 1,8 positions to investigate the effect of each substitution position on the pyrene core group. The triphenylbenzene group was selected for use as a bulky side group and was substituted at each position to synthesize three derivatives: 1,6-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,6-DTBP), 4,9-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (4,9-DTBP), 1,8-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,8-DTBP). The optical properties of the three compounds were compared with the EL properties of non-doped OLED devices using as the emitting layer (EML). 2. Experimental 2.1. Synthesis Compound 1. 1,3,5-Tribromobenzene (20.0 g, 63.5 mmol), phenyl boronic acid (18.8 g, 154 mmol), and Pd(PPh3)4 (3.75 g, 3.25 mmol) were added to 300 mL of anhydrous THF. Then, 2 M K2CO3 solution (100 mL), which was dissolved in H2O, was added to the reaction mixture. The mixture was heated to 65 °C for 12 h under nitrogen. After completion of the reaction, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated via silica gel column chromatography by using CHCl3: n-hexane (1: 20) as the eluent to afford a white solid (12.4 g, 63%). 1H NMR (300 MHz, CDCl3): δ = 7.70 (s, 3H), 7.60 (d, 4H), 7.44(t, 4H), 7.36 (t, 2H); EI + Mass: 310. Compound 2. Compound 1 (12.0 g, 38.8 mmol) was added to 300 mL of anhydrous THF and stirred at −78 °C under a nitrogen atmosphere, then 2 M n-BuLi (29.0 mL, 58.1 mmol) was added to the reaction mixture. Next, isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (11.9 mL, 58.1 mmol) was added to the reaction mixture after 30 min. After the reaction was finished, the reaction mixture was extracted with diethyl ether and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The residue was re-dissolved in CHCl3 and MeOH was added to the solution. The precipitate was filtered and washed with MeOH to afford a white solid (12.9 g, 93%). 1H NMR (300 MHz, CDCl3): δ = 8.03 (s, 2H), 7.90 (s, 1H), 7.69 (d, 4H), 7.44(t, 4H), 7.35 (t, 2H), 1.37 (s, 12H); EI + Mass: 356. Compound 3. 1,4-dibromobenzene (1.65 g, 7.00 mmol), compound 2 (2.49 g, 7.00 mmol), and Pd(PPh3)4 (0.40 g, 0.35 mmol) were added to 100 mL of anhydrous THF. Then, 2 M K2CO3 solution (20 mL), which was dissolved in H2O, was added to the reaction mixture. The reaction mixture was heated to 65 °C for 12 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated by silica gel column chromatography using CHCl3: n-hexane (1: 10) eluent to afford a white solid (2.07 g, Yield 77%). 1H NMR (300 MHz, CDCl3): δ = 7.80 (s, 1H), 7.74 (s, 2H), 7.69 (d, 4H), 7.58 (m, 4H), 7.48 (t, 4H), 7.39 (t, 2H); EI + -Mass: 384. 43
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were added to 240 mL of anhydrous toluene under nitrogen atmosphere and stirred for reflux. After 4 h of reaction, the temperature was reduced to room temperature. The solution was refined by column chromatography using CHCl3. The solvent was vaporized and recrystallized using CHCl3. A yellow solid was obtained (4.51 g, 38%). 1H NMR (300 MHz, CDCl3): δ = 8.57 (d, 2H), 8.40 (s, 2H), 8.13 (d, 2H), 8.04 (t, 2H); EI + -Mass: 360. 4,9-DTBP. Compound 9 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered, and the solvent was evaporated. The product was isolated by silica gel column chromatography using a CHCl3: n-hexane (1: 10) eluent to produce a beige solid (1.15 g, 51%). 1H NMR (300 MHz, [D8]THF): δ = 8.35 (m, 4H), 8.20 (s, 2H), 8.07 (m, 10H), 7.94 (s, 2H), 7.87 (m, 12H), 7.53 (t, 8H), 7.41 (t, 4H); 13C NMR (125 MHz, CDCl3): δ = 142.67, 142.17, 141.33, 140.56, 140.27, 139.16, 131.26, 130.81, 130.54, 129.07, 128.31, 127.78, 127.57, 126.27, 125.67, 125.53, 125.37, 124.81, 124.06; HRMS (EI, m/z): [M+] calcd for C64H42, 810.3287; found, 810.3291; elemental analysis calcd (%) for C64H42: C 94.78, H 5.22; found: C 94.48, H 5.29%.
Fig. 1. Chemical structures of the pyrene derivatives.
electrometer. Light intensities were obtained with a Minolta CS-1000A. The operational stability of the devices was measured under encapsulation in a glove box. 3. Results and discussion
2.2. Measurements and OLED fabrication Fig. 1 and Scheme 1 show the chemical structure and synthetic methods of the synthesized materials. In this study, optical characteristics and OLED performance according to different substitution positions were observed with the same core and side groups. Pyrene chromophore was used as the core group in a blue OLED emitter and a triphenylbenzene group was used as a side group to reduce intermolecular interactions and to prevent excimer formation through a bulky chemical structure. The substitution position was either 1,6, 4,9, or 1,8, with 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP, respectively, synthesized as model compounds. In organic synthesis, conventional Suzuki-Miyaura cross-coupling reactions and boronylation were used. In particular, the bromination of pyrene resulted in two isomers, 1,6-dibromopyrene and 1,8-dibromopyrene. These two isomers were purified several times through recrystallization. 4,9Dibromopyrene cannot be synthesized by direct bromination due to its regioselectivity. In order to obtain 4,9-dibromopyrene, it was synthesized from pyrene through the three synthetic steps shown in Scheme 1 [14]. All compounds were purified by recrystallization and column chromatography. The synthesized compounds were characterized using nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, and elemental analysis. The synthetic methods are described in more detail in the Experimental section. Figs. 2 and 3 show the UV–vis absorption and PL spectra of the synthesized compounds in 1 × 10−5 M THF solution and vacuum-deposited film states. Table 1 summarizes the optical properties, absolute photoluminescence quantum yield, energy levels of molecular orbitals, and thermal properties of synthesized compounds. The tendencies of the UV–vis absorption and PL spectra were similar in the solution and film states. The photophysical properties of 1,6-DTBP and 1,8-DTBP were similar, and 4,9-DTBP showed a relatively blue-shifted absorption and PL spectrum. The UVmax and PLmax of 1,6-DTBP and 1,8-DTBP in the THF solution were the same at 365 nm and 425 nm, and were 373 nm and 461 nm for the film state, respectively. 4,9-DTBP exhibited UVmax of 349 nm and PLmax of 384 nm in the solution state, and UVmax of 358 nm and PLmax of 428 nm in the film state, respectively. The PL spectra in various solvents in order to investigate the intramolecular charge transfer of compounds were shown in Fig. S1. Stokes shifts of three compounds in various solvents were small. The blue-shifted value of 4,9-DTBP can be explained by the π-electron partition of pyrene,
13
C NMR data of the 1,6-DTBP could not be obtained because of the very low solubility. The 1H NMR spectra and 13C NMR spectra were recorded on Bruker Avance 300 (300 MHz for 1H) and JEOL ECA 500 (125 MHz for 13C) spectrometers. The FAB+-mass and EI+-spectra were recorded on a JEOL, JMS-AX505WA, HP5890 series II. The optical absorption spectra were obtained using a Lambda 1050 UV/vis/NIR spectrometer (PerkinElmer). A PerkinElmer luminescence spectrometer LS50 (Xenon flash tube) was used to perform photoluminescence (PL) spectroscopy. PL quantum yield was measured by an absolute PL quantum yield measurement system (QM-400 Spectrofluorometer). The glass-transition temperatures (Tg) of the compounds were determined with differential scanning calorimetry (DSC) under a nitrogen atmosphere by using a DSC4000 (PerkinElmer). Degradation temperatures (Td) were determined with thermogravimetric analysis (TGA) using a TGA4000 (PerkinElmer). HOMO energy levels were determined by ultraviolet photoelectron yield spectroscopy (Riken Keiki AC-2). LUMO energy levels were derived from HOMO energy levels and band gaps. The redox potentials of the compounds were determined with cyclic voltammetry (CV) using IVIUM STAT system. The synthesized materials were dissolved in acetonitrile with 0.1 M tetrabutylammonium perchlorate as an electrolyte. We used a carbon working electrode, a platinum wire counter electrode and a saturated Ag/AgCl reference electrode. Ferrocene was used for potential calibration. DFT calculations for optimization of the geometries were conducted using the Gaussian 09 program [13]. In each of the EL devices, N,N′-bis(naphthalen-1-yl)-N,N′-bis (phenyl)benzidine (NPB) was used for the hole transport and injection layer (HTL), tris(4-carbazoyl-9-ylphenyl)amine (TCTA) was used for the hole transport and electron blocking layer, one of the synthetic materials 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP was used as the EML, 1,3,5-tri (1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) was used for the electron transport and hole blocking layer (ETL), lithium fluoride (LiF) was used for the electron injection layer (EIL), and ITO was used as the anode and aluminum (Al) as the cathode. All organic layers were deposited under 10−6 Torr, with a rate of deposition of 1.0 Å/s to create an emitting area of 4 mm2. LiF and aluminum layers were continuously deposited under the same vacuum conditions. Luminance efficiency data for the fabricated EL devices were obtained using a Keithley 2400 44
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Scheme 1. Synthetic routes of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP.
Fig. 5 shows the calculation results of 1,6-DTBP, 4,9-DTBP, and 1,8DTBP using the B3LYP/6-31G (d) level. In the side view of the optimized molecular structure, 1,6-DTBP had a more planar structure than the other two materials. On the other hand, in the case of 4,9-DTBP and 1,8-DTBP, the pyrene core and triphenylbenzene side group were highly twisted. Such a highly twisted structure prevents intermolecular interactions in the film state [16]. This difference in intermolecular interactions may also affect the absolute photoluminescence quantum yields (PLQYs), which were 43%, 55%, and 69% for 1,6-DTBP, 4,9DTBP and 1,8-DTBP, respectively. As shown in Table 1, The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and band gap of
which was reported by A. T. Balabana et al. [15] As shown in Fig. 4, the relative π-electron partition values are described in pyrene. The upper and lower phenyl rings inside of pyrene have an π-electron partition value of 4.67, while the value of the two phenyl rings in the middle of pyrene is 3.33, indicating relatively deficient π-electron partition. The FWHM values of 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP in solution state were 54 nm, 54 nm, and 35 nm, respectively, and 4,9-DTBP had the narrowest PL spectrum among the three compounds. In case of the film state, the FWHM values of 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP were 68 nm, 54 nm, and 46 nm. 1,8-DTBP in the film state had the same FWHM value, while the other two compounds exhibited increased FWHM values in the film state.
Fig. 2. (a) UV–vis absorption and (b) PL spectra of 1 ⅹ10−5 M THF solution for the synthesized materials. 45
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Fig. 3. (a) UV–vis absorption and (b) PL spectra of the vacuum-deposited film for the synthesized materials.
structure. The OLED device properties and luminance – current density plot are summarized in Fig. 7, Fig. 8, Table 2, and Fig. S3. Fig. 7 (a) shows the energy levels of the organic materials used in OLED devices. The HOMO and LUMO levels of the synthesized materials matched the HOMO levels of the carrier transporting materials. Fig. 7 (b) shows the EL spectra of the EL device. The ELmax of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP exhibited similar maximum values to the PL spectrum at 455 nm, 433 nm, and 456 nm, respectively. Such a maximum emission wavelength value suggests that the light emission of the device is generated from the synthesized material within under normal device operation. In Fig. 8, 1,8-DTBP showed higher efficiency than 1,6DTBP or 4,9-DTBP. Table 2 shows the turn-on voltage, luminance efficiency (LE), power efficiency (PE), external quantum efficiency (EQE), and color coordinates values for OLED devices based on the three materials. 1,6DTBP had a turn-on voltage of 4.0 V, LE of 1.82 cd/A, PE of 1.08 lm/W, EQE of 1.59%, and color coordinate of (0.168, 0.152). For 4,9-DTBP and 1,8-DTBP, these values were 4.2 V, 1.58 cd/A, 0.92 lm/W, 3.47%, and (0.158, 0.063), and 4.0 V, 6.89 cd/A, 3.03 lm/W, 7.10%, and (0.151, 0.125), respectively. All three materials exhibited a similar turn-on voltage of 4.0 V at 1 cd/m2. The LE of 1,8-DTBP showed the highest efficiency of 6.89 cd/A among the three materials. This can be explained by the suppressed intermolecular interactions and consequent high PLQY of 69%, which are related to the bulky triphenyl benzene group. The PE of 1,8-DTBP was 3.03 lm/W, which was three times higher than those of the other two materials. The EQE value was generally affected by the LE and CIE coordinate values. Thus, 1,8-DTBP exhibited an excellent EQE of 7.10%, which was approximately 4.5 times higher than that of 1,6DTBP. As shown in Fig. 9, in the case of mobile phones, attaining a blue color requires a y value of 0.15 or less for the color coordinates and
the synthesized materials were −5.70 eV, −2.76 eV, and 2.94 eV for 1,6-DTBP, −5.69 eV, −2.74 eV, and 2.95 eV for 1,8-DTBP, and −5.78 eV, −2.65 eV, and 3.13 eV for 4,9-DTBP, respectively. In the calculation results of Fig. 5, the electron density distributions of HOMO and LUMO of the newly synthesized materials were mainly localized to the pyrene core. Also, the tendency of the calculation results for HOMO, LUMO, and band gap was similar to those obtained by experiments. The CV of the compounds were measured to further investigate their electrochemical properties as shown in Fig. S2. HOMO level calculated by CV results were −5.95 eV for 1,6-DTBP, −6.02 eV for 4,9-DTBP, and −5.98 eV for 1,8-DTBP, respectively. To determine the thermal properties of the synthesized molecules, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used as shown in Fig. 6 and Table 1. OLED materials require high thermal stability. Materials with a high glass transition temperature (Tg) and decomposition temperature (Td) are advantageous in that their morphology does not easily change due to the heat generated during OLED device operation [17]. As shown in Table 1, Td of the newly synthesized materials showed excellent thermal stability over 500 °C. In the case of Tg, 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP had high values of 159 °C, 146 °C, and 142 °C. These values indicated better thermal properties than Tg (120 °C) of MADN, which is a well-known blue-emitting material with an anthracene core. In order to observe the change in EL performance according to the substitution position on the pyrene core, an OLED device was fabricated using the synthesized materials as non-doped EMLs according to the following structure: ITO/NPB (40 nm)/TCTA (20 nm)/EML (30 nm)/ TPBi (20 nm)/LiF (1 nm)/Al (200 nm). NPB was used as the hole transport layer, and the TCTA layer played a hole transport and electron blocking role. TPBi was used as the electron transport and hole blocking layers. Emitting materials were used as a single layer with a non-doped
Table 1 Physical properties of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP. Compounds
1,6-DTBP 4,9-DTBP 1,8-DTBP a b c d e
Solutiona
Film on glass
λmax abs (nm)
λmax PL (nm)
FWHMb (nm)
λmax abs (nm)
λmax PL (nm)
FWHMb (nm)
365 349 365
425 384, 405 425
54 35 54
373 358 373
461 428 461
68 46 54
Measured in 1 × 10−5 M THF. Full width at half maximum of PL. Absolute photoluminescence quantum yield in neat films. Ultraviolet photoelectron spectroscopy (Riken-keiki, AC-2). LUMO obtained from HOMO and the optical band gap. 46
Φf c
HO MOd (eV)
LU MOe (eV)
Band gap (eV)
Tg (°C)
Td (°C)
43 55 69
−5.70 −5.78 −5.69
−2.76 −2.65 −2.74
2.94 3.13 2.95
159 146 142
508 530 527
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Fig. 4. The π-electron partition of pyrene.
high definition (HD)-TV requires a y value of 0.08 or less. Three compounds met the requirement specifications of mobile phones in case of color purity. Also, although EL efficiency of 4,9-DTBP was lower than that of 1,8-DTBP, the color coordinate y value was 0.063, showing excellent color purity exceeding 0.08. Fig. 6. TGA curves of the synthesized materials.
4. Conclusion the HD-TV requirements due to its wide band gap. These results are expected to contribute to the development of new blue-emitting materials.
1,6-DTBP, 4,9-DTBP, and 1,8-DTBP were successfully synthesized using a pyrene core group and triphenyl benzene side group with different link positions. The PLQYs of the 1,6-DTBP, 4,9-DTBP and 1,8DTBP materials were 43%, 55%, and 69%, respectively. 1,8-DTBP showed the highest PLQY value among the three compounds, likely due to its highly twisted structure. When these synthesized materials were applied to non-doped OLEDs as an EML, the EQE of 1,6-DTBP, 4,9DTBP, and 1,8-DTBP was 1.59%, 3.47%, and 7.10%. 1,8-DTBP showed remarkably improved EL performance for blue emission. In addition, 4,9-DTBP had a deep-blue emission of CIE x, y (0.158, 0.063), satisfying
Acknowledgements This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science & ICT (No. 2017M3A7B4041699). This research was supported by the Basic Science Research Program through the
Fig. 5. Pictorial presentation of the frontier orbitals, a plot of the Kohn-Sham HOMO and LUMO energy levels for pyrene, triphenylbenzene (TB), 1,6-DTBP, 4,9DTBP, and 1,8-DTBP (calculated at the B3LYP/6-31G(d) level of theory). 47
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Fig. 7. EL characteristics of devices using 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP as EMLs: (a) energy levels of materials, (b) EL spectra.
Fig. 8. EL performance of devices using 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP as EMLs: (a) luminance efficiency versus current density, (b) external quantum efficiency versus current density.
Table 2 The EL performance of the ITO/NPB (40 nm)/TCTA (20 nm)/EML (30 nm)/ TPBi (20 nm)/LiF/Al device. Compounds
Von (V)a
L (cd/ m2)b
LE (Cd/ A)b
PE (lm/ W)b
EQE (%)b
CIE (x,y)b
λmax EL (nm)
1,6-DTBP
4.0
182
1.82
1.08
1.59
455
4,9-DTBP
4.2
158
1.58
0.92
3.47
1,8-DTBP
4.0
689
6.89
3.03
7.10
(0.168, 0.152) (0.158, 0.063) (0.151, 0.125)
433 456
a
Turn-on voltage at 1 cd/m2. Luminance, luminance efficiency, power efficiency, external quantum efficiency, Commission Internationale de l’Eclairage at 10 mA/cm2. b
National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1A09082138). Appendix A. Supplementary data Fig. 9. CIE x, y values of the synthesized compounds: (a) y value of 0.15 for mobile phone specifications, (b) y value of 0.08 for HD-TV specifications.
Supplementary data related to this chapter can be found at http:// dx.doi.org/10.1016/j.dyepig.2018.05.024. References [1] (a) Tang C, Vanslyke S. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913; (b) Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic
48
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M. Jung et al. light-emitting diodes from delayed fluorescence. Nature 2012;492:234. [2] Katz HE, Bao Z, Gilat SL. Synthetic chemistry for ultrapure, processable, and highmobility organic transistor semiconductors. Acc Chem Res 2001;34:359. [3] Li W, Hendriks KH, Wienk MM, Janssen RAJ. Diketopyrrolopyrrole polymers for organic solar cells. Acc Chem Res 2016;49:78. [4] (a) Sun YR, Giebink N, Kanno C, Ma B, Thompson ME, Forrest SR. Management of singlet and triplet excitons for efficient white organic light-emitting devices. Nature 2006;440:908; (b) Higuchi T, Nakanotani H, Adachi C. High-efficiency white organic light-emitting diodes based on a blue thermally activated delayed fluorescent emitter combined with green and red fluorescent emitters. Adv Mater 2015;27:2019. [5] Zhang S, Yao L, Peng Q, Li W, Pan Y, Xiao R, Gao Y, Gu C, Wang Z, Lu P, Li F, Su S, Yang B, Ma Y. Achieving a significantly increased efficiency in nondoped pure blue fluorescent OLED: a quasi-equivalent hybridized excited state. Adv Funct Mater 2015;25:1755. [6] Zhang Q, Li J, Shizu K, Huang S, Hirata S, Miyazaki H, Adachi C. Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes. J Am Chem Soc 2012;134:14706 (b) Yokoyama D, Park Y, Kim B, Kim S, Pu Y, Kido J, Park J. Dual efficiency enhancement by delayed fluorescence and dipole orientation in high-efficiency fluorescent organic light-emitting diodes. Appl Phys Lett 2011;99:123303. [7] (a) Lee J, Jung H, Shin H, Kim J, Yokoyama D, Nishimura H, Wakamiya A, Park J. Excimer emission based on the control of molecular structure and intermolecular interactions. J Mater Chem C 2016;2:2784. [8] Lee H, Kim B, Kim S, Kim J, Lee J, Shin H, Lee J, Park J. Synthesis and electroluminescence properties of highly efficient dual core chromophores with side groups for blue emission. J Mater Chem C 2014;2:4737. [9] (a) Kim M, Lee J. Engineering of interconnect position of bicarbazole for high
[10]
[11] [12]
[13] [14] [15] [16]
[17]
49
external quantum efficiency in green and blue phosphorescent organic light-emitting diodes. ACS Appl Mater Interfaces 2014;6:14874; (b) Jang S, Joo C, Jeon S, Yook K, Lee J. The relationship between the substitution position of the diphenylphosphine oxide on the spirobifluorene and device performances of blue phosphorescent organic light-emitting diodes. Org Electron 2010;11:1059. Park Y, Kim S, Lee J, Jung D, Wu CC, Park J. New blue-violet emitters based on an indenopyrazine core for OLEDs: effects of the position of m-terphenyl side group substitution on optical and electroluminescence properties. Org Electron 2010;11:864. Figueira-Duarte TM, Müllen K. Pyrene-based materials for organic electronics. Chem Rev 2011;111:7260. Wang Z, Xu C, Wang W, Dong X, Zhao B, Ji B. Novel pyrene derivatives: synthesis, properties and highly efficient non-doped deep-blue electroluminescent device. Dyes Pigments 2011;92:732. Frisch MJ, et al. Gaussian, Inc. 2009. Wallingford CT. Lee J, Park J. Synthesis and electroluminescence of novel pyrene-fused chromophores. Org Lett 2015;17:3960. Balabana AT, Randić M. Correlations between various ways of accounting for the distribution of π-electrons in benzenoids. New J Chem 2008;32:1071. (a) Danel K, Huang TH, Lin JT, Tao YT, Chuen CH. Blue-emitting anthracenes with end-capping diarylamines. Chem Mater 2002;14:3860; (b) Shih PI, Chuang CY, Chien CH, Diau EWG, Shu CF. Highly efficient non-doped blue-light-emitting diodes based on an anthrancene derivative end-capped with tetraphenylethylene groups. Adv Funct Mater 2007;17:3141. Park Y, Kim B, Lee C, Hyun A, Jang S, Lee JH, Gal YS, Kim TH, Kim KS, Park J. Highly efficient new hole injection materials for OLEDs based on dimeric phenothiazine and phenoxazine derivatives. J Phys Chem C 2011;115:4843.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Highly efficient pyrene blue emitters for OLEDs based on substitution position effect
T
Mina Junga,1, Jaehyun Leea,c,1, Hyocheol Jungb, Seokwoo Kangb, Atsushi Wakamiyac, Jongwook Parkb,∗ a
Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, South Korea Department of Chemical Engineering, Kyung Hee University, Gyeonggi, 17104, South Korea c Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan b
A B S T R A C T
To investigate the effect of substitution position of the side group on a pyrene core, three derivatives having a triphenylbenzene group as a bulky side group at the 1,6-position, 4,9-position, and 1,8-position were successfully synthesized: 1,6-bis(5′-phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,6-DTBP), 4,9-bis(5′-phenyl-[1,1':3′,1″terphenyl]-4-yl)pyrene (4,9-DTBP), and 1,8-bis(5′-phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,8-DTBP). The three synthesized materials showed excellent thermal stability with a high Tg of > 140 °C and a high Td of > 500 °C. Due to the highly twisted structure of 1,8-DTBP in the film state, the absolute photoluminescence quantum yield value was improved. Of the three synthesized materials used as an emitter in a non-doped organic light-emitting diode device, 1,8-DTBP showed highly efficient electroluminescence performance, with a luminance efficiency of 6.89 cd/A, power efficiency of 3.03 lm/W, and external quantum efficiency of 7.10% at 10 mA/cm2. In addition, 4,9-DTBP showed a deep-blue emission of CIE x, y (0.158, 0.063) suitable for HD-TV.
1. Introduction In the past two decades, interest in π-conjugated compounds in the optoelectronic field, such as organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic photovoltaic cells (OPVCs), has increased dramatically [1–3]. In particular, OLEDs have been intensively studied due to potential applications such as full-color flat-panel displays, next-generation lighting, and flexible displays [4]. The current research trends in OLED technologies have focused on optimizing device structure and developing new emitting materials. Among the various development methods for OLEDs, new blue-emitting materials that satisfy high electroluminescence (EL) efficiencies, good thermal stability, long device lifetime, and pure color coordinates remains a hot button issue [5]. Predicting the properties of emitting materials and finding effective ways to develop high-performance materials depends on the relationship between molecular structure and properties [6,7]. These molecular structures consist of core groups and side groups. As a core group, for example, anthracene and pyrene are typically used as blue light-emitting materials, and a variety of derivatives combined side groups have been reported in the blue light-emitting field [8]. Among the many blue
∗
1
Corresponding author. E-mail address: [email protected] (J. Park). Mina Jung and Jaehyun Lee contributed equally to this work as the first coauthor.
https://doi.org/10.1016/j.dyepig.2018.05.024 Received 19 March 2018; Received in revised form 12 May 2018; Accepted 13 May 2018 Available online 14 May 2018 0143-7208/ © 2018 Published by Elsevier Ltd.
emitters using core and side groups, substitution positions can affect optical and electrical properties including conjugation length, electronic state, photophysical properties, and intramolecular and intermolecular interactions [9]. Therefore, when designing the emitting material, an understanding of the substitution position effect is needed. However, few studies have investigated substitution positions between core and side groups and their effects. Therefore, we report the efficiency and color purity of blue light emitters when using different link positions with the same core and side groups. In our previous study, the m-terphenyl side groups were introduced in the ortho, meta, and para link positions on 6,12-dihydro-diindeno [1,2-b; 1′, 2′-e] pyrazine (indenopyrazine) and the optical and EL properties were investigated according to changes in the substitution position [10]. Larger blue shifts in the UV–visible (UV–vis) absorption and photoluminescence (PL) spectra occur in the synthesized ortho- and para-substituted derivatives compared to the meta-substituted derivative. In particular, the OLED device containing the para-substituted derivative exhibited excellent characteristics, with maximum EL emission at 423 nm, a full width at half maximum of 42 nm, pure violet emission with CIE coordinates (0.173, 0.063), and an external quantum efficiency of 1.88%.
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Compound 4. Compound 3 (3.00 g, 7.79 mmol) was added to 13 mL of anhydrous THF and stirred at −78 °C under a nitrogen atmosphere, then 2 M n-BuLi (5.85 mL, 11.7 mmol) was added to the reaction mixture. Next, isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.18 mL, 15.6 mmol) was added to the reaction mixture after 30 min. After the reaction was finished, the reaction mixture was extracted with ethyl acetate and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was removed by evaporation. The product was purified by silica gel column chromatography using THF: n-hexane (1: 10) eluent to afford a white solid (3.01 g, 89%). 1 H -NMR (300 MHz, CDCl3): δ = 7.94 (d, 2H), 7.79 (m, 3H), 7.69 (m, 6H), 7.46 (t, 4H), 7.37 (t, 2H), 1.24 (d, 12H); EI + -Mass: 432. Compounds 5 and 6. Bromine (10.0 mL, 195 mmol) in CHCl3 (500 mL) was dropped into a solution of pyrene (20.0 g, 98.9 mmol) in CHCl3 (500 mL) over 5 h while stirring. The precipitate was collected after 12 h and recrystallized from xylene to give compound 5 (5.01 g, 14%) and compound 6 (2.17 g, 6%). Compound 5: 1H NMR (300 MHz, CDCl3): δ = 8.46 (d, 2H), 8.26 (d, 2H), 8.11 (d, 2H), 8.05 (d, 2H); EI + -Mass: 360. Compound 6: 1H NMR (300 MHz, CDCl3): δ = 8.56 (s, 2H), 8.27 (d, 2H), 8.05 (m, 4H); EI + -Mass: 360. 1,6-DTBP. Compound 5 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was filtered. The product was isolated using silica gel column chromatography with CHCl3 as the solvent. The eluent was removed by evaporation. Recrystallization of the residue from CHCl3 afforded a beige solid product (0.51 g, 23%). 1H NMR (300 MHz, [D8]THF): δ = 8.33 (m, 4H), 8.16 (d, 2H), 8.07 (d, 2H), 8.05 (m, 8H), 7.93 (s, 2H), 7.86 (m, 12H), 7.52 (t, 8H), 7.41 (t, 4H); 13C NMR data could not be obtained because of the low solubility of the 1,6-DTBP; TOF-MS 810.3 m/z. 1,8-DTBP. Compound 6 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated by silica gel column chromatography using the CHCl3: n-hexane (1: 10) eluent to afford a beige solid (1.51 g, 67%). 1H NMR (300 MHz, [D8]THF): δ = 8.35 (d, 2H), 8.29 (s, 2H), 8.20 (s, 2H), 8.11 (d, 2H), 8.03 (m, 8H), 7.92 (s, 2H), 7.83 (m, 12H), 7.50 (t, 8H), 7.39 (t, 4H); 13C NMR (125 MHz, CDCl3): δ = 142.63, 142.10, 141.31, 140.58, 140.20, 137.30, 131.28, 131.20, 129.04, 128.57, 127.89, 127.75, 127.69, 127.54, 127.48, 125.63, 125.48, 125.33, 125.10; HRMS (EI, m/z): [M+] calcd for C64H42, 810.3287; found, 810.3291; elemental analysis calcd (%) for C64H42: C 94.78, H 5.22; found: C 94.39, H 5.31%. Compound 7. Pyrene (20.0 g, 98.9 mmol) was added to 500 mL of 1-pentanol and stirred while refluxing. 32.0 g of sodium was slowly added for 1 h and additionally stirred for 2 h. Temperature was reduced to room temperature and the solution was extracted using water. After vaporizing the solvent, ethanol was used for recrystallization to obtain a white solid (9.25 g, 45%). 1H NMR (300 MHz, CDCl3): δ = 7.12 (s, 4H), 3.04 (t, 8H), 2.00 (m, 4H); EI + -Mass: 208. Compound 8. Compound 7 (42.0 g, 144 mmol) was added to 100 mL of CH2Cl2 and stirred. 15 mL of Br2 was added to 30 mL of CH2Cl2, and this solution was slowly added by dropping. After 30 min of reaction, the material to be extracted was filtered. A white solid was obtained (16.9 g, 32%). 1H NMR (300 MHz, CDCl3): δ = 7.40 (s, 2H), 3.06 (t, 4H), 2.97 (t, 4H), 1.99 (m, 4H), EI + -Mass: 366. Compound 9. Compound 8 (12.0 g, 32.8 mmol) and 22.3 g of DDQ
The pyrene core group is one of the most efficient core groups among organic compounds, and has been used to replace anthracene recently due to advantages such as high quantum efficiency and thermal and chemical stability [11]. With regard to the link positions of pyrene, the 1, 3, 6, and 8 positions are easily accessible in organic synthesis, as established in previous research [12]. However, because of the synthetic challenge, research on pyrene derivatives in terms of the 4 and 9 positions has been not been investigated. Also, it is very difficult to separate 1,6 and 1,8 disubstituted pyrene derivatives because of chemical similarity of two compounds. Especially, we introduce 1,8 disubstituted pyrene in this study which has twisted chemical structure. Thus, pyrene derivatives which are substituted for various positions, can provide not only numerous new chromophores but also an effective strategy for performance optimization of pyrene-based emitters. In this study, we successfully synthesized Br-substituted pyrene compounds at the 1,6, 4,9, and 1,8 positions to investigate the effect of each substitution position on the pyrene core group. The triphenylbenzene group was selected for use as a bulky side group and was substituted at each position to synthesize three derivatives: 1,6-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,6-DTBP), 4,9-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (4,9-DTBP), 1,8-bis(5′phenyl-[1,1':3′,1″-terphenyl]-4-yl)pyrene (1,8-DTBP). The optical properties of the three compounds were compared with the EL properties of non-doped OLED devices using as the emitting layer (EML). 2. Experimental 2.1. Synthesis Compound 1. 1,3,5-Tribromobenzene (20.0 g, 63.5 mmol), phenyl boronic acid (18.8 g, 154 mmol), and Pd(PPh3)4 (3.75 g, 3.25 mmol) were added to 300 mL of anhydrous THF. Then, 2 M K2CO3 solution (100 mL), which was dissolved in H2O, was added to the reaction mixture. The mixture was heated to 65 °C for 12 h under nitrogen. After completion of the reaction, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated via silica gel column chromatography by using CHCl3: n-hexane (1: 20) as the eluent to afford a white solid (12.4 g, 63%). 1H NMR (300 MHz, CDCl3): δ = 7.70 (s, 3H), 7.60 (d, 4H), 7.44(t, 4H), 7.36 (t, 2H); EI + Mass: 310. Compound 2. Compound 1 (12.0 g, 38.8 mmol) was added to 300 mL of anhydrous THF and stirred at −78 °C under a nitrogen atmosphere, then 2 M n-BuLi (29.0 mL, 58.1 mmol) was added to the reaction mixture. Next, isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (11.9 mL, 58.1 mmol) was added to the reaction mixture after 30 min. After the reaction was finished, the reaction mixture was extracted with diethyl ether and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The residue was re-dissolved in CHCl3 and MeOH was added to the solution. The precipitate was filtered and washed with MeOH to afford a white solid (12.9 g, 93%). 1H NMR (300 MHz, CDCl3): δ = 8.03 (s, 2H), 7.90 (s, 1H), 7.69 (d, 4H), 7.44(t, 4H), 7.35 (t, 2H), 1.37 (s, 12H); EI + Mass: 356. Compound 3. 1,4-dibromobenzene (1.65 g, 7.00 mmol), compound 2 (2.49 g, 7.00 mmol), and Pd(PPh3)4 (0.40 g, 0.35 mmol) were added to 100 mL of anhydrous THF. Then, 2 M K2CO3 solution (20 mL), which was dissolved in H2O, was added to the reaction mixture. The reaction mixture was heated to 65 °C for 12 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered. The solvent was evaporated. The product was isolated by silica gel column chromatography using CHCl3: n-hexane (1: 10) eluent to afford a white solid (2.07 g, Yield 77%). 1H NMR (300 MHz, CDCl3): δ = 7.80 (s, 1H), 7.74 (s, 2H), 7.69 (d, 4H), 7.58 (m, 4H), 7.48 (t, 4H), 7.39 (t, 2H); EI + -Mass: 384. 43
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were added to 240 mL of anhydrous toluene under nitrogen atmosphere and stirred for reflux. After 4 h of reaction, the temperature was reduced to room temperature. The solution was refined by column chromatography using CHCl3. The solvent was vaporized and recrystallized using CHCl3. A yellow solid was obtained (4.51 g, 38%). 1H NMR (300 MHz, CDCl3): δ = 8.57 (d, 2H), 8.40 (s, 2H), 8.13 (d, 2H), 8.04 (t, 2H); EI + -Mass: 360. 4,9-DTBP. Compound 9 (1.00 g, 2.78 mmol), compound 4 (2.63 g, 6.08 mmol), Pd(OAc)2 (0.06 g, 0.28 mmol), and (cyclohexyl)3P (0.12 g, 0.42 mmol) were added to 30 mL of anhydrous THF and 170 mL of anhydrous toluene mixture. Then tetraethylammonium hydroxide (20 wt%) (35 mL) was added to the reaction mixture at 50 °C. The mixture was heated to 80 °C for 2 h under nitrogen. After the reaction was finished, the reaction mixture was extracted with CHCl3 and water. The organic layer was dried with anhydrous MgSO4 and filtered, and the solvent was evaporated. The product was isolated by silica gel column chromatography using a CHCl3: n-hexane (1: 10) eluent to produce a beige solid (1.15 g, 51%). 1H NMR (300 MHz, [D8]THF): δ = 8.35 (m, 4H), 8.20 (s, 2H), 8.07 (m, 10H), 7.94 (s, 2H), 7.87 (m, 12H), 7.53 (t, 8H), 7.41 (t, 4H); 13C NMR (125 MHz, CDCl3): δ = 142.67, 142.17, 141.33, 140.56, 140.27, 139.16, 131.26, 130.81, 130.54, 129.07, 128.31, 127.78, 127.57, 126.27, 125.67, 125.53, 125.37, 124.81, 124.06; HRMS (EI, m/z): [M+] calcd for C64H42, 810.3287; found, 810.3291; elemental analysis calcd (%) for C64H42: C 94.78, H 5.22; found: C 94.48, H 5.29%.
Fig. 1. Chemical structures of the pyrene derivatives.
electrometer. Light intensities were obtained with a Minolta CS-1000A. The operational stability of the devices was measured under encapsulation in a glove box. 3. Results and discussion
2.2. Measurements and OLED fabrication Fig. 1 and Scheme 1 show the chemical structure and synthetic methods of the synthesized materials. In this study, optical characteristics and OLED performance according to different substitution positions were observed with the same core and side groups. Pyrene chromophore was used as the core group in a blue OLED emitter and a triphenylbenzene group was used as a side group to reduce intermolecular interactions and to prevent excimer formation through a bulky chemical structure. The substitution position was either 1,6, 4,9, or 1,8, with 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP, respectively, synthesized as model compounds. In organic synthesis, conventional Suzuki-Miyaura cross-coupling reactions and boronylation were used. In particular, the bromination of pyrene resulted in two isomers, 1,6-dibromopyrene and 1,8-dibromopyrene. These two isomers were purified several times through recrystallization. 4,9Dibromopyrene cannot be synthesized by direct bromination due to its regioselectivity. In order to obtain 4,9-dibromopyrene, it was synthesized from pyrene through the three synthetic steps shown in Scheme 1 [14]. All compounds were purified by recrystallization and column chromatography. The synthesized compounds were characterized using nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, and elemental analysis. The synthetic methods are described in more detail in the Experimental section. Figs. 2 and 3 show the UV–vis absorption and PL spectra of the synthesized compounds in 1 × 10−5 M THF solution and vacuum-deposited film states. Table 1 summarizes the optical properties, absolute photoluminescence quantum yield, energy levels of molecular orbitals, and thermal properties of synthesized compounds. The tendencies of the UV–vis absorption and PL spectra were similar in the solution and film states. The photophysical properties of 1,6-DTBP and 1,8-DTBP were similar, and 4,9-DTBP showed a relatively blue-shifted absorption and PL spectrum. The UVmax and PLmax of 1,6-DTBP and 1,8-DTBP in the THF solution were the same at 365 nm and 425 nm, and were 373 nm and 461 nm for the film state, respectively. 4,9-DTBP exhibited UVmax of 349 nm and PLmax of 384 nm in the solution state, and UVmax of 358 nm and PLmax of 428 nm in the film state, respectively. The PL spectra in various solvents in order to investigate the intramolecular charge transfer of compounds were shown in Fig. S1. Stokes shifts of three compounds in various solvents were small. The blue-shifted value of 4,9-DTBP can be explained by the π-electron partition of pyrene,
13
C NMR data of the 1,6-DTBP could not be obtained because of the very low solubility. The 1H NMR spectra and 13C NMR spectra were recorded on Bruker Avance 300 (300 MHz for 1H) and JEOL ECA 500 (125 MHz for 13C) spectrometers. The FAB+-mass and EI+-spectra were recorded on a JEOL, JMS-AX505WA, HP5890 series II. The optical absorption spectra were obtained using a Lambda 1050 UV/vis/NIR spectrometer (PerkinElmer). A PerkinElmer luminescence spectrometer LS50 (Xenon flash tube) was used to perform photoluminescence (PL) spectroscopy. PL quantum yield was measured by an absolute PL quantum yield measurement system (QM-400 Spectrofluorometer). The glass-transition temperatures (Tg) of the compounds were determined with differential scanning calorimetry (DSC) under a nitrogen atmosphere by using a DSC4000 (PerkinElmer). Degradation temperatures (Td) were determined with thermogravimetric analysis (TGA) using a TGA4000 (PerkinElmer). HOMO energy levels were determined by ultraviolet photoelectron yield spectroscopy (Riken Keiki AC-2). LUMO energy levels were derived from HOMO energy levels and band gaps. The redox potentials of the compounds were determined with cyclic voltammetry (CV) using IVIUM STAT system. The synthesized materials were dissolved in acetonitrile with 0.1 M tetrabutylammonium perchlorate as an electrolyte. We used a carbon working electrode, a platinum wire counter electrode and a saturated Ag/AgCl reference electrode. Ferrocene was used for potential calibration. DFT calculations for optimization of the geometries were conducted using the Gaussian 09 program [13]. In each of the EL devices, N,N′-bis(naphthalen-1-yl)-N,N′-bis (phenyl)benzidine (NPB) was used for the hole transport and injection layer (HTL), tris(4-carbazoyl-9-ylphenyl)amine (TCTA) was used for the hole transport and electron blocking layer, one of the synthetic materials 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP was used as the EML, 1,3,5-tri (1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) was used for the electron transport and hole blocking layer (ETL), lithium fluoride (LiF) was used for the electron injection layer (EIL), and ITO was used as the anode and aluminum (Al) as the cathode. All organic layers were deposited under 10−6 Torr, with a rate of deposition of 1.0 Å/s to create an emitting area of 4 mm2. LiF and aluminum layers were continuously deposited under the same vacuum conditions. Luminance efficiency data for the fabricated EL devices were obtained using a Keithley 2400 44
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Scheme 1. Synthetic routes of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP.
Fig. 5 shows the calculation results of 1,6-DTBP, 4,9-DTBP, and 1,8DTBP using the B3LYP/6-31G (d) level. In the side view of the optimized molecular structure, 1,6-DTBP had a more planar structure than the other two materials. On the other hand, in the case of 4,9-DTBP and 1,8-DTBP, the pyrene core and triphenylbenzene side group were highly twisted. Such a highly twisted structure prevents intermolecular interactions in the film state [16]. This difference in intermolecular interactions may also affect the absolute photoluminescence quantum yields (PLQYs), which were 43%, 55%, and 69% for 1,6-DTBP, 4,9DTBP and 1,8-DTBP, respectively. As shown in Table 1, The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and band gap of
which was reported by A. T. Balabana et al. [15] As shown in Fig. 4, the relative π-electron partition values are described in pyrene. The upper and lower phenyl rings inside of pyrene have an π-electron partition value of 4.67, while the value of the two phenyl rings in the middle of pyrene is 3.33, indicating relatively deficient π-electron partition. The FWHM values of 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP in solution state were 54 nm, 54 nm, and 35 nm, respectively, and 4,9-DTBP had the narrowest PL spectrum among the three compounds. In case of the film state, the FWHM values of 1,6-DTBP, 1,8-DTBP, and 4,9-DTBP were 68 nm, 54 nm, and 46 nm. 1,8-DTBP in the film state had the same FWHM value, while the other two compounds exhibited increased FWHM values in the film state.
Fig. 2. (a) UV–vis absorption and (b) PL spectra of 1 ⅹ10−5 M THF solution for the synthesized materials. 45
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Fig. 3. (a) UV–vis absorption and (b) PL spectra of the vacuum-deposited film for the synthesized materials.
structure. The OLED device properties and luminance – current density plot are summarized in Fig. 7, Fig. 8, Table 2, and Fig. S3. Fig. 7 (a) shows the energy levels of the organic materials used in OLED devices. The HOMO and LUMO levels of the synthesized materials matched the HOMO levels of the carrier transporting materials. Fig. 7 (b) shows the EL spectra of the EL device. The ELmax of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP exhibited similar maximum values to the PL spectrum at 455 nm, 433 nm, and 456 nm, respectively. Such a maximum emission wavelength value suggests that the light emission of the device is generated from the synthesized material within under normal device operation. In Fig. 8, 1,8-DTBP showed higher efficiency than 1,6DTBP or 4,9-DTBP. Table 2 shows the turn-on voltage, luminance efficiency (LE), power efficiency (PE), external quantum efficiency (EQE), and color coordinates values for OLED devices based on the three materials. 1,6DTBP had a turn-on voltage of 4.0 V, LE of 1.82 cd/A, PE of 1.08 lm/W, EQE of 1.59%, and color coordinate of (0.168, 0.152). For 4,9-DTBP and 1,8-DTBP, these values were 4.2 V, 1.58 cd/A, 0.92 lm/W, 3.47%, and (0.158, 0.063), and 4.0 V, 6.89 cd/A, 3.03 lm/W, 7.10%, and (0.151, 0.125), respectively. All three materials exhibited a similar turn-on voltage of 4.0 V at 1 cd/m2. The LE of 1,8-DTBP showed the highest efficiency of 6.89 cd/A among the three materials. This can be explained by the suppressed intermolecular interactions and consequent high PLQY of 69%, which are related to the bulky triphenyl benzene group. The PE of 1,8-DTBP was 3.03 lm/W, which was three times higher than those of the other two materials. The EQE value was generally affected by the LE and CIE coordinate values. Thus, 1,8-DTBP exhibited an excellent EQE of 7.10%, which was approximately 4.5 times higher than that of 1,6DTBP. As shown in Fig. 9, in the case of mobile phones, attaining a blue color requires a y value of 0.15 or less for the color coordinates and
the synthesized materials were −5.70 eV, −2.76 eV, and 2.94 eV for 1,6-DTBP, −5.69 eV, −2.74 eV, and 2.95 eV for 1,8-DTBP, and −5.78 eV, −2.65 eV, and 3.13 eV for 4,9-DTBP, respectively. In the calculation results of Fig. 5, the electron density distributions of HOMO and LUMO of the newly synthesized materials were mainly localized to the pyrene core. Also, the tendency of the calculation results for HOMO, LUMO, and band gap was similar to those obtained by experiments. The CV of the compounds were measured to further investigate their electrochemical properties as shown in Fig. S2. HOMO level calculated by CV results were −5.95 eV for 1,6-DTBP, −6.02 eV for 4,9-DTBP, and −5.98 eV for 1,8-DTBP, respectively. To determine the thermal properties of the synthesized molecules, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used as shown in Fig. 6 and Table 1. OLED materials require high thermal stability. Materials with a high glass transition temperature (Tg) and decomposition temperature (Td) are advantageous in that their morphology does not easily change due to the heat generated during OLED device operation [17]. As shown in Table 1, Td of the newly synthesized materials showed excellent thermal stability over 500 °C. In the case of Tg, 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP had high values of 159 °C, 146 °C, and 142 °C. These values indicated better thermal properties than Tg (120 °C) of MADN, which is a well-known blue-emitting material with an anthracene core. In order to observe the change in EL performance according to the substitution position on the pyrene core, an OLED device was fabricated using the synthesized materials as non-doped EMLs according to the following structure: ITO/NPB (40 nm)/TCTA (20 nm)/EML (30 nm)/ TPBi (20 nm)/LiF (1 nm)/Al (200 nm). NPB was used as the hole transport layer, and the TCTA layer played a hole transport and electron blocking role. TPBi was used as the electron transport and hole blocking layers. Emitting materials were used as a single layer with a non-doped
Table 1 Physical properties of 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP. Compounds
1,6-DTBP 4,9-DTBP 1,8-DTBP a b c d e
Solutiona
Film on glass
λmax abs (nm)
λmax PL (nm)
FWHMb (nm)
λmax abs (nm)
λmax PL (nm)
FWHMb (nm)
365 349 365
425 384, 405 425
54 35 54
373 358 373
461 428 461
68 46 54
Measured in 1 × 10−5 M THF. Full width at half maximum of PL. Absolute photoluminescence quantum yield in neat films. Ultraviolet photoelectron spectroscopy (Riken-keiki, AC-2). LUMO obtained from HOMO and the optical band gap. 46
Φf c
HO MOd (eV)
LU MOe (eV)
Band gap (eV)
Tg (°C)
Td (°C)
43 55 69
−5.70 −5.78 −5.69
−2.76 −2.65 −2.74
2.94 3.13 2.95
159 146 142
508 530 527
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Fig. 4. The π-electron partition of pyrene.
high definition (HD)-TV requires a y value of 0.08 or less. Three compounds met the requirement specifications of mobile phones in case of color purity. Also, although EL efficiency of 4,9-DTBP was lower than that of 1,8-DTBP, the color coordinate y value was 0.063, showing excellent color purity exceeding 0.08. Fig. 6. TGA curves of the synthesized materials.
4. Conclusion the HD-TV requirements due to its wide band gap. These results are expected to contribute to the development of new blue-emitting materials.
1,6-DTBP, 4,9-DTBP, and 1,8-DTBP were successfully synthesized using a pyrene core group and triphenyl benzene side group with different link positions. The PLQYs of the 1,6-DTBP, 4,9-DTBP and 1,8DTBP materials were 43%, 55%, and 69%, respectively. 1,8-DTBP showed the highest PLQY value among the three compounds, likely due to its highly twisted structure. When these synthesized materials were applied to non-doped OLEDs as an EML, the EQE of 1,6-DTBP, 4,9DTBP, and 1,8-DTBP was 1.59%, 3.47%, and 7.10%. 1,8-DTBP showed remarkably improved EL performance for blue emission. In addition, 4,9-DTBP had a deep-blue emission of CIE x, y (0.158, 0.063), satisfying
Acknowledgements This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science & ICT (No. 2017M3A7B4041699). This research was supported by the Basic Science Research Program through the
Fig. 5. Pictorial presentation of the frontier orbitals, a plot of the Kohn-Sham HOMO and LUMO energy levels for pyrene, triphenylbenzene (TB), 1,6-DTBP, 4,9DTBP, and 1,8-DTBP (calculated at the B3LYP/6-31G(d) level of theory). 47
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Fig. 7. EL characteristics of devices using 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP as EMLs: (a) energy levels of materials, (b) EL spectra.
Fig. 8. EL performance of devices using 1,6-DTBP, 4,9-DTBP, and 1,8-DTBP as EMLs: (a) luminance efficiency versus current density, (b) external quantum efficiency versus current density.
Table 2 The EL performance of the ITO/NPB (40 nm)/TCTA (20 nm)/EML (30 nm)/ TPBi (20 nm)/LiF/Al device. Compounds
Von (V)a
L (cd/ m2)b
LE (Cd/ A)b
PE (lm/ W)b
EQE (%)b
CIE (x,y)b
λmax EL (nm)
1,6-DTBP
4.0
182
1.82
1.08
1.59
455
4,9-DTBP
4.2
158
1.58
0.92
3.47
1,8-DTBP
4.0
689
6.89
3.03
7.10
(0.168, 0.152) (0.158, 0.063) (0.151, 0.125)
433 456
a
Turn-on voltage at 1 cd/m2. Luminance, luminance efficiency, power efficiency, external quantum efficiency, Commission Internationale de l’Eclairage at 10 mA/cm2. b
National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1A09082138). Appendix A. Supplementary data Fig. 9. CIE x, y values of the synthesized compounds: (a) y value of 0.15 for mobile phone specifications, (b) y value of 0.08 for HD-TV specifications.
Supplementary data related to this chapter can be found at http:// dx.doi.org/10.1016/j.dyepig.2018.05.024. References [1] (a) Tang C, Vanslyke S. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913; (b) Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic
48
Dyes and Pigments 158 (2018) 42–49
M. Jung et al. light-emitting diodes from delayed fluorescence. Nature 2012;492:234. [2] Katz HE, Bao Z, Gilat SL. Synthetic chemistry for ultrapure, processable, and highmobility organic transistor semiconductors. Acc Chem Res 2001;34:359. [3] Li W, Hendriks KH, Wienk MM, Janssen RAJ. Diketopyrrolopyrrole polymers for organic solar cells. Acc Chem Res 2016;49:78. [4] (a) Sun YR, Giebink N, Kanno C, Ma B, Thompson ME, Forrest SR. Management of singlet and triplet excitons for efficient white organic light-emitting devices. Nature 2006;440:908; (b) Higuchi T, Nakanotani H, Adachi C. High-efficiency white organic light-emitting diodes based on a blue thermally activated delayed fluorescent emitter combined with green and red fluorescent emitters. Adv Mater 2015;27:2019. [5] Zhang S, Yao L, Peng Q, Li W, Pan Y, Xiao R, Gao Y, Gu C, Wang Z, Lu P, Li F, Su S, Yang B, Ma Y. Achieving a significantly increased efficiency in nondoped pure blue fluorescent OLED: a quasi-equivalent hybridized excited state. Adv Funct Mater 2015;25:1755. [6] Zhang Q, Li J, Shizu K, Huang S, Hirata S, Miyazaki H, Adachi C. Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes. J Am Chem Soc 2012;134:14706 (b) Yokoyama D, Park Y, Kim B, Kim S, Pu Y, Kido J, Park J. Dual efficiency enhancement by delayed fluorescence and dipole orientation in high-efficiency fluorescent organic light-emitting diodes. Appl Phys Lett 2011;99:123303. [7] (a) Lee J, Jung H, Shin H, Kim J, Yokoyama D, Nishimura H, Wakamiya A, Park J. Excimer emission based on the control of molecular structure and intermolecular interactions. J Mater Chem C 2016;2:2784. [8] Lee H, Kim B, Kim S, Kim J, Lee J, Shin H, Lee J, Park J. Synthesis and electroluminescence properties of highly efficient dual core chromophores with side groups for blue emission. J Mater Chem C 2014;2:4737. [9] (a) Kim M, Lee J. Engineering of interconnect position of bicarbazole for high
[10]
[11] [12]
[13] [14] [15] [16]
[17]
49
external quantum efficiency in green and blue phosphorescent organic light-emitting diodes. ACS Appl Mater Interfaces 2014;6:14874; (b) Jang S, Joo C, Jeon S, Yook K, Lee J. The relationship between the substitution position of the diphenylphosphine oxide on the spirobifluorene and device performances of blue phosphorescent organic light-emitting diodes. Org Electron 2010;11:1059. Park Y, Kim S, Lee J, Jung D, Wu CC, Park J. New blue-violet emitters based on an indenopyrazine core for OLEDs: effects of the position of m-terphenyl side group substitution on optical and electroluminescence properties. Org Electron 2010;11:864. Figueira-Duarte TM, Müllen K. Pyrene-based materials for organic electronics. Chem Rev 2011;111:7260. Wang Z, Xu C, Wang W, Dong X, Zhao B, Ji B. Novel pyrene derivatives: synthesis, properties and highly efficient non-doped deep-blue electroluminescent device. Dyes Pigments 2011;92:732. Frisch MJ, et al. Gaussian, Inc. 2009. Wallingford CT. Lee J, Park J. Synthesis and electroluminescence of novel pyrene-fused chromophores. Org Lett 2015;17:3960. Balabana AT, Randić M. Correlations between various ways of accounting for the distribution of π-electrons in benzenoids. New J Chem 2008;32:1071. (a) Danel K, Huang TH, Lin JT, Tao YT, Chuen CH. Blue-emitting anthracenes with end-capping diarylamines. Chem Mater 2002;14:3860; (b) Shih PI, Chuang CY, Chien CH, Diau EWG, Shu CF. Highly efficient non-doped blue-light-emitting diodes based on an anthrancene derivative end-capped with tetraphenylethylene groups. Adv Funct Mater 2007;17:3141. Park Y, Kim B, Lee C, Hyun A, Jang S, Lee JH, Gal YS, Kim TH, Kim KS, Park J. Highly efficient new hole injection materials for OLEDs based on dimeric phenothiazine and phenoxazine derivatives. J Phys Chem C 2011;115:4843.