1. Introduction

The OLED technology has some very attractive features that are likely to give it prominent place in full-color displays and solid-state lighting market of the future, since Tang and Slyke introduced an efficient organic light emitting diode (OLED) [1–4]. Full-color displays require red, green and blue emission of relatively equal stability, efficiency and color purity. Especially, Efficient blue-light OLEDs are of particular interest, because they are desired for use as blue light sources in full color display applications. Compared with red or green emitting materials, the electroluminescene (EL) characteristics of blue-emitting material have to be improved, particularly in terms of efficiency, color purity and lifetime [5–11]. Anthracene derivatives have been used widely as blue emitting materials because they possess outstanding photoluminescence and electroluminescence properties. High efficiency blue OLEDs with high color purity have been reported based on anthracene derivatives [12–18]. Recently, our group has reported that the whole molecule comes to be highly twisted by introducing bulky substituents at the 9-and 10-position of anthracene. Consequently, the fluorescence-quenching interactions caused by intermolecular interactions can be suppressed to some extent and non-radiative energy decay can be decreased [19–28]. We also reported that asymmetric limb-structured blue light emitting material, composed of anthracene main core, naphthalene units at 9,10-position of anthracene and m-xylene units at 2,3-positon of anthracene [29]. The introduction of m-xylene substituents at 2,3-positions can reduce highly twisted structure due to steric hindrance between xylenes than the inhibited. In this study, we designed and developed new asymmetric limb-structured blue light emitting material, composed of anthracene main core, naphthalene units at 9,10-position of anthracene and m-(di(trifluoromethylphenyl)) units at 2,3-positon of anthracene. The introduction of m-(di(trifluoromethylphenyl)) substituents at 2,3-positions can make highly twisted structure due to steric hindrance between bulky di(trifluoromethylphenyl) groups. Moreover, the introduction of trifluoro group can increase thermal stability as well as can tune the bang gap due to electron withdrawing ability.

2. Experimental

All starting materials were purchased from Aldrich and Strem. All reagents purchased commercially were used without further purification.

2.1. Instrument

¹H NMR spectra were recorded using a Bruker Avance-300 MHz FT-NMR spectrometer, and chemical shifts were reported in ppm with tetramethylsilane as internal standard. FT-IR spectra were recorded using a Bruker IFS66 spectrometer. Thermogravimetric analysis (TGA) was performed under nitrogen using a TA instruments 2050 thermogravimetric analyzer. Differential scanning calorimeter (DSC) was conducted under nitrogen using a TA instrument DSC Q10. The both samples were heated using a 10 °C/min. UV-visible spectra and photoluminescence (PL) spectra were measured by Shimadsu UV-1065PC UV-visible spectrophotometer and Perkin Elmer LS50B fluorescence spectrophotometer, respectively. The electrochemical properties of the materials were measured by cyclic voltammetry (Epsilon C3) in a 0.1 M solution of tetrabutyl ammonium perchlorate in acetonitrile. The organic EL devices were fabricated using successive vacuum-deposition of 4,4',4”-Tris-(N-(naphthylen-2-yl)-N-phenylamine)triphenylamine (2-TNATA, 600 Å), N,N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB, 150 Å), 9.10-di(naphthalene-2-yl)anthracene (mFADN), tris(8-hydroxyquinoline)aluminum (Alq3, 200 Å), LiF (10 Å), and Al electrode on top of the ITO glass substrate. The ITO glass with a sheet resistance of about 10 Ω was etched for the anode electrode pattern and cleaned in ultrasonic baths of isopropyl alcohol and acetone. The overlap area of Al and ITO electrodes is about 4 mm². A UV zone cleaner (Jeilight Company) was used for further cleaning before vacuum deposition of the organic materials. Vacuum deposition of the organic materials was carried out under a pressure of 2 × 10 –7 torr. The deposition rate for organic materials was about 0.1 nm/s. The evaporation rate and the thickness of the film were measured with a quartz oscillator. OLED performance was studied by measuring the current-voltage-luminescence (I-V-L) characteristics, EL, and PL spectra at room temperature. I-V-L characteristics and CIE color coordinates were measured with a Keithley SMU238 and Spectrascan PR650. EL spectra of the devices were measured utilizing a diode array rapid analyzer system (Professional Scientific Instrument Corp.) Fluorescence spectra of the solutions in chloroform were measured using a spectro fluorimeter (Shimadzu Corp.).

2.2. Synthesis of 3,4-Dibromothiophene-1,1-dioxide (1)

Trifluoroacetic acid (11.01 mmol, 153 mL) was slowly added into the 30% H₂O₂ (60 mL) at 0 °C. After addition of trifluoroacetic acid, the solution of 3,4-dibromothiophene (70.40 mmol, 8.14 mL) in CH₂Cl₂ (90 mL) was added in the mixture. The mixture was stirred for 3 h at room temperature. The reaction was worked up by adding Na₂CO₃ aqueous solution at 0 °C. The organic layer was separated by methylene dichloride, and the solvent was evaporated. The crude product was recrystallized in co-solvent of CH₂Cl₂ and ethanol Yield: (12.4 g, 49%). Mp 103-104 °C. IR (KBr): 1309, 1146 cm^-1 (S = O), 3015 cm⁻¹ (sp² C-H). ¹H-NMR (300MHz, CDCl₃, ppm): δ 6.85 (1).

2.3. 2,3-Dibromoanthraquinone (2)

3,4-Dibromothiophene-1.1-dioxide (5.65 g, 20.62 mmol) and 1,4-naphthoquinone (16.31 g, 103.13 mmol) were dissolved in 350 mL of acetic acid. The reaction mixture was refluxed with for 48 h. The reaction was worked up by adding Na₂CO₃ aqueous solution at 0 °C. The organic layer was separated by methylene dichloride, and the solvent was evaporated. The crude product was purified by column chromatography with n-hexane/ CH₂Cl₂ (1:1, v/v) as eluent. Yield: (3.2 g, 40.65%). Mp 278-279 °C. IR (KBr): 1681 cm^-1 (C = O), 3075 cm⁻¹ (sp² C-H), 1656 cm^-1 (C = C). ¹H-NMR (300MHz, CDCl₃, ppm): δ 8.49 (2), 8.38-8.40 (2), 7.86-7.89 (2).

2.4. 2,3-Dibromo-9,10-dihydroxy-9,10-dinaphthylanthracene (3)

2,3-Dibromoanthraquinone (2.10g, 5.74 mmol) were dissolved in 70mL of THF. Naphthylmagnesium bromide (10.2 mL) was added to the reaction mixture at 0-15 °C. The mixture was stirred for 3 h at room temperature. The organic layer was extracted with diethyl ether. The crude product was purified by column chromatography with n-hexane/ CH₂Cl₂ (1:1, v/v) as eluent. Yield: (1.99g, 68%). Mp 226 °C. IR (KBr): 3521 cm^-1 (-OH), 3017-3029 cm⁻¹ (sp² C-H), 1582cm^-1 (C = C). ¹H-NMR(300MHz, CDCl₃, ppm): δ8.52(2), 8.38-8.40 (4), 7.86-7.89 (5), 6.94 (2).

2.5. 2,3-Dibromo-9,10-dinaphthylanthracene (4)

2,3-Dibromo-9,10-dihydroxy-9,10-dinaphtylanthracene (2.4 g, 3.86 mmol), NaH₂PO₄‧H2O (4.09 g, 38.56 mmol) and KI (1.92 g, 11.58 mmol) were dissolved in 50 ml of acetic acid. After the reaction mixture was refluxed for 4 h, the reaction was worked up by water. The crude solid product was several time filtered by water. Yield: (2.02 g, 83%), Mp 298 °C. IR (KBr): 3011-3043 cm⁻¹ (sp² C-H), 1752-1734 cm^-1 (C = C). ¹H-NMR (300MHz, CDCl₃, ppm): δ 8.07-7.96 (10), 7.72-7.67 (8), 7.64-7.59 (2).

2.5. 2,3-Bis(3,5-bis(trifluoromethyl)phenyl)-9,10-di(naphthalen-2-yl)anthracene (mFADN)

2,3-Dibromo-9,10-dinaphthalene anthracene (1g, 1.7 mmol) and 3,5-bis(trifluoromethyl) phenylboronic acid (1.97 g, 7.65 mmol) and 2M K₂CO₃ was added to the 50 mL of THF. After Pd(pph₃)₄ (0.06 g, 3 mol/%) was added to the mixture, the mixture was stirred at 90 °C for 48 h. After the reaction was worked up by adding water, the organic layer was extracted with chloroform. After the solvent was evaporated, the crude product was purified by column chromatography using n-hexane/ethyl acetate (5:1, v/v) as eluent. Yield: (0.85 g, 59%). mp 279°C; IR (KBr): 3051-3023 cm⁻¹ (sp² C-H), 1738-1702 cm^-1 (C = C); ¹H-NMR (300MHz, CDCl₃, ppm): δ8.14 (2), 7.93-7.67 (18), 7.58-7.39 (6); ¹³C-NMR (300MHz, CDCl₃, ppm):142.3, 138.0, 135.6, 135.0, 133.4, 133.0, 131.7, 131.2, 130.4, 129.9, 129.2, 128.9, 128.4,1 28.0, 127.3, 126.3, 126.6, 126.2, 124.7, 121.0, 120.6 EI-MS: m/z 854

3. Results and discussion

The synthetic scheme of the three limb structured anthracene derivatives is shown in Fig. 1. 2,3-Dibromoanthraquinone was obtained by oxidation and the Diels-Alder reaction following degradation of SO₂. 2,3-Dibromo-9,10-dihydroxy-9,10-dinaphthylanthracene was synthesized by reaction of 2-naphthyl magnesium bromide and 2,3-dibromoanthraquinone. 2,3-Dibromo-9,10-dinaphthylanthracene obtained through reduction of 2,3-dibromo-9,10-dihydroxy-9,10-dinaphthylanthracene was reacted with 3,5-bis(trifluoromethyl)phenyl- boronic acid to afford 2,3-bis(3,5-bis(trifluoromethyl)phenyl)-9,10-di(naphthalen-2-yl)anthracene (mFADN).

 

Fig. 1 Synthetic Scheme of mFADN.

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The structure of new limb structured materials was confirmed by various spectroscopic methods such as FT-NMR, IR and mass spectroscopies. Theoretical calculations using Spartan08 software in order to fully optimize the molecular structure, were carried out for the characterization of 3-dimensional structures and the energy densities of the HOMO and LUMO states of material (Fig. 2). As we expected, mFADN had large band gap with 3.3 eV.

 

Fig. 2 HOMO and LUMO states by using DFT calculation.

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The thermal properties of mFAND were determined by differential scanning calorimetry (DSC) and thermogravimetry (TGA) measurements. (Fig. 3) mFAND exhibit high thermal stabilities with decomposition temperatures (5% weight loss) of 398 °C. The melting transition temperature (T_m) of mFAND was not observed at heating up to 300 °C. As a results, it is suggested that new limb structured material had high thermal stability and inhibited self-aggregation. This suggests that the emitting layer composed with mFAND has stable morphological properties, which is desirable for OLEDs with high stability and efficiency.

 

Fig. 3 TGA and DSC thermograms of mFADN.

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Figure 4 shows the UV-visible absorption and photoluminescence (PL) spectra of mFAND in chloroform solution. The UV-visible absorption spectrum of mFAND shows the characteristic vibrational patterns of an isolated anthracene group (λmax = 366, 387, 410 nm). Upon irradiation at 387 nm, the PL spectrum of mFAND in solution exhibited an excellent blue emission with the peak maximum at λmax 447 nm. In the PL spectrum of mFAND in a film, the peak maximum of the blue emission is found at 459 nm without a shoulder emission, which implies the blue emission with inhibited intermolecular interaction. mFAND containing an m-di(trifluoromethylphenyl) showed a blue shifted emission compared with BMPNA with xylene groups. It may be due to the increased steric hindrance of bulky trifluoro methyl groups and electron withdrawing ability of trifluoro methyl group, which reduced HOMO energy level. The electrochemical behaviors of mFAND were investigated by cyclic voltammetry (CV). The HOMO, LUMO, and bandgap (Eg) are summarized Table 1.The oxidation peak potentials are high, Eox = 1.2 for mFAND. HOMO levels of mFAND are 5.65 eV. The phtoluminescence quantum yield (PLQY) of BMPNA was 0.49, which was measured with diphenylanthracene reference.

 

Fig. 4 UV-visible and PL spectra of mFADN.

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Table 1. The optical, thermal and electrical properties of mFADN and mMADN

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Non-doped and doped OLED device was fabricated using the synthesized material as the emitting layer in the following structure: ITO/4,4',4”-Tris-(N-(naphthylen-2-yl)-N-phenylamine)triphenylamine (2-TNATA, 600 Å)/ N,N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB, 150 Å)/ 9.10-di(naphthalene-2-yl)anthracene (mFADN)/tris(8-hydroxyquinoline)aluminum (Alq3, 200 Å) / LiF/Al. In this device, ITO (indium tin oxide) and Al were the anode and the cathode, respectively. 2-TNATA was the hole injection layer (HIL); NPB was the hole transporting layer (HTL); Alq3 was the electron transporting layer (ETL) and the newly prepared limb-type anthracene derivative was used as the emitting layer (EML). The current-voltage-brightness curve of non-doped devices is displayed in Fig. 5(a).The turn-on voltage of device was 17.01 V and maximum brightness was reached to 4, 000 cd/ m². Figure 5(b) shows the current efficiency and power efficiency of device. The non-doped device using mFADN exhibits the maximum external quantum efficiency of 0.67% (1.72 cd/A). The EL spectra as increasing voltage were shown in the Fig. 6.The color index of non-doped device was (0.28, 0.46). Although the HOMO and LUMO levels are well matched with NPB as HTL and Alq3 as ETL, the device shows high turn on voltage and low color purity. From the results, it is suggested that the emission zone is formed between emitter and Alq3. The further optimized device structure can increase efficiency and color purity.

 

Fig. 5 Current density-voltage-luminance characteristics (left) and efficiency curves (right).

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Fig. 6 EL spectra of devices.

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4. Conclusion

We designed new limb structured anthracene derivatives, mFADN, which are composed of an anthracene core and naphthalene units at the 9,10-positions of anthracene and trifluoro units at the 2,3-position of anthracene. We synthesized asymmetric limb-structured blue light emitting material through oxidation, Diels-Alder reaction following degradation of SO₂, Suzuki coupling reaction. The theoretical calculation supports that the synthesized material has 3-dimensionally highly twisted non-coplanar structures due to steric hindrance of the introduced limbs. The non-doped blue EL device using mFADN as an emitting material showed maximum brigntness of 4, 000 cd/ m² and maximum external quantum efficiency of 0.67% (1.72 cd/A).