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A trifluoromethyl-substituted 9,9′-bianthracene derivative named 10,10′-bis(3-trifluoromethylphenyl)-9,9′-bianthracene (BAn-(3)-CF3) has been designed and synthesized for organic light-emitting device (OLED). The compound is thermally stable with high decomposition temperature (T d = 356 °C) and glass transition temperature (T g = 266 °C). Non-doped deep blue OLED using BAn-(3)-CF3 as the emitter shows the Commission Internationale de l'Éclairage (CIE) coordinates of (0.156, 0.142), which is indicative of excellent blue color purity. Furthermore, BAn-(3)-CF3 serves as an excellent host material doped for 4,4-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) to get high-performance blue OLED with low turn-on voltage of 3.8 V, high luminance of over 12000 cd/m2, high current efficiency of 5.88 cd/A and high external quantum efficiency (EQE) of 3.15%.
© 2015 Optical Society of America
1. Introduction
Organic light-emitting devices (OLEDs) have attracted sustaining attention in large-scale consumer electronics applications, including general illumination and displays, because of their advantages over current technologies in terms of luminescent property, power consumption, shape and size variability. For a full-color display and white lighting, it is essential to have the three primary colors, red (R), green (G), and blue (B). However, efficient and stable blue electroluminescent organic emitters are still lacking compared to green and red emitters reported by now, due to the difficulty in developing blue-emitting materials with a high efficiency, color purity and long operation time [1–4 ].
Anthracene derivatives possessing excellent photoluminescence (PL) and electroluminescence (EL) properties as well as good thermal properties have been studied intensively for efficient blue emitting materials [5–10 ]. However, anthracene with its intrinsic planarity and grid structure easily causes fluorescence concentration quenching and emission wavelength bathochromic shift in the solid state. Considerable effort has thus been directed at developing efficient blue emitting materials by preventing molecular aggregation through the incorporation of sterically bulky moieties within the anthracene-derived blue emitters [11,12 ]. Recently, 9,9′-bianthracene (BAn) and its derivatives that showed high quantum efficiency and good thermal stability have been reported to improve the EL performance by preventing intermolecular interactions and thus reducing the self-quenching effect [13–16 ]. Because of the intramolecular H−H repulsion between hydrogen atoms at the 1,1′- and 8,8′-positions, the two anthracene rings of BAn are oriented perpendicular to each other with dihedral angles of 82°-90° depending on the substituents [13–17 ]. For example, Liu et al. reported an efficient 9,9′-bianthracene-cored molecule for highly efficient deep-blue OLED with a current efficiency of 3.7 cd/A and an external quantum efficiency (EQE) of 3.9% [16]. In our previous work, a series of fluorinated 9,9′-bianthracene derivatives (BAnFs) with varied fluorinated phenyl rings attached to the 9,9′-bianthracene core, were first presented as highly efficient pure blue emitters for OLEDs, especially the device based on 10,10'-bis(3,5-bis(trifluoromethyl) phenyl)-9,9'-bianthracene (BAn-(3,5)-CF3). The maximum current efficiency and power efficiency of the device based on BAn-(3,5)-CF3 are 3.05 cd/A and 2.62 lm/W, respectively, corresponding to a EQE of 5.02% [17]. However, it's a pity that all these devices based on BAnFs show a low luminance of no more than 4000 cd/m2.
In this paper, we have designed and synthesized a new 9,9′-bianthracene derivative named 10,10′-bis(3-trifluoromethylphenyl)-9,9′-bianthracene (BAn-(3)-CF3), in which the 3-trifluoromethylphenyl moieties are covalently incorporated to the BAn-core to lower the energy level. The OLED employing BAn-(3)-CF3 as the host material doped with 4,4-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi) shows a high current efficiency of 5.88 cd/A, low turn-on voltage of 3.8 V, high EQE of 3.15% and high luminance of over 12000 cd/m2. Notably, the luminance has been greatly improved when compared with our previous work (4000 cd/m2).
2. Experimental
General information: Commercially available reagents were used without further purication. 9,10-Anthracenedione, [3-(trifluoromethyl)phenyl]boronic acid and tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) were purchased and used as received. The photoluminescence (PL) and absorption spectra (UV) were obtained by a Horiba Jobin Yvon Fluoromax-4 spectrophotometer and a Hitachi U-3900 spectrophotometer, respectively. To measure the fluorescence quantum yields (Φ f), degassed solutions of the compound in CH2Cl2 was prepared and the excitation was performed at 390 nm. Glass transition temperature (T g) and thermal decomposition temperature (T d) were determined with a STA 409 PC instrument, heating rate of 10 °C/min under a nitrogen atmosphere. Cyclic voltammetry (CV) was performed using a CHI 660E workstation at a scan rate of 100 mV/s. The CV experiment was carried out in a three electrode compartment cell with a Pt-sheet counter electrode, a glassy carbon working electrode and a Pt-wire reference electrode. The supporting electrolyte used was a 0.1 M tetrabutylammonium perchlorate ([Bu4N]ClO4) solution in dry acetonitrile (CH3CN). The cell containing the solution of the sample (1 mM) and the supporting electrolyte was purged with nitrogen thoroughly before scanning for its oxidation and reduction properties. Ferrocene was used for the potential calibration in each measurement. The potential was reported relative to the ferrocene-ferrocenium (Fc/Fc+) couple, whose onset potential was + 0.13 V relative to the reference electrode. The oxidation potential was determined by taking the onset of the anodic potentials. The HOMO and LUMO values were estimated by using the following general equation: ; , where the 4.8 is the energy level of ferrocene below the vacuum level and is calculated from UV-vis absorption spectra.
Materials and synthesis: 10,10′-bis(3-(trifluoromethyl)phenyl)-9,9′-bianthracene (BAn-(3)-CF3) was synthesized according to Suzuki coupling reaction. 9,9′-Bianthracene (BAn) and 10,10'-dibromo-9,9'-bianthracene (BAn-2Br) were synthesized referring to the literature [18,19 ]. THF (60 mL) and 2.0 M aqueous solution of K2CO3 (15 mL) were added to a flask containing BAn-2Br (3 mmol), [3-(trifluoromethyl)phenyl]boronic acid (6.3 mmol) and Pd(PPh3)4 (0.35 mmol) under nitrogen. The reaction mixture was heated to reflux and maintained at this temperature for 24 h. When the reaction came to an end (inspected by thin-layer chromatography), water was added to quench the reaction. Then the product was extracted with THF, washed with brine, dried over anhydrous MgSO4, concentrated by evaporating off the solvent for further purification by column chromatography on silica gel in solvent as mobile phase. BAn-(3)-CF3 was obtained as a pale powder. Yield: 83%. 1H NMR (CDCl3, 400 MHz): δ 7.19-7.27 (m, 6H), 7.36-8.00 (m, 16H), 8.88-8.89 (m, 2H). Anal. Calcd for C42H24F6: C, 78.50%; H, 3.74%. Found: C, 78.52%; H, 3.75%.
Device fabrication and testing: The OLEDs were fabricated by thermal evaporation onto a cleaned glass substrate, pre-coated with transparent and conductive indium tin oxide (ITO). Before loading into a deposition chamber, the ITO substrates were exposed to a UV-ozone flux for 10 min, following ultrasonic cleaning in acetone and isopropyl alcohol (IPA). The devices were fabricated by the conventional vacuum deposition of the organic layers, LiF and an Al cathode onto an ITO-coated glass substrate under a base pressure lower than 1 × 10−3 Pa. The testing area of device was 3 × 3 mm2. The thickness of each layer was determined by a quartz thickness monitor. The voltage–current density (V–J) and voltage–brightness (V–L) as well as the current density–current efficiency (J–η c) and current density–power efficiency (J–η p) curve characteristics of the devices were measured with a Keithley 2602 and Source Meter. All the measurements were carried out at room temperature under ambient conditions.
3. Results and discussions
Synthesis and calculation: BAn-(3)-CF3 was readily obtained with a one-step Suzuki coupling between brominated 9,9'-bianthracene (BAn-2Br) and [3-(trifluoromethyl)phenyl]boronic acid in the presence of a palladium catalyst, with a yield of 83%. The chemical structure and the synthetic route of BAn-(3)-CF3 in this study is shown in Fig. 1 . Then the product was purified by column chromatography and recrystallization. The compound had been fully characterized with 1H NMR spectroscopy and elemental analysis.
The three-dimensional geometry and the frontier molecular orbital energy level of this compound were calculated using the density functional theory (DFT) at the B3LYP/6-31G (d, p) level in the Gaussian 09 program. As shown in Fig. 2 , in the highest occupied molecular orbital (HOMO), π-electrons locate only on one anthracene ring, while in the lowest unoccupied molecular orbital (LUMO), the π-electrons delocalize on the two anthracene rings, which indicates that the absorption and emission processes are mainly attributed to the π-π* transition of the 9,9'-bianthracene moiety. Thus, the excellent luminescence efficiency of the 9,9'-bianthracene moiety can be maintained. The calculated HOMO and LUMO energy levels of BAn-(3)-CF3 are listed in Table 1 . In the molecule, the two adjacent planar anthracene units are nearly perpendicular to each other with the dihedral angle of 89° owing to a steric repulsion of the anthracene peri-hydrogen atoms (1,1'- and 8,8'-positions) and the dihedral angle between the functionalized phenyl ring and the adjacent phenyl ring of the 9,9'-bianthracene core is 103°. The three-dimensional geometry of the molecular is shown in Fig. 2. This can be explained by the intramolecular CF3/H interactions of the substitute group CF3 in the meta-positions of the peripheral substituted phenyl with the H atoms of the anthracene unit in BAn-(3)-CF3 [14–17 ]. The calculation result indicates that the BAn-(3)-CF3 has extremely twisted geometry configuration, which can prevent intramolecular extending of the π-electrons and suppress intermolecular interactions, conjugation, and molecular recrystallization.
Fig. 2 The optimized geometry and the molecular orbital surface of the HOMO and LUMO for BAn-(3)-CF3 obtained at the B3LYP/6-31G level.
Table 1. Physical properties of the BAn-(3)-CF3.
Thermal properties: The thermal property of BAn-(3)-CF3 was determined by thermal gravimetric analysis (TGA) and a differential scanning calorimeter (DSC). It can be seen in Figs. 3(a) and 3(b) that BAn-(3)-CF3 possesses excellent thermal stability with a thermal decomposition temperature (T d) (5% weight loss) over 350 °C and a high glass temperature (T g) up to 266 °C. Therefore, thermal analysis indicates that this compound is thermally stable and suitable for vapor deposition in OLEDs fabrication.
Fig. 3 TGA (a) and DSC (b) measurements for BAn-(3)-CF3 (DSC measurements: 2nd scan after N2 treatment, 10 °C/min) .
Photophysical properties: Figs. 4(a) and 4(b) show the normalized UV-vis absorption and PL spectra of BAn-(3)-CF3 in dichloromethane (CH2Cl2) dilute solution and film (ca. 50 nm) obtained by thermal evaporation on a pre-cleaned quartz substrate. Their photophysical data are summarized in Table 1. The UV-vis absorption spectra in CH2Cl2 solution shown in Fig. 4(a) indicate that BAn-(3)-CF3 exhibits the common isolated characteristic vibronic structure at approximately 400, 379, 358, 340 nm, owing to the 9,9'-bianthracene core [15–17 ]. The absorption spectrum of BAn-(3)-CF3 in film is only red-shifted by 4 nm relative to its in solution. The similarity between the absorption spectra of the dilute solution and thin film suggest that the conformation of the solid state of BAn-(3)-CF3 is attributed to its non-coplanar structure, reducing the degree of intermolecular aggregation and π-delocalization [14,15,17 ]. Also, the optical energy gap (E g = 3.0 eV) of BAn-(3)-CF3 was calculated according to the absorption edge of the optical absorption spectra in film, which indicates BAn-(3)-CF3 should be suitable candidate for blue emitter. From the PL spectra, we can clearly observe a significant blue shift in the emission maximum for BAn-(3)-CF3 in film state (λ max = 439 nm) relative to that in solution state (λ max = 448 nm). And the full width at half maximum (FWHM) in the film state (ca. 51 nm) is apparently smaller than that in solution (ca. 83 nm), which indicates that aryl substituents at the 9- and 10- positions of the anthracene disable the molecular packing in the solid state and effectively inhibit excimer formation as well as fluorescence quenching [15,17 ]. We measured the absolute photoluminescence quantum yield (Φ f) that determined in CH2Cl2 using integrating sphere at room temperature. BAn-(3)-CF3 exhibited high Φ f of 0.548, which is comparable to the 9,9′-bianthracene-cored compound (CzBACz) and the fluorene derivatives [24–27 ].
Fig. 4 UV-vis absorption spectra (a) and PL spectra (b) of BAn-(3)-CF3 in dilute CH2Cl2 solution and solid film.
Electrochemical properties: Cyclic voltammetric (CV) study was performed to calculate the HOMO and LUMO values for the BAn-(3)-CF3. Trace of cyclic voltammetric measurement of BAn-(3)-CF3 is shown in Fig. 5 . The electrochemical property of BAn-(3)-CF3 was carried out in solution of a 0.1 M supporting electrolyte ([Bu4N]ClO4) and 1 mM substrate in CH3CN under an nitrogen atmosphere using ferrocene as an internal standard. The HOMO and LUMO values, as well as the energy level parameter of BAn-(3)-CF3, are listed in Table 1. From the oxidation onset potential, the HOMO energy level of the BAn-(3)-CF3 is estimated to be −5.7 eV, and the LUMO energy level of BAn-(3)-CF3 is calculated to be −2.7 eV from the absorption edge of the optical absorption spectra. As seen from Table 1, the experimental HOMO and LUMO values are in excellent agreement with the calculated values.
Electroluminescent properties: The OLED was fabricated with the following configuration: indium tin oxide (ITO)/dipyrazino[2,3-f:2',3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) (5 nm)/1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) (40 nm)/BAn-(3)-CF3 (20 nm)/1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) (40 nm)/LiF (1 nm)/Al (150 nm). HAT-CN was used as a hole injection layer (HIL), TAPC was used as a hole transporting layer (HTL), BAn-(3)-CF3 was used as an emitting layer (EML), TPBi was used as an electron transporting layer (ETL) and exciton blocking layer, and LiF was used as an electron injection layer (EIL). Figures 6(a)-6(d) exhibit the current density–voltage–luminance–efficiency (J–V–L–η) characteristics of the BAn-(3)-CF3-based device. The device performance parameters and EL emission characteristics are summarized in Table 2 . The device with BAn-(3)-CF3 as the emitter shows excellent deep blue emitting light performance with a CIE (0.156, 0.142) at 7 V, maximum current efficiency of 1.11 cd/A, power efficiency of 0.46 lm/W and EQE of 1.01%. External quantum efficiency-current density curves for BAn-(3)-CF3 devices is shown in Fig. 7(b) .
Fig. 6 (a) Current density-voltage curves, (b) Brightness-voltage curves, (c) Current efficiency-current density curves, and (d) Power efficiency-current density curves.
Table 2. EL performance of BAn-(3)-CF3 devices.
Fig. 7 (a) Normalized EL spectra of DPAVBi-doped device using BAn-(3)-CF3 recorded at various driving voltages, and (b) External quantum efficiency-current density curves for BAn-(3)-CF3 devices.
In order to improve the performance of device, we choose BAn-(3)-CF3 as a host material doped with 3 wt% DPAVBi in EML. The structure of the doped device is depicted in Fig. 8 . Figure 7(a) shows the normalized EL spectra for the doped device at various driving voltages. Noticeably, the doped device exhibits striking blue EL color stability at various driving voltages. With increasing driving voltages from 6 to 12 V, the EL spectra of the doped device remain nearly unchanged, suggesting a remarkable voltage-independent EL emission. As seen from the Table 2, the doped device has a low turn-on voltage (at a luminance of 1 cd/m2) no greater than 4 V, which shows small injection barriers in the device. As shown in Figs. 6(a)-6(d) and Fig. 7(b), the doped device displays a high current efficiency of 5.88 cd/A, high power efficiency of 2.69 lm/W and high EQE of 3.15%, especially an extremely high luminance of over 12000 cd/m2 in the blue visible region with a CIE chromaticity coordinates (0.151, 0.344). Luminance has been greatly improved when compared with the devices based on BAnFs in our previous work (no more than 4000 cd/m2) [17]. Also, the performance of device using BAn-(3)-CF3 as host is excellent compared with the device using other famous host material as host [11,24–27 ]. For example, Xia et al. reported blue OLEDs with a turn-on voltage 5.47 V, a power efficiency of 2.98 lm/W, and a maximum EQE of 3.7% from the device using anthracene derivatives as host [11]; Li et al. reported blue OLEDs with a turn-on voltage 4.2 V, a power efficiency of 2.47 lm/W, and a maximum EQE of 3.02% from the device using spirobifluorene derivatives as host [27].
We owe the excellent efficiencies to the excellent injection of carrier charges and good confinement of carriers and excitons within EML. Since the HOMO levels of the host (−5.7 eV for BAn-(3)-CF3) and TAPC (−5.5 eV), the LUMO levels of the host (−2.7 eV for BAn-(3)-CF3) and TPBi (−2.7 eV) are almost the same, the barriers for hole injection from the TAPC layer to EML and for electron injection from the TPBi layer to EML can be neglected. Therefore, the carrier charges will easily transport into the host and recombine within EML. Meanwhile, carriers and excitons are well confined within EML due to the higher LUMO levels of TAPC (−2.0 eV) and the lower HOMO levels of TPBi (−6.2 eV). Also, the dopant HOMO energy level (−5.3 eV for DPVABi) is higher than that of the host HOMO energy level (−5.7 eV for BAn-(3)-CF3), prompting the DPAVBi molecule to serve as hole-traps [15,21 ]. In generally, the synthesized new fluorinated 9,9'-bianthracene derivative shows potential application as high efficient blue emitter for OLEDs.
4. Conclusion
In summary, a new blue light emitting material of 9,9′-bianthracene derivative (BAn-(3)-CF3) has been successfully prepared by Suzuki coupling reaction in high yield of 83%. The compound showed excellent thermal stabilities and pronounced PL efficiencies. The device with BAn-(3)-CF3 as the emitter presented excellent deep blue emitting light performance with a CIE (0.156, 0.142) at 7 V. Moreover, the doped device using BAn-(3)-CF3 as the host achieved a maximum EQE of 3.15%, a high current efficiency of 5.88 cd/A and an EQE of over 12000 cd/m2 in the blue visible region.
Acknowledgments
This research work was financially supported by National High Technology Research and Development Program (“863” Program) of China (2015AA016901) and National Natural Scientific Foundation of China (61308093, 11204202, 11204205, 61274056, 61475109, 61571317). The New Teachers' Fund for Doctor Stations (20131402120020) and the China Postdoctoral Science Foundation (Grant no. 2015M572454).
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