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Efficient deep blue emitter based on carbazole-pyrene hybrid for non-doped electroluminescent device
Optical Materials 100 (2020) 109632
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Efficient deep blue emitter based on carbazole-pyrene hybrid for non-doped electroluminescent device Ting Zhang a, b, *, Jieyang Ye a, Ansheng Luo a, Di Liu b, ** a b
School of Chemistry and Material Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, China State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Non-doped electroluminescent device Fluorescent materials Deep-blue Carbazole Pyrene
A new deep blue fluorescent material based on carbazole-pyrene hybrid, namely PyEtCz, was synthesized by palladium catalyzed C–C Suzuki coupling reaction, and structurally characterized by NMR, mass spectra and elemental analysis. The thermal stability, photophysical and optoelectronic properties were fully investigated. The material shows good thermal stability and can form an amorphous glass state. Its CH2Cl2 solution exhibits strong deep-blue emission with high fluorescent quantum yield of 75%. PyEtCz also possesses high HOMO level of 5.48 eV that would facilitate efficient hole injection into the emitting layer. The non-doped OLED using PyEtCz as emitter through vacuum evaporation was fabricated for purpose of investigating its electrolumines cence properties. The device exhibited stable blue electroluminescence with CIE coordinate of (0.15, 0.10), a good performance with maximum luminance of 6573 cd m 2 and maximum luminance efficiency of 3.24 cd A 1 (3.35%) was achieved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted extensive attention in both industries and academia owing to their importance applications in flat-panel display and solid-state lighting [1–3]. Among the three primary emission materials (red, green and blue) to realize full-color displays and white lighting, an abundant number of red and green emitters have been realized in commercial use, and are gradually occupied a certain percentage of the domestic market [4,5]. However, blue materials with high performances remain as a major challenge because of their intrinsically wide band gap. Although many blue phosphorescent and thermally activated decayed fluorescence (TADF) device with high efficiencies can be achieved because both kinds of emitters can utilize both singlet and triplet exciton states and obtain internal quantum efficiency as high as 100%, these emitters currently are mainly for non-commercial use due to the poor color stability and purity [6–8]. Furthermore, the efficiency and the value of CIE coordi nate y for the blue emitters are also essential to determine the overall performance of the white OLEDs. Generally, the standard blue coordi nate of the National Television System Committee (NTSC) standard is (0.14, 0.08), and the CIE coordinates in the range of (x � 0.15, y � 0.15)
which was usually defined as a deep blue color and saturated deep-blue emission with a CIE coordinate of y � 0.10 [9,10]. Therefore, the development of efficient deep-blue fluorescent materials and devices with low color coordinates of y � 0.10 and good spectral stability is still a subject of current interest. Moreover, in comparison with the doped OLEDs, the non-doped blue devices possess the easy device fabrication and can avoid the absence of phase separation that has been proved to be an important factor to cause the performance degradation [11,12]. Among the various building blocks, pyrene has been intensively studied for constructing organic light-emitting materials because of its intense blue emission, excellent thermal and electroluminescence properties, and easier modification with functional groups [13–15]. In recent years, various pyrene-based materials have been explored and some of them were successfully employed as blue emitters for OLEDs [16–19]. Promarak et al. used the pyrene-functionalized materials as non-doped emitting layer to construct blue devices, which exhibited the maximum luminance efficiency and CIE coordinates of 2.06 cd A 1 and (0.15, 0.10) [17]. Ni et al. reported a group of dipolar 1,3,6,8-tetrasub stituted pyrene-based benzimidazole derivatives, and a pure blue CIE coordinates of (0.16, 0.10) with the maximum current and power effi ciencies of 2.70 cd A 1 and 1.97 lm W 1 were obtained for the
* Corresponding author. School of Chemistry and Material Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, China. ** Corresponding author. E-mail addresses: [email protected] (T. Zhang), [email protected] (D. Liu). https://doi.org/10.1016/j.optmat.2019.109632 Received 20 September 2019; Received in revised form 10 November 2019; Accepted 21 December 2019 Available online 28 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Optical Materials 100 (2020) 109632
Scheme 1. The molecular structure and synthetic routes of PyEtCz.
non-doped device by employing the meta-linked compound as a fluo rescent emitter [18]. A high external quantum efficiency of 7.10% was achieved for non-doped blue OLEDs by Park and his group using the pyrene-cored derivatives, but the devices show insufficient deep-blue emission, the CIE coordinates is (0.151, 0.125) [19]. Considering the research results above, in this paper, we designed and synthesized a new fluorescent material based on carbazole-pyrene hybrid with deep blue emission, namely PyEtCz (Scheme 1). Carbazole-based materials have been famous for their high temperature resistance, sufficient hole-transporting ability, and unique ability to form an amorphous glass state [20,21]. The carbazole unit was intro duced in the molecular structure with the expectations to enhance the possible hole-transporting ability of pyrene as well as thermal stability of the molecule. In addition, the selection of pyrene in the C2 position of carbazole ring is in the purpose of forming a highly twisted structure of the molecule, which significantly impacts the thermal and photo physical behavior of material. It was also demonstrated that the 2-substituted carbazole compounds had higher and more balanced electron and hole mobilities than their 3-substituted analogues [22,23]. The molecule designed in this way processes good thermal stability, the ability to form an amorphous glass state, strong blue fluorescence, and high HOMO energy level. The non-doped OLEDs containing PyEtCz as an emitter shows fantastic performances with the maximum external quantum efficiency of 3.35%, and stable blue electroluminescence with CIE coordinate of (0.15, 0.10).
voltammetry (CV) was carried out on a CHI–600C electro-chemical workstation comprising of a glassy carbon as working surface, a platinum-wire as counter electrode, and a saturated calomel electrode (SCE) as reference electrode by dissolving in anhydrous dichloro methane solution of sample and [n-Bu4N]PF6 at a sealed electrolytic cell. After carefully deoxygenating with argon at room temperature, the so lution was tested at a sweeping rate of 100 mV s 1 with ferrocene as the internal standard. The geometry optimized structure was calculated by density functional theory (DFT) using the B3LYP/6-31G(d) atomic basis sets through the Gaussian 09 software. The imaginary frequency in optimized structure is zero [25]. The devices were constructed by using the vacuum deposition method. At first, the glass substrates pre-coated with indium tin oxide (ITO) were carefully washed with ultrapure water, acetone, and iso propanol in sequence by using an ultrasonic machine. Then, the ITO glass substrates were vigorously scrubbed with detergent and oven-dried in a drying oven for 40 min. Before being transferred to a deposition system, three pre-cleaned substrates were treated with UV-ozone for 25 min. The organic layers were sequentially deposited onto the substrates under high vacuum (~1 � 10 5 Pa). A metal cathode layer containing 1 nm LiF and 200 nm Al was patterned by employing a shadow mask with an active area of 9 mm2 openings. The luminance, electroluminescence (EL) spectra and CIE coordinates at different driving voltage were recorded with a program-controlled PR-705 spectrometer. The current density-voltage property of device was measured with a Keithley 256 instrument. The electroluminescent measurements were made at room temperature without further encapsulations.
2. Experimental section 2.1. General information
2.2. Synthesis
The DMF, toluene and methanol used as solvents in synthesis re actions were strictly distilled from metallic Na and CaH2 by following the conventional methods. All reagents were from domestic and foreign reagent manufactures, which were directly used in experiments without having to go through the purification process. The 1H NMR and 13C NMR spectra were obtained by using a Varian INOVA 400 MHz and 100 MHz spectrometer, respectively. The sample was dissolved in CDCl3 at ambient temperature. Mass spectra were received from a HP1100LC/ MSD MS (TOF-MS-EI). The compounds were characterized by a CarloEriba-1106 elemental analysis system. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 with the rate of increasing temperature at 10 � C min 1 under nitrogen flushing during heating between 25 and 600 � C. Differential scanning calorimetry (DSC) ana lyses was undertaken on a Netzsch DSC Q2000 under nitrogen flushing. The sample was heated for two times and quickly cooled with liquid nitrogen after completion of the first heating. Ultra-violet to visible absorption spectra was obtained on a UV-265 spectrometer. Photo luminescence (PL) spectra and fluorescence quantum yields (ΦF) in dilute solution were measured on a Shimadzu RF-5301PC fluorescence spectrometer referring to the relevant literature report [24]. Cyclic
The compounds 4-bromo-2-nitrobiphenyl (1), 2-bromocarbazole (2) and 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane (4) were synthesized and characterized according to the related report [26,27]. 2.2.1. Synthesis of 2-bromo-N-ethylcarbazole (3) A 100 mL 2-necked bottle was added 2-bromocarbazole 2 (1.5 g, 6.09 mmol), pre-milled potassium hydroxide (1.71 g, 30.5 mmol) and 20 mL DMF. After the mixture was stirred for 1 h at room temperature, bromoethane (1.0 g, 9.18 mmol) was added by dropper, and the solution was then reacted continuously for 12 h at room temperature. After the reaction is completed, the solvent was removed by decompressing distillation process and the residual solid was extracted with CH2Cl2 for at least two times. Finally, the important intermediate 3 was gained by silica gel column chromatography to produce a white solid (1.64 g, 98%). MS (TOF-MS-EI, m/z): calcd. for C14H12NBr 274.1588, found 273.0156. 2.2.2. Synthesis of 2-(pyren-1-yl)-N-ethylcarbazole (PyEtCz) Under nitrogen atmosphere, a 100 mL 2-necked bottle was added 2bromo-N-ethylcarbazole 3 (0.500 g, 1.82 mmol), 4,4,5,5-tetramethyl-22
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Optical Materials 100 (2020) 109632
Fig. 1. (a) TGA and (b) DSC (the second heating cycle) curves of PyEtCz.
(pyren-1-yl)-1,3,2-dioxaborolane 4 (0.719 g, 2.19 mmol), Pd(PPh3)4 (0.105 g, 0.091 mmol), and 18 mL mixed solvent of toluene and meth anol (5:1 vol ratio). After the mixture was degassed by pump for 10 min, a 2 M aqueous potassium carbonate solution (4.6 mL, 9.2 mmol) was added, the reaction mixture was then heated to 80 � C and kept at this temperature for 8 h. When the reaction is finished, the cooled solution was extracted with CH2Cl2 for at least two times. The organic layer was collected and dried over anhydrous magnesium sulfate before being evaporated in vacuum. The crude product was finally treated by silica gel column chromatography and then recrystallized from ethanol/ chloroform to afford PyEtCz (0.612 g, 85%) as a white solid. 1H NMR (400 MHz, CDCl3, δ): 8.29–8.27 (m, 1H, ArH), 8.26–8.23 (m, 2H, ArH), 8.19–8.17 (m, 2H, ArH), 8.15–8.07 (m, 4H, ArH), 8.02–7.97 (m, 2H, ArH), 7.64 (s, 1H, ArH), 7.53–7.43 (m, 3H, ArH), 7.30–7.26 (m, 1H, ArH), 4.41–4.36 (m, 2H, CH2), 1.46 (t, J ¼ 7.2 Hz, 3H, CH3). 13C NMR (100 MHz, CDCl3, δ): 140.50, 140.15, 138.88, 138.81, 131.59, 131.08, 130.55, 128.84, 128.06, 127.53, 127.47, 127.41, 126.06, 125.82, 125.68, 125.12, 125.01, 124.82, 124.62, 122.87, 122.12, 121.91, 120.60, 120.19, 119.08, 110.60, 108.61, 37.66, 13.94. MS (TOF-MS-EI, m/z): calcd. for C30H21N 395.4944, found 395.1673. Anal. calcd. for C30H21N: C, 91.11; H, 5.35; N, 3.54. Found: C, 91.02; H, 5.42; N, 3.43.
yl)-1,3,2-dioxaborolane (4) were attained according to relevant litera ture report [26,27] and slightly modified. Subsequent, the important intermediate 2-bromo-N-ethylcarbazole (3) was generated by N-alkyl ation of 2-bromocarbazole. Finally, the 2-bromo-N-ethylcarbazole (3) was reacted with excess molar ratio of borate ester 4,4,5,5-tetrame thyl-2-(pyren-1-yl)-1,3,2-dioxaborolane (4) to produce the target com pound PyEtCz at a good yield of 85% via the Suzuki reaction. In addition, the final product was purified by column chromatography and recrystallization to reach a purity of at least >99.5% for achieving the requirement of device fabrication. The molecular structure of PyEtCz was carefully confirmed by 1H spectroscopy, 13C NMR spectroscopy, mass spectra and elemental analysis. 3.2. Thermal properties The thermal stability of PyEtCz was determined by means of ther mogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As displayed by the TGA curve in Fig. 1a, the onset decomposi tion temperatures (Td) of PyEtCz was calculated to be 363 � C according to 5% weight loss of initial weight. Such good thermal property is benefit to avoid degradation of sample, and hence render it suitable for a dry vacuum deposition process. During the DSC measurement of the first heating run in Fig. 1b, only one endothermic peak due to melting point was observed at 199 � C. When a quick-cooling sample by liquid nitrogen was reheated, a endothermic peak at 81 � C corresponds to the glass transition temperature (Tg) was detected in the second run. The ability of PyEtCz to form a molecular glass is highly desirable for fabricating stable devices.
3. Results and discussion 3.1. Synthesis and structural characterization Scheme 1 outlines the synthetic method used for the final material (PyEtCz). First, the intermediate products including 4-bromo-2-nitrobi phenyl (1), 2-bromocarbazole (2) and 4,4,5,5-tetramethyl-2-(pyren-1-
Fig. 2. (a) The UV–vis absorption and fluorescence spectra in dilute dichloromethane solutions and in solid films, and (b) geometry optimized structure of PyEtCz.
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Table 1 Physical parameters of compound PyEtCz. Comp.
λabs (nm) CH2Cl2
film
CH2Cl2
film
PyEtCz
302, 348
305, 353
435
443
a b
λem (nm)
ΦFa (%)
Tg (� C)
Td (� C)
75
81
363
HOMO (eV) 5.48
Egb (eV) 3.06
LUMO (eV) 2.42
Calculated in CH2Cl2 solution by giving the 0.1 M quinine sulfate as a standard (ΦF ¼ 0.55). The optical energy band gap, obtained from onset of absorption spectra in thin film.
Fig. 4. The energy level diagram for PyEtCz based device. Fig. 3. Cyclic voltammograms of PyEtCz measured in CH2Cl2 solution under an argon atmosphere at a scan rate of 100 mV s 1.
3.4. Electrochemical properties In order to evaluate the redox activity of PyEtCz and to estimate the practical HOMO energy level, the electrochemical behavior of PyEtCz was investigated by cyclic voltammetry method using a conventional three-electrode cell in CH2Cl2 solution under an argon atmosphere. As seen by the cyclic voltammetry curve in Fig. 3, two one-electron oxidation processes were detected during the anodic scan, the first of which is reversible for the oxidation process of the carbazole moiety. And the second oxidation process at more positive potential is irre versible probably because this oxidation intermediate is so reactive and captures one electron through a too fast process to detect. In addition, one extra reduction peak was observed at 0.74 V for PyEtCz during the corresponding reduction process. This additional reduction peak was often appeared in previous reports due to the instability of radical cat ions of carbazole moiety for the carbazole derivatives with unprotected 3,6-positions [29]. The onset potential (Eonset ox ) of the first oxidation wave is 0.68 V relative to ferrocene. From the onset potential of the first oxidation wave, the HOMO energy level of PyEtCz was calculated to be 5.48 eV, according to the empirical formula of HOMO ¼ - (Eonset ox þ4.8) eV [30,31]. The high-lying HOMO energy level may result from its native electron-rich characteristic of the carbazole moiety within the whole molecule. Obviously, the HOMO energy level of PyEtCz is 0.18 eV lower than the common hole transporting material NPB ( 5.30 eV) [27], suggesting the hole injection from NBP to the blue emitting layer is feasible they are used together for the device fabrication. The LUMO energy level is 2.42 eV for PyEtCz from the empirical formula of LUMO ¼ (HOMO þ Eg) eV, where Eg is the optical bandgap determined by the absorption edge, which is taken from the onset of absorption spectra recorded in thin film [25]. The electrochemical parameters of HOMO, LUMO and Eg are listed in Table 1.
3.3. Photophysical properties The UV–vis absorption and PL spectra of PyEtCz for both dilute so lution in dichloromethane and the solid film on quartz plate were shown in Fig. 2a. The detailed data for optical properties were collected in Table 1. In the ultra-violet region, the absorption spectrum of PyEtCz in dilute solution can be attributed to two major absorption bands, the peak at 302 nm for the π-π* local electron transition of the carbazole moiety and the stronger peak at 348 nm for the π–π* electron transition of the whole conjugated backbone. The similar absorption spectrum profile and position with a slight red shift of 3–5 nm was detected in thin film in comparison with those of the solution. Upon photoexcitation at 348 nm, the compound in dilute solution exhibits narrow and deep-blue fluorescence with peak centered at 435 nm. In consistence with the absorption spectra, the PL spectrum in film state has same profile as that of solution and exhibits an 8 nm red-shift probably due to the rather trivial solid-state effect. The small red shift in both absorption and PL spectra of the thin film of PyEtCz reflects that the unwanted intermo lecular interaction in solid states is very tiny from the non-coplanar conformation of the molecule by the introduction of pyrene in the C2 position of carbazole ring [28]. In order to support this reason, theo retical molecular orbital calculation was carried out to characterize optimized geometry. The three-dimensional optimized structure for PyEtCz is shown in Fig. 2b, the torsion angle between pyrene moiety and the neighbouring carbazole ring is 53.9� , thus leading to a twisted mo lecular configuration and obtain the deep-blue emission color in solid state. The fluorescence quantum yield (ΦF) of PyEtCz for dilute solution in dichloromethane is calculated as 75% by giving the 0.1 M quinine sulfate as a standard referring to the relevant literature report [24]. The high fluorescence quantum yield should be conducive to obtain efficient blue emission when the material is used as the emitting lay in devices.
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Optical Materials 100 (2020) 109632
voltage (the driving voltage at the luminance of about 1 cd m 2) at 3.7 V that could potentially benefit from the efficient hole injection from NPB to the emitting layer because of the small injection barrier at NPB/ EML interface. With the increasing of driving voltage to 8.0 V, the PyEtCz based device showed a maximum luminance of 6573 cd m 2. A maximum luminance efficiency (ηL) of 3.24 cd A 1 at 5.5 V, a peak power efficiency (ηp) of 1.91 lm W 1 at 5.0 V and an external quantum efficiency (ηext) of 3.35% were recorded. The primary device perfor mance was summarized in Table 2. Kim et al. reported a group of new anthracenes obtained using rational molecular design, which exhibited the maximum luminance efficiency of 3.64 cd A 1 with CIE coordinates of (0.16, 0.09) for the deep-blue OLED [33]. Tang et al. used two novel star-shaped compounds based on the triphenylamine-cored and pe ripheral blue emitters as non-doped emitting layer to construct blue devices, which exhibited the maximum luminance efficiency and CIE coordinates of 3.36 cd A 1 and (0.16, 0.07) [34]. A high external quantum efficiency of 9.23% (8.22 cd A 1) with CIE coordinates of (0.14, 0.10) and low efficiency roll-off was achieved for nondoped blue fluorescent OLEDs based on benzonitrile-anthracene derivative by Ma et al., which was the state-of-the-art in comparison with previously re ported non-doped blue OLEDs [35]. Evidently, the performance of de vice based on PyEtCz is comparable with the reported non-doped fluorescent OLEDs with deep-blue CIE coordinate y � 0.10 to date in consideration of spectral stability and device efficiency [33–36].
Fig. 5. EL spectra at different voltages of the blue device based on PyEtCz.
3.5. Electroluminescent properties Given the good thermal stability, the ability to form molecular glass, efficient deep blue emission and suitable HOMO energy level, the EL performance of PyEtCz was evaluated in non-doped OLED. Using the vacuum-deposited technique, a blue fluorescent device with a structure of ITO/NPB (45 nm)/PyEtCz (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (200 nm) was constructed according to the continuous optimization. For this device, NPB is 4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl selected as the hole-transporting layer, and TPBI is 1,3,5-tris[N-(phenyl) benzimidazole]-benzene played a electron-transporting and holeblocking role. The structure and energy level diagram of device was shown in Fig. 4. The device based on PyEtCz exhibited strong deep-blue emission with a unique emission at 452 nm, and a CIE color coordinate of (0.15, 0.10). Obviously, not only the CIE coordinate of device is quite close to the standard blue coordinate of the National Television System Committee (NTSC) standard of (0.14, 0.08), but also its emission spectra emitted by the emitting layer is very stable by increasing the driving voltages from 5 to 8 V, as show by the EL spectra at different voltages in Fig. 5. In addition, compared to the PL spectrum in film state, the EL spectrum of device has the identical profile except with a small red-shift of 9 nm. The red-shift in EL is a commonly observed phenomenon in most of OLEDs, which is typically because the additional electrical field may reduce the excited state energy of the emitting molecule [32]. The current density-voltage-luminance (J-V-L) and luminance effi ciency (LE)-current density-external quantum efficiency (EQE) charac teristics of device were shown in Fig. 6. The device had a low turn-on
4. Conclusion In conclusion, a new deep blue emitter based on carbazole-pyrene hybrid, PyEtCz, was synthesized and fully characterized. The material is thermally and morphologically stabilities, and possess high solution fluorescence quantum yield, as well as appropriate HOMO energy level ( 5.48 eV). The non-doped OLEDs employing PyEtCz as an emitter shows fantastic performances with a maximum external quantum effi ciency of 3.35%, and an almost standard blue coordinate of (0.15, 0.10). These results are expected to contribute to the development of a new deep blue fluorescent material. Table 2 Key electroluminescent characteristics of the non-doped multilayer device. Comp.
Vturnon
[V] PyEtCz
3.7
Lmax [cd m 2, V]
ηL(max)
6573, 8.0
3.24, 5.5
[cd A 1, V]
ηp(max) [lm W 1, V]
1.91, 5.0
ηext
λmax (nm)
CIE (x, y)
452
0.15,0.10
(max)
(%)
3.35
Fig. 6. (a) The current density-voltage-luminance (J-V-L) and (b) luminance efficiency (LE)-current density-external-external quantum efficiency (EQE) charac teristics of the blue device based on PyEtCz. 5
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Author contribution section
[16] V. Joseph, K.R.J. Thomas, S. Sahoo, M. Singh, J.H. Jou, Cyano-functionalized carbazole substituted pyrene derivatives for promising organic light-emitting diodes, Dyes Pigments 158 (2018) 295–305. [17] A. Thangthong, N. Prachumrak, R. Tarsang, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk, V. Promarak, Blue light-emitting and hole-transporting materials based on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices, J. Mater. Chem. 22 (2012) 6869–6877. [18] R. Zhang, H. Sun, Y. Zhao, X. Tang, Z. Ni, Dipolar 1,3,6,8-tetrasubstituted pyrenebased blue emitters containing electro-transporting benzimidazole moieties: syntheses, structures, optical properties, electrochemistry and electroluminescence, Dyes Pigments 152 (2018) 1–13. [19] M. Jung, J. Lee, H. Jung, S. Kang, A. Wakamiya, Highly efficient pyrene blue emitters for OLEDs based on substitution position effect, Dyes Pigments 158 (2018) 42–49. [20] C.L. Wu, C.T. Chen, C.T. Chen, Synthesis and characterization of heteroatombridged bis-spirobifluorenes for the application of organic light-emitting diodes, Org. Lett. 16 (2014) 2114–2117. [21] Y. Xu, X. Liang, X. Zhou, P. Yuan, J. Zhou, C. Wang, B. Li, D. Hu, X. Qiao, X. Jiang, L. Liu, S.J. Su, D. Ma, Y. Ma, Highly efficient blue fluorescent OLEDs based on upper level triplet-singlet intersystem crossing, Adv. Mater. 12 (1–8) (2019) 1807388. [22] J.Y. Shen, X.L. Yang, T.H. Huang, J.T. Lin, T.H. Ke, L.Y. Chen, C.C. Wu, M.C.P. Yeh, Ambipolar conductive 2,7-carbazole derivatives for electroluminescent devices, Adv. Funct. Mater. 17 (2007) 983–995. [23] W. Jiang, L. Duan, J. Qiao, G. Dong, D. Zhang, L. Wang, Y. Qiu, High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices, J. Mater. Chem. 21 (2011) 4918–4926. [24] Z.J. Si, Y. Shao, C.X. Li, Q. Liu, Synthesis and fluorescence study of sodium-2-(4’dimethyl-aminocinnamicacyl) -3,3-(1’,3’-alkylenedithio) acrylate, J. Lumin. 124 (2007) 365–369. [25] T. Zhang, X. Cheng, X. Wang, C. Song, Bipolarfluorene-cored derivatives containing carbazole-benzothiazole hybrids as non-doped emitters for deep-blue electroluminescence, Opt. Mater. 89 (2019) 498–504. [26] S.H. Kim, I. Cho, M.K. Sim, S. Park, S.Y. Park, Highly efficient deep-blue emitting organic light emitting diode based on the multifunctional fluorescent molecule comprising covalently bonded carbazole and anthracene moieties, J. Mater. Chem. 21 (2011) 9139–9148. [27] T. Zhang, D. Liu, Q. Wang, R.J. Wang, H.C. Ren, J.Y. Li, Deep-blue and white organic light-emitting diodes based on novelfluorene-cored derivatives with naphthylanthracene endcaps, J. Mater. Chem. 21 (2011) 12969–12976. [28] J. Huang, J.H. Su, X. Li, M.K. Lam, K.M. Fuang, H.H. Fan, K.W. Cheah, C.H. Chen, H. Tian, Bipolar anthracene derivatives containing hole- and electron-transporting moieties for highly efficient blue electroluminescence devices, J. Mater. Chem. 21 (2011) 2957–2964. [29] W. Li, J.Y. Li, F. Wang, Z. Gao, S.F. Zhang, Universal host materials for highefficiency phosphorescent and delayed-fluorescence OLEDs, ACS Appl. Mater. Interfaces 7 (2015) 26206–26216. [30] S. Zhuang, R. Shangguan, J. Jin, G. Tu, L. Wang, J. Chen, D. Ma, X. Zhu, Efficient nondoped blue organic light-emitting diodes based on phenanthroimidazolesubstituted anthracene derivatives, Org. Electron. 13 (2012) 3050–3059. [31] S. Zhuang, R. Shangguan, H. Huang, G. Tu, L. Wang, X. Zhu, Synthesis, characterization, physical properties, and blue electroluminescent device applications of phenanthroimidazole derivatives containing anthracene or pyrene moiety, Dyes Pigments 101 (2014) 93–102. [32] A. Wada, T. Yasuda, Q. Zhang, Y.S. Yang, I. Takasu, S. Enomoto, C. Adachi, A host material consisting of a phosphinic amide directly linked donor-acceptor structure for efficient blue phosphorescent organic light-emitting diodes, J. Mater. Chem. C 1 (2013) 2404–2407. [33] S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J.W. Park, Exceedingly efficient deep-blue electroluminescence from new anthracenes obtained using rational molecular design, J. Mater. Chem. 18 (2008) 3376–3384. [34] S. Tang, W. Li, F. Shen, D. Liu, B. Yang, Y. Ma, Highly efficient deep-blue electroluminescence based on the triphenylaminecored and peripheral blue emitters with segregative HOMO-LUMO characteristics, J. Mater. Chem. 1 (2012) 4401–4408. [35] W. Liu, S. Ying, R. Guo, X. Qiao, P. Leng, Q. Zhang, Y. Wang, D. Ma, L. Wang, Nondoped blue fluorescent organic light-emitting diodes based on benzonitrileanthracene derivative with 10.06% external quantum efficiency and low efficiency roll-off, J. Mater. Chem. C 7 (2019) 1014–1021. [36] J.Y. Hu, Y.J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Bisanthracenebased donor–acceptor-type light-emitting dopants: highly efficient deep-blue emission in organic light-emitting devices, Adv. Funct. Mater. 24 (2014) 2064–2071.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We appreciate support from the Natural Science Foundation of Anhui Education Department (KJ2018A0525), the Excellent Talents Program of Anhui Province (gxyq2019062), and the innovative projects for un dergraduate student of Anhui Province S201910879289). References [1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913–915. [2] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Synthesis of light emitting conjugated polymers for applications in electroluminescent devices, Chem. Rev. 109 (2009), 897-1091. [3] L.X. Guo, X.J. Wang, L.H. Feng, Synthesis, photophysical and electrochemical properties of a blue emitter with binaphthalene and carbazole units, Spectroc. Acta Pt. A-Molec. Biomolec. Spectr. 201 (2018) 376–381. [4] L. Chen, C. Zhang, G. Lin, H. Nie, W. Luo, Z. Zhuang, S. Ding, R. Hu, S.J. Su, F. Huang, A. Qin, Z. Zhao, B.Z. Tang, Solution-processable, starshaped bipolar tetraphenylethene derivatives for the fabrication of efficient nondoped OLEDs, J. Mater. Chem. C 4 (2016) 2775–2783. [5] C. Raffaella, T. Stefano, G. Gianluca, U. Hakan, F. Antonio, M. Michele, Organic light emitting transistors with an efficiency that outperforms the equivalent lightemitting diodes, Nat. Mater. 9 (2010) 496–503. [6] P. Sun, K. Wang, B. Zhao, T. Yang, H. Xu, Y. Miao, H. Wang, B. Xu, Blue-emitting Ir (III) complexes usingfluorinated bipyridyl as main ligand and 1,2,4-triazol as ancillary ligand: photophysical properties and performances in devices, Tetrahedron 72 (2016) 8335–8341. [7] T.L. Wu, M.J. Huang, C.C. Lin, P.Y. Huang, T.Y. Chou, R.W.C. Cheng, H.W. Lin, R. S. Liu, C.H. Cheng, Diboron compound-based organic light-emitting diodes with high efficiency and reduced efficiency roll-off, Nat. Photonics 12 (2018) 235–240. [8] Q.S. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T. Takahashi, S.Y. Lee, T. Yasuda, C. Adachi, Nearly 100% internal quantum efficiency in undoped electroluminescent devices employing pure organic emitters, Adv. Mater. 27 (2015) 2096–2100. [9] I. Cho, S.H. Kim, J.H. Kim, S. Park, S.Y. Park, Highly efficient and stable deep-blue emitting anthracene-derived molecular glass for versatile types of non-doped OLED applications, J. Mater. Chem. 22 (2012) 123–129. [10] X. Xing, L. Xiao, L. Zheng, S. Hu, Z. Chen, B. Qu, Q. Gong, Spirobifluorene derivative: a pure blue emitter (CIEy � 0.08) with high efficiency and thermal stability, J. Mater. Chem. 22 (2012) 15136–15140. [11] G.Y. Zhong, Z. Xu, S.T. Zhang, W. Huang, X.Y. Hou, Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq, Appl. Phys. Lett. 81 (2002) 1122–1124. [12] J.R. Gong, L.J. Wan, S.B. Lei, C.L. Bai, X.H. Zhang, S.T. Lee, Direct evidence of molecular aggregation and degradation mechanism of organic light-emitting diodes under joule heating: an STM and photoluminescence study, J. Phys. Chem. B 109 (2005) 1675–1682. [13] R. Zhang, Y. Zhao, G.L. Li, D.S. Yang, Z.H. Ni, A new series of pyrenyl-based triarylamines: syntheses, structures, optical properties, electrochemistry and electroluminescence, RSC Adv. 6 (2016) 9037–9048. [14] J.N. Moorthy, P. Natarajan, P. Venkatakrishnan, D.F. Huang, T.J. Chow, Steric inhibition of π-stacking: 1,3,6,8-tetraarylpyrenes as efficient blue emitters in organic light emitting diodes (OLEDs), Org. Lett. 9 (2007) 5215–5218. [15] T.M. Figueira-Duarte, K. Müllen, Pyrene-based materials for organic electronics, Chem. Rev. 111 (2011) 7260–7314.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Efficient deep blue emitter based on carbazole-pyrene hybrid for non-doped electroluminescent device Ting Zhang a, b, *, Jieyang Ye a, Ansheng Luo a, Di Liu b, ** a b
School of Chemistry and Material Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, China State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Non-doped electroluminescent device Fluorescent materials Deep-blue Carbazole Pyrene
A new deep blue fluorescent material based on carbazole-pyrene hybrid, namely PyEtCz, was synthesized by palladium catalyzed C–C Suzuki coupling reaction, and structurally characterized by NMR, mass spectra and elemental analysis. The thermal stability, photophysical and optoelectronic properties were fully investigated. The material shows good thermal stability and can form an amorphous glass state. Its CH2Cl2 solution exhibits strong deep-blue emission with high fluorescent quantum yield of 75%. PyEtCz also possesses high HOMO level of 5.48 eV that would facilitate efficient hole injection into the emitting layer. The non-doped OLED using PyEtCz as emitter through vacuum evaporation was fabricated for purpose of investigating its electrolumines cence properties. The device exhibited stable blue electroluminescence with CIE coordinate of (0.15, 0.10), a good performance with maximum luminance of 6573 cd m 2 and maximum luminance efficiency of 3.24 cd A 1 (3.35%) was achieved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted extensive attention in both industries and academia owing to their importance applications in flat-panel display and solid-state lighting [1–3]. Among the three primary emission materials (red, green and blue) to realize full-color displays and white lighting, an abundant number of red and green emitters have been realized in commercial use, and are gradually occupied a certain percentage of the domestic market [4,5]. However, blue materials with high performances remain as a major challenge because of their intrinsically wide band gap. Although many blue phosphorescent and thermally activated decayed fluorescence (TADF) device with high efficiencies can be achieved because both kinds of emitters can utilize both singlet and triplet exciton states and obtain internal quantum efficiency as high as 100%, these emitters currently are mainly for non-commercial use due to the poor color stability and purity [6–8]. Furthermore, the efficiency and the value of CIE coordi nate y for the blue emitters are also essential to determine the overall performance of the white OLEDs. Generally, the standard blue coordi nate of the National Television System Committee (NTSC) standard is (0.14, 0.08), and the CIE coordinates in the range of (x � 0.15, y � 0.15)
which was usually defined as a deep blue color and saturated deep-blue emission with a CIE coordinate of y � 0.10 [9,10]. Therefore, the development of efficient deep-blue fluorescent materials and devices with low color coordinates of y � 0.10 and good spectral stability is still a subject of current interest. Moreover, in comparison with the doped OLEDs, the non-doped blue devices possess the easy device fabrication and can avoid the absence of phase separation that has been proved to be an important factor to cause the performance degradation [11,12]. Among the various building blocks, pyrene has been intensively studied for constructing organic light-emitting materials because of its intense blue emission, excellent thermal and electroluminescence properties, and easier modification with functional groups [13–15]. In recent years, various pyrene-based materials have been explored and some of them were successfully employed as blue emitters for OLEDs [16–19]. Promarak et al. used the pyrene-functionalized materials as non-doped emitting layer to construct blue devices, which exhibited the maximum luminance efficiency and CIE coordinates of 2.06 cd A 1 and (0.15, 0.10) [17]. Ni et al. reported a group of dipolar 1,3,6,8-tetrasub stituted pyrene-based benzimidazole derivatives, and a pure blue CIE coordinates of (0.16, 0.10) with the maximum current and power effi ciencies of 2.70 cd A 1 and 1.97 lm W 1 were obtained for the
* Corresponding author. School of Chemistry and Material Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, China. ** Corresponding author. E-mail addresses: [email protected] (T. Zhang), [email protected] (D. Liu). https://doi.org/10.1016/j.optmat.2019.109632 Received 20 September 2019; Received in revised form 10 November 2019; Accepted 21 December 2019 Available online 28 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Scheme 1. The molecular structure and synthetic routes of PyEtCz.
non-doped device by employing the meta-linked compound as a fluo rescent emitter [18]. A high external quantum efficiency of 7.10% was achieved for non-doped blue OLEDs by Park and his group using the pyrene-cored derivatives, but the devices show insufficient deep-blue emission, the CIE coordinates is (0.151, 0.125) [19]. Considering the research results above, in this paper, we designed and synthesized a new fluorescent material based on carbazole-pyrene hybrid with deep blue emission, namely PyEtCz (Scheme 1). Carbazole-based materials have been famous for their high temperature resistance, sufficient hole-transporting ability, and unique ability to form an amorphous glass state [20,21]. The carbazole unit was intro duced in the molecular structure with the expectations to enhance the possible hole-transporting ability of pyrene as well as thermal stability of the molecule. In addition, the selection of pyrene in the C2 position of carbazole ring is in the purpose of forming a highly twisted structure of the molecule, which significantly impacts the thermal and photo physical behavior of material. It was also demonstrated that the 2-substituted carbazole compounds had higher and more balanced electron and hole mobilities than their 3-substituted analogues [22,23]. The molecule designed in this way processes good thermal stability, the ability to form an amorphous glass state, strong blue fluorescence, and high HOMO energy level. The non-doped OLEDs containing PyEtCz as an emitter shows fantastic performances with the maximum external quantum efficiency of 3.35%, and stable blue electroluminescence with CIE coordinate of (0.15, 0.10).
voltammetry (CV) was carried out on a CHI–600C electro-chemical workstation comprising of a glassy carbon as working surface, a platinum-wire as counter electrode, and a saturated calomel electrode (SCE) as reference electrode by dissolving in anhydrous dichloro methane solution of sample and [n-Bu4N]PF6 at a sealed electrolytic cell. After carefully deoxygenating with argon at room temperature, the so lution was tested at a sweeping rate of 100 mV s 1 with ferrocene as the internal standard. The geometry optimized structure was calculated by density functional theory (DFT) using the B3LYP/6-31G(d) atomic basis sets through the Gaussian 09 software. The imaginary frequency in optimized structure is zero [25]. The devices were constructed by using the vacuum deposition method. At first, the glass substrates pre-coated with indium tin oxide (ITO) were carefully washed with ultrapure water, acetone, and iso propanol in sequence by using an ultrasonic machine. Then, the ITO glass substrates were vigorously scrubbed with detergent and oven-dried in a drying oven for 40 min. Before being transferred to a deposition system, three pre-cleaned substrates were treated with UV-ozone for 25 min. The organic layers were sequentially deposited onto the substrates under high vacuum (~1 � 10 5 Pa). A metal cathode layer containing 1 nm LiF and 200 nm Al was patterned by employing a shadow mask with an active area of 9 mm2 openings. The luminance, electroluminescence (EL) spectra and CIE coordinates at different driving voltage were recorded with a program-controlled PR-705 spectrometer. The current density-voltage property of device was measured with a Keithley 256 instrument. The electroluminescent measurements were made at room temperature without further encapsulations.
2. Experimental section 2.1. General information
2.2. Synthesis
The DMF, toluene and methanol used as solvents in synthesis re actions were strictly distilled from metallic Na and CaH2 by following the conventional methods. All reagents were from domestic and foreign reagent manufactures, which were directly used in experiments without having to go through the purification process. The 1H NMR and 13C NMR spectra were obtained by using a Varian INOVA 400 MHz and 100 MHz spectrometer, respectively. The sample was dissolved in CDCl3 at ambient temperature. Mass spectra were received from a HP1100LC/ MSD MS (TOF-MS-EI). The compounds were characterized by a CarloEriba-1106 elemental analysis system. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 with the rate of increasing temperature at 10 � C min 1 under nitrogen flushing during heating between 25 and 600 � C. Differential scanning calorimetry (DSC) ana lyses was undertaken on a Netzsch DSC Q2000 under nitrogen flushing. The sample was heated for two times and quickly cooled with liquid nitrogen after completion of the first heating. Ultra-violet to visible absorption spectra was obtained on a UV-265 spectrometer. Photo luminescence (PL) spectra and fluorescence quantum yields (ΦF) in dilute solution were measured on a Shimadzu RF-5301PC fluorescence spectrometer referring to the relevant literature report [24]. Cyclic
The compounds 4-bromo-2-nitrobiphenyl (1), 2-bromocarbazole (2) and 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxaborolane (4) were synthesized and characterized according to the related report [26,27]. 2.2.1. Synthesis of 2-bromo-N-ethylcarbazole (3) A 100 mL 2-necked bottle was added 2-bromocarbazole 2 (1.5 g, 6.09 mmol), pre-milled potassium hydroxide (1.71 g, 30.5 mmol) and 20 mL DMF. After the mixture was stirred for 1 h at room temperature, bromoethane (1.0 g, 9.18 mmol) was added by dropper, and the solution was then reacted continuously for 12 h at room temperature. After the reaction is completed, the solvent was removed by decompressing distillation process and the residual solid was extracted with CH2Cl2 for at least two times. Finally, the important intermediate 3 was gained by silica gel column chromatography to produce a white solid (1.64 g, 98%). MS (TOF-MS-EI, m/z): calcd. for C14H12NBr 274.1588, found 273.0156. 2.2.2. Synthesis of 2-(pyren-1-yl)-N-ethylcarbazole (PyEtCz) Under nitrogen atmosphere, a 100 mL 2-necked bottle was added 2bromo-N-ethylcarbazole 3 (0.500 g, 1.82 mmol), 4,4,5,5-tetramethyl-22
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Fig. 1. (a) TGA and (b) DSC (the second heating cycle) curves of PyEtCz.
(pyren-1-yl)-1,3,2-dioxaborolane 4 (0.719 g, 2.19 mmol), Pd(PPh3)4 (0.105 g, 0.091 mmol), and 18 mL mixed solvent of toluene and meth anol (5:1 vol ratio). After the mixture was degassed by pump for 10 min, a 2 M aqueous potassium carbonate solution (4.6 mL, 9.2 mmol) was added, the reaction mixture was then heated to 80 � C and kept at this temperature for 8 h. When the reaction is finished, the cooled solution was extracted with CH2Cl2 for at least two times. The organic layer was collected and dried over anhydrous magnesium sulfate before being evaporated in vacuum. The crude product was finally treated by silica gel column chromatography and then recrystallized from ethanol/ chloroform to afford PyEtCz (0.612 g, 85%) as a white solid. 1H NMR (400 MHz, CDCl3, δ): 8.29–8.27 (m, 1H, ArH), 8.26–8.23 (m, 2H, ArH), 8.19–8.17 (m, 2H, ArH), 8.15–8.07 (m, 4H, ArH), 8.02–7.97 (m, 2H, ArH), 7.64 (s, 1H, ArH), 7.53–7.43 (m, 3H, ArH), 7.30–7.26 (m, 1H, ArH), 4.41–4.36 (m, 2H, CH2), 1.46 (t, J ¼ 7.2 Hz, 3H, CH3). 13C NMR (100 MHz, CDCl3, δ): 140.50, 140.15, 138.88, 138.81, 131.59, 131.08, 130.55, 128.84, 128.06, 127.53, 127.47, 127.41, 126.06, 125.82, 125.68, 125.12, 125.01, 124.82, 124.62, 122.87, 122.12, 121.91, 120.60, 120.19, 119.08, 110.60, 108.61, 37.66, 13.94. MS (TOF-MS-EI, m/z): calcd. for C30H21N 395.4944, found 395.1673. Anal. calcd. for C30H21N: C, 91.11; H, 5.35; N, 3.54. Found: C, 91.02; H, 5.42; N, 3.43.
yl)-1,3,2-dioxaborolane (4) were attained according to relevant litera ture report [26,27] and slightly modified. Subsequent, the important intermediate 2-bromo-N-ethylcarbazole (3) was generated by N-alkyl ation of 2-bromocarbazole. Finally, the 2-bromo-N-ethylcarbazole (3) was reacted with excess molar ratio of borate ester 4,4,5,5-tetrame thyl-2-(pyren-1-yl)-1,3,2-dioxaborolane (4) to produce the target com pound PyEtCz at a good yield of 85% via the Suzuki reaction. In addition, the final product was purified by column chromatography and recrystallization to reach a purity of at least >99.5% for achieving the requirement of device fabrication. The molecular structure of PyEtCz was carefully confirmed by 1H spectroscopy, 13C NMR spectroscopy, mass spectra and elemental analysis. 3.2. Thermal properties The thermal stability of PyEtCz was determined by means of ther mogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As displayed by the TGA curve in Fig. 1a, the onset decomposi tion temperatures (Td) of PyEtCz was calculated to be 363 � C according to 5% weight loss of initial weight. Such good thermal property is benefit to avoid degradation of sample, and hence render it suitable for a dry vacuum deposition process. During the DSC measurement of the first heating run in Fig. 1b, only one endothermic peak due to melting point was observed at 199 � C. When a quick-cooling sample by liquid nitrogen was reheated, a endothermic peak at 81 � C corresponds to the glass transition temperature (Tg) was detected in the second run. The ability of PyEtCz to form a molecular glass is highly desirable for fabricating stable devices.
3. Results and discussion 3.1. Synthesis and structural characterization Scheme 1 outlines the synthetic method used for the final material (PyEtCz). First, the intermediate products including 4-bromo-2-nitrobi phenyl (1), 2-bromocarbazole (2) and 4,4,5,5-tetramethyl-2-(pyren-1-
Fig. 2. (a) The UV–vis absorption and fluorescence spectra in dilute dichloromethane solutions and in solid films, and (b) geometry optimized structure of PyEtCz.
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Table 1 Physical parameters of compound PyEtCz. Comp.
λabs (nm) CH2Cl2
film
CH2Cl2
film
PyEtCz
302, 348
305, 353
435
443
a b
λem (nm)
ΦFa (%)
Tg (� C)
Td (� C)
75
81
363
HOMO (eV) 5.48
Egb (eV) 3.06
LUMO (eV) 2.42
Calculated in CH2Cl2 solution by giving the 0.1 M quinine sulfate as a standard (ΦF ¼ 0.55). The optical energy band gap, obtained from onset of absorption spectra in thin film.
Fig. 4. The energy level diagram for PyEtCz based device. Fig. 3. Cyclic voltammograms of PyEtCz measured in CH2Cl2 solution under an argon atmosphere at a scan rate of 100 mV s 1.
3.4. Electrochemical properties In order to evaluate the redox activity of PyEtCz and to estimate the practical HOMO energy level, the electrochemical behavior of PyEtCz was investigated by cyclic voltammetry method using a conventional three-electrode cell in CH2Cl2 solution under an argon atmosphere. As seen by the cyclic voltammetry curve in Fig. 3, two one-electron oxidation processes were detected during the anodic scan, the first of which is reversible for the oxidation process of the carbazole moiety. And the second oxidation process at more positive potential is irre versible probably because this oxidation intermediate is so reactive and captures one electron through a too fast process to detect. In addition, one extra reduction peak was observed at 0.74 V for PyEtCz during the corresponding reduction process. This additional reduction peak was often appeared in previous reports due to the instability of radical cat ions of carbazole moiety for the carbazole derivatives with unprotected 3,6-positions [29]. The onset potential (Eonset ox ) of the first oxidation wave is 0.68 V relative to ferrocene. From the onset potential of the first oxidation wave, the HOMO energy level of PyEtCz was calculated to be 5.48 eV, according to the empirical formula of HOMO ¼ - (Eonset ox þ4.8) eV [30,31]. The high-lying HOMO energy level may result from its native electron-rich characteristic of the carbazole moiety within the whole molecule. Obviously, the HOMO energy level of PyEtCz is 0.18 eV lower than the common hole transporting material NPB ( 5.30 eV) [27], suggesting the hole injection from NBP to the blue emitting layer is feasible they are used together for the device fabrication. The LUMO energy level is 2.42 eV for PyEtCz from the empirical formula of LUMO ¼ (HOMO þ Eg) eV, where Eg is the optical bandgap determined by the absorption edge, which is taken from the onset of absorption spectra recorded in thin film [25]. The electrochemical parameters of HOMO, LUMO and Eg are listed in Table 1.
3.3. Photophysical properties The UV–vis absorption and PL spectra of PyEtCz for both dilute so lution in dichloromethane and the solid film on quartz plate were shown in Fig. 2a. The detailed data for optical properties were collected in Table 1. In the ultra-violet region, the absorption spectrum of PyEtCz in dilute solution can be attributed to two major absorption bands, the peak at 302 nm for the π-π* local electron transition of the carbazole moiety and the stronger peak at 348 nm for the π–π* electron transition of the whole conjugated backbone. The similar absorption spectrum profile and position with a slight red shift of 3–5 nm was detected in thin film in comparison with those of the solution. Upon photoexcitation at 348 nm, the compound in dilute solution exhibits narrow and deep-blue fluorescence with peak centered at 435 nm. In consistence with the absorption spectra, the PL spectrum in film state has same profile as that of solution and exhibits an 8 nm red-shift probably due to the rather trivial solid-state effect. The small red shift in both absorption and PL spectra of the thin film of PyEtCz reflects that the unwanted intermo lecular interaction in solid states is very tiny from the non-coplanar conformation of the molecule by the introduction of pyrene in the C2 position of carbazole ring [28]. In order to support this reason, theo retical molecular orbital calculation was carried out to characterize optimized geometry. The three-dimensional optimized structure for PyEtCz is shown in Fig. 2b, the torsion angle between pyrene moiety and the neighbouring carbazole ring is 53.9� , thus leading to a twisted mo lecular configuration and obtain the deep-blue emission color in solid state. The fluorescence quantum yield (ΦF) of PyEtCz for dilute solution in dichloromethane is calculated as 75% by giving the 0.1 M quinine sulfate as a standard referring to the relevant literature report [24]. The high fluorescence quantum yield should be conducive to obtain efficient blue emission when the material is used as the emitting lay in devices.
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voltage (the driving voltage at the luminance of about 1 cd m 2) at 3.7 V that could potentially benefit from the efficient hole injection from NPB to the emitting layer because of the small injection barrier at NPB/ EML interface. With the increasing of driving voltage to 8.0 V, the PyEtCz based device showed a maximum luminance of 6573 cd m 2. A maximum luminance efficiency (ηL) of 3.24 cd A 1 at 5.5 V, a peak power efficiency (ηp) of 1.91 lm W 1 at 5.0 V and an external quantum efficiency (ηext) of 3.35% were recorded. The primary device perfor mance was summarized in Table 2. Kim et al. reported a group of new anthracenes obtained using rational molecular design, which exhibited the maximum luminance efficiency of 3.64 cd A 1 with CIE coordinates of (0.16, 0.09) for the deep-blue OLED [33]. Tang et al. used two novel star-shaped compounds based on the triphenylamine-cored and pe ripheral blue emitters as non-doped emitting layer to construct blue devices, which exhibited the maximum luminance efficiency and CIE coordinates of 3.36 cd A 1 and (0.16, 0.07) [34]. A high external quantum efficiency of 9.23% (8.22 cd A 1) with CIE coordinates of (0.14, 0.10) and low efficiency roll-off was achieved for nondoped blue fluorescent OLEDs based on benzonitrile-anthracene derivative by Ma et al., which was the state-of-the-art in comparison with previously re ported non-doped blue OLEDs [35]. Evidently, the performance of de vice based on PyEtCz is comparable with the reported non-doped fluorescent OLEDs with deep-blue CIE coordinate y � 0.10 to date in consideration of spectral stability and device efficiency [33–36].
Fig. 5. EL spectra at different voltages of the blue device based on PyEtCz.
3.5. Electroluminescent properties Given the good thermal stability, the ability to form molecular glass, efficient deep blue emission and suitable HOMO energy level, the EL performance of PyEtCz was evaluated in non-doped OLED. Using the vacuum-deposited technique, a blue fluorescent device with a structure of ITO/NPB (45 nm)/PyEtCz (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (200 nm) was constructed according to the continuous optimization. For this device, NPB is 4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl selected as the hole-transporting layer, and TPBI is 1,3,5-tris[N-(phenyl) benzimidazole]-benzene played a electron-transporting and holeblocking role. The structure and energy level diagram of device was shown in Fig. 4. The device based on PyEtCz exhibited strong deep-blue emission with a unique emission at 452 nm, and a CIE color coordinate of (0.15, 0.10). Obviously, not only the CIE coordinate of device is quite close to the standard blue coordinate of the National Television System Committee (NTSC) standard of (0.14, 0.08), but also its emission spectra emitted by the emitting layer is very stable by increasing the driving voltages from 5 to 8 V, as show by the EL spectra at different voltages in Fig. 5. In addition, compared to the PL spectrum in film state, the EL spectrum of device has the identical profile except with a small red-shift of 9 nm. The red-shift in EL is a commonly observed phenomenon in most of OLEDs, which is typically because the additional electrical field may reduce the excited state energy of the emitting molecule [32]. The current density-voltage-luminance (J-V-L) and luminance effi ciency (LE)-current density-external quantum efficiency (EQE) charac teristics of device were shown in Fig. 6. The device had a low turn-on
4. Conclusion In conclusion, a new deep blue emitter based on carbazole-pyrene hybrid, PyEtCz, was synthesized and fully characterized. The material is thermally and morphologically stabilities, and possess high solution fluorescence quantum yield, as well as appropriate HOMO energy level ( 5.48 eV). The non-doped OLEDs employing PyEtCz as an emitter shows fantastic performances with a maximum external quantum effi ciency of 3.35%, and an almost standard blue coordinate of (0.15, 0.10). These results are expected to contribute to the development of a new deep blue fluorescent material. Table 2 Key electroluminescent characteristics of the non-doped multilayer device. Comp.
Vturnon
[V] PyEtCz
3.7
Lmax [cd m 2, V]
ηL(max)
6573, 8.0
3.24, 5.5
[cd A 1, V]
ηp(max) [lm W 1, V]
1.91, 5.0
ηext
λmax (nm)
CIE (x, y)
452
0.15,0.10
(max)
(%)
3.35
Fig. 6. (a) The current density-voltage-luminance (J-V-L) and (b) luminance efficiency (LE)-current density-external-external quantum efficiency (EQE) charac teristics of the blue device based on PyEtCz. 5
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Optical Materials 100 (2020) 109632
Author contribution section
[16] V. Joseph, K.R.J. Thomas, S. Sahoo, M. Singh, J.H. Jou, Cyano-functionalized carbazole substituted pyrene derivatives for promising organic light-emitting diodes, Dyes Pigments 158 (2018) 295–305. [17] A. Thangthong, N. Prachumrak, R. Tarsang, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk, V. Promarak, Blue light-emitting and hole-transporting materials based on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices, J. Mater. Chem. 22 (2012) 6869–6877. [18] R. Zhang, H. Sun, Y. Zhao, X. Tang, Z. Ni, Dipolar 1,3,6,8-tetrasubstituted pyrenebased blue emitters containing electro-transporting benzimidazole moieties: syntheses, structures, optical properties, electrochemistry and electroluminescence, Dyes Pigments 152 (2018) 1–13. [19] M. Jung, J. Lee, H. Jung, S. Kang, A. Wakamiya, Highly efficient pyrene blue emitters for OLEDs based on substitution position effect, Dyes Pigments 158 (2018) 42–49. [20] C.L. Wu, C.T. Chen, C.T. Chen, Synthesis and characterization of heteroatombridged bis-spirobifluorenes for the application of organic light-emitting diodes, Org. Lett. 16 (2014) 2114–2117. [21] Y. Xu, X. Liang, X. Zhou, P. Yuan, J. Zhou, C. Wang, B. Li, D. Hu, X. Qiao, X. Jiang, L. Liu, S.J. Su, D. Ma, Y. Ma, Highly efficient blue fluorescent OLEDs based on upper level triplet-singlet intersystem crossing, Adv. Mater. 12 (1–8) (2019) 1807388. [22] J.Y. Shen, X.L. Yang, T.H. Huang, J.T. Lin, T.H. Ke, L.Y. Chen, C.C. Wu, M.C.P. Yeh, Ambipolar conductive 2,7-carbazole derivatives for electroluminescent devices, Adv. Funct. Mater. 17 (2007) 983–995. [23] W. Jiang, L. Duan, J. Qiao, G. Dong, D. Zhang, L. Wang, Y. Qiu, High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices, J. Mater. Chem. 21 (2011) 4918–4926. [24] Z.J. Si, Y. Shao, C.X. Li, Q. Liu, Synthesis and fluorescence study of sodium-2-(4’dimethyl-aminocinnamicacyl) -3,3-(1’,3’-alkylenedithio) acrylate, J. Lumin. 124 (2007) 365–369. [25] T. Zhang, X. Cheng, X. Wang, C. Song, Bipolarfluorene-cored derivatives containing carbazole-benzothiazole hybrids as non-doped emitters for deep-blue electroluminescence, Opt. Mater. 89 (2019) 498–504. [26] S.H. Kim, I. Cho, M.K. Sim, S. Park, S.Y. Park, Highly efficient deep-blue emitting organic light emitting diode based on the multifunctional fluorescent molecule comprising covalently bonded carbazole and anthracene moieties, J. Mater. Chem. 21 (2011) 9139–9148. [27] T. Zhang, D. Liu, Q. Wang, R.J. Wang, H.C. Ren, J.Y. Li, Deep-blue and white organic light-emitting diodes based on novelfluorene-cored derivatives with naphthylanthracene endcaps, J. Mater. Chem. 21 (2011) 12969–12976. [28] J. Huang, J.H. Su, X. Li, M.K. Lam, K.M. Fuang, H.H. Fan, K.W. Cheah, C.H. Chen, H. Tian, Bipolar anthracene derivatives containing hole- and electron-transporting moieties for highly efficient blue electroluminescence devices, J. Mater. Chem. 21 (2011) 2957–2964. [29] W. Li, J.Y. Li, F. Wang, Z. Gao, S.F. Zhang, Universal host materials for highefficiency phosphorescent and delayed-fluorescence OLEDs, ACS Appl. Mater. Interfaces 7 (2015) 26206–26216. [30] S. Zhuang, R. Shangguan, J. Jin, G. Tu, L. Wang, J. Chen, D. Ma, X. Zhu, Efficient nondoped blue organic light-emitting diodes based on phenanthroimidazolesubstituted anthracene derivatives, Org. Electron. 13 (2012) 3050–3059. [31] S. Zhuang, R. Shangguan, H. Huang, G. Tu, L. Wang, X. Zhu, Synthesis, characterization, physical properties, and blue electroluminescent device applications of phenanthroimidazole derivatives containing anthracene or pyrene moiety, Dyes Pigments 101 (2014) 93–102. [32] A. Wada, T. Yasuda, Q. Zhang, Y.S. Yang, I. Takasu, S. Enomoto, C. Adachi, A host material consisting of a phosphinic amide directly linked donor-acceptor structure for efficient blue phosphorescent organic light-emitting diodes, J. Mater. Chem. C 1 (2013) 2404–2407. [33] S.K. Kim, B. Yang, Y. Ma, J.H. Lee, J.W. Park, Exceedingly efficient deep-blue electroluminescence from new anthracenes obtained using rational molecular design, J. Mater. Chem. 18 (2008) 3376–3384. [34] S. Tang, W. Li, F. Shen, D. Liu, B. Yang, Y. Ma, Highly efficient deep-blue electroluminescence based on the triphenylaminecored and peripheral blue emitters with segregative HOMO-LUMO characteristics, J. Mater. Chem. 1 (2012) 4401–4408. [35] W. Liu, S. Ying, R. Guo, X. Qiao, P. Leng, Q. Zhang, Y. Wang, D. Ma, L. Wang, Nondoped blue fluorescent organic light-emitting diodes based on benzonitrileanthracene derivative with 10.06% external quantum efficiency and low efficiency roll-off, J. Mater. Chem. C 7 (2019) 1014–1021. [36] J.Y. Hu, Y.J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Bisanthracenebased donor–acceptor-type light-emitting dopants: highly efficient deep-blue emission in organic light-emitting devices, Adv. Funct. Mater. 24 (2014) 2064–2071.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We appreciate support from the Natural Science Foundation of Anhui Education Department (KJ2018A0525), the Excellent Talents Program of Anhui Province (gxyq2019062), and the innovative projects for un dergraduate student of Anhui Province S201910879289). References [1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913–915. [2] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Synthesis of light emitting conjugated polymers for applications in electroluminescent devices, Chem. Rev. 109 (2009), 897-1091. [3] L.X. Guo, X.J. Wang, L.H. Feng, Synthesis, photophysical and electrochemical properties of a blue emitter with binaphthalene and carbazole units, Spectroc. Acta Pt. A-Molec. Biomolec. Spectr. 201 (2018) 376–381. [4] L. Chen, C. Zhang, G. Lin, H. Nie, W. Luo, Z. Zhuang, S. Ding, R. Hu, S.J. Su, F. Huang, A. Qin, Z. Zhao, B.Z. Tang, Solution-processable, starshaped bipolar tetraphenylethene derivatives for the fabrication of efficient nondoped OLEDs, J. Mater. Chem. C 4 (2016) 2775–2783. [5] C. Raffaella, T. Stefano, G. Gianluca, U. Hakan, F. Antonio, M. Michele, Organic light emitting transistors with an efficiency that outperforms the equivalent lightemitting diodes, Nat. Mater. 9 (2010) 496–503. [6] P. Sun, K. Wang, B. Zhao, T. Yang, H. Xu, Y. Miao, H. Wang, B. Xu, Blue-emitting Ir (III) complexes usingfluorinated bipyridyl as main ligand and 1,2,4-triazol as ancillary ligand: photophysical properties and performances in devices, Tetrahedron 72 (2016) 8335–8341. [7] T.L. Wu, M.J. Huang, C.C. Lin, P.Y. Huang, T.Y. Chou, R.W.C. Cheng, H.W. Lin, R. S. Liu, C.H. Cheng, Diboron compound-based organic light-emitting diodes with high efficiency and reduced efficiency roll-off, Nat. Photonics 12 (2018) 235–240. [8] Q.S. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T. Takahashi, S.Y. Lee, T. Yasuda, C. Adachi, Nearly 100% internal quantum efficiency in undoped electroluminescent devices employing pure organic emitters, Adv. Mater. 27 (2015) 2096–2100. [9] I. Cho, S.H. Kim, J.H. Kim, S. Park, S.Y. Park, Highly efficient and stable deep-blue emitting anthracene-derived molecular glass for versatile types of non-doped OLED applications, J. Mater. Chem. 22 (2012) 123–129. [10] X. Xing, L. Xiao, L. Zheng, S. Hu, Z. Chen, B. Qu, Q. Gong, Spirobifluorene derivative: a pure blue emitter (CIEy � 0.08) with high efficiency and thermal stability, J. Mater. Chem. 22 (2012) 15136–15140. [11] G.Y. Zhong, Z. Xu, S.T. Zhang, W. Huang, X.Y. Hou, Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq, Appl. Phys. Lett. 81 (2002) 1122–1124. [12] J.R. Gong, L.J. Wan, S.B. Lei, C.L. Bai, X.H. Zhang, S.T. Lee, Direct evidence of molecular aggregation and degradation mechanism of organic light-emitting diodes under joule heating: an STM and photoluminescence study, J. Phys. Chem. B 109 (2005) 1675–1682. [13] R. Zhang, Y. Zhao, G.L. Li, D.S. Yang, Z.H. Ni, A new series of pyrenyl-based triarylamines: syntheses, structures, optical properties, electrochemistry and electroluminescence, RSC Adv. 6 (2016) 9037–9048. [14] J.N. Moorthy, P. Natarajan, P. Venkatakrishnan, D.F. Huang, T.J. Chow, Steric inhibition of π-stacking: 1,3,6,8-tetraarylpyrenes as efficient blue emitters in organic light emitting diodes (OLEDs), Org. Lett. 9 (2007) 5215–5218. [15] T.M. Figueira-Duarte, K. Müllen, Pyrene-based materials for organic electronics, Chem. Rev. 111 (2011) 7260–7314.
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