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Thermally stable fluorescent blue organic light-emitting diodes using spirobifluorene based anthracene host materials with different substitution position
Synthetic Metals 160 (2010) 1184–1188
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Thermally stable fluorescent blue organic light-emitting diodes using spirobifluorene based anthracene host materials with different substitution position Sang Eok Jang a , Chul Woong Joo a , Kyoung Soo Yook a , Joon-Woo Kim b , Chil-Won Lee b , Jun Yeob Lee a,∗ a b
Department of Polymer Science and Engineering, Dankook University Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea OLED Team, Daejoo Electronic Materials, Siheung, Gyeonggi-do 429-848, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 February 2010 Accepted 8 March 2010 Available online 3 April 2010 Keywords: Blue organic light-emitting diode Spirobifluorene Anthracene High efficiency Thermal stability Bandgap
a b s t r a c t Thermally stable blue organic light-emitting diodes (OLEDs) were developed using anthracene based host materials with a spirobifluorene group. 4-Bromospirobifluorene and 2-bromospirobifluorene were attached to the anthracene core and the effect of the substitution position on the physical properties and device performances of the blue fluorescent OLEDs was investigated. The 4-spirobifluorene substitution was better than the 2-spirobifluorene substitution in terms of thermal stability and widened the bandgap of the anthracene based host material due to the geometrical structure of the material. However, the wide bandgap of the host material with 4-spirobifluorene had negative effect on the current density and efficiency of the blue devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have been developed for more than 20 years and there has been much progress in the device performances. A theoretical maximum internal quantum efficiency of 100% was already achieved in the red and green devices [1]. However, the efficiency of deep blue phosphorescent OLEDs is not good enough and the lifetime is also too short for commercialization [2]. Therefore, the blue fluorescent OLEDs have been actively studied in spite of low efficiency compared with blue phosphorescent devices. Many host and dopant materials for blue fluorescent devices were developed, which include diarylanthracene [3], di(styryl)arylene [4,5], fluorene [6], pyrene [7], and fluoranthene [8]. One of the most well known blue host material is 2-methyl9,10-di(2 -naphthyl)anthracene (MADN) with an anthracene core structure [9]. It showed good device performances as a host in the blue fluorescent OLEDs. Other than the MADN, many blue emitting materials with the anthracene core structure were developed and showed good device performances in the blue device [10–12]. The spirobifluorene is also a group which can be introduced in the molecular structure of the anthracene based blue emitting
∗ Corresponding author. Tel.: +82 31 8005 3585; fax: +82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.03.006
materials. A spirofluorenebenzofluorene was used as the substituent of the anthracene type blue material and high efficiency was reported [13]. A spirobifluorene attached anthracene host material was also reported to be effective as the host material in the fluorescent blue OLEDs [14]. The t-butyl substituted spirobifluorene unit was connected to the anthracene through the 2-position of the spirobifluorene. Other than the 2-spirobifluorene, no spirobifluorene based anthracene host material was synthesized due to the difficulty of the synthesis of 3-bromospirobifluorene and 4bromospirobifluorene. In this work, 4-bromospirobifluorene was synthesized and it was attached to the 1-naphthylanthracene. The effect of the substitution position of the spirobifluorene on the physical properties and device performances of the anthracene based host materials was investigated. 2. Experimental 2.1. Materials 9H-fluoren-9-one, n-butyllithium, triethyl borate, and tetrakis(triphenylphosphine)palladium(0) (Aldrich Chem. Co.) were used as received. 2,2 -Dibromobiphenyl (TCI Chem. Co.) and sodium hydrogen carbonate (Junsei Chem. Co.) were used without further purification. Tetrahydrofuran was distilled over sodium and calcium hydride
S.E. Jang et al. / Synthetic Metals 160 (2010) 1184–1188
2.2. Synthesis of 2-bromo-9,9 -spirobi[fluorene] (1) The 2-bromo-9,9 -spirobi[fluorene] and 4-bromo-9,9 -spirobi [fluorene] were synthesized according to the synthetic procedure reported earlier [15]. 2.3. Synthesis of 9-(2-naphthylanthracene)-10-boronic acid (3) Into a 250 mL, two-neck flask, was placed a 9-bromo-10(naphthalen-1-yl)anthracene (3.00 g, 7.82 mmol) in THF (60 mL). The reaction flask was cooled to −78 ◦ C and n-BuLi (10 M in hexane, 1.01 mL) was added dropwise slowly. The whole solution was stirred at this temperature for 1 h, followed by addition of triethyl borate (1.48 g, 16.7 mmol) under argon atmosphere. After stirring for another 12 h, the solution was warmed to room temperature slowly and the solution was then quenched with 2 N HCl. The mixture was extracted with ethyl acetate. The combined organic layers were dried over magnesium sulfate, filtered, and evaporated under reduced pressure. The resulting powdery product was purified by reprecipitation from ethyl acetate and n-hexane to give yellow powder. 2.4. Synthesis of 2-(10-(naphthalen-1-yl)anthracen-9-yl)-9,9 (SPAN1)
-spirobi[fluorene]
A solution of 2-bromo-9,9 -spirobi[fluorene] (1) (2.00 g, 5.059 mmol), 9-(2-naphthylanthracene)-10-boronic acid (3) (2.11 g, 6.070 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.29 g, 0.253 mmol) dissolved in THF (30 mL) was stirred in a two-necked flask under a argon atmosphere for 30 min. To the reaction mixture, potassium carbonate (8.29 g, 59.98 mmol) in distilled water (30 mL) was added dropwise over a period of 20 min. The resulting solution was refluxed overnight at 80 ◦ C. The reaction mixture was extracted with dichloromethane and the organic layer was separated. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography using n-hexane as the eluent to give a white crystalline solid. Yield 44%. Tg 180 ◦ C. 1 H NMR (200 MHz, CDCl3 ): ı 8.12–7.94 (m, 4H), 7.78–7.63 (m, 6H), 7.56–7.41 (m, 5H), 7.36–6.79 (m, 12H). MS (FAB) m/z 618 [(M+1)+]. 2.5. Synthesis of 4-(10-(naphthalen-1-yl)anthracen-9-yl)-9,9 (SPAN11)
-spirobi[fluorene]
A solution of 4-bromo-9,9 -spirobi[fluorene] (2) (2.29 g, 5.793 mmol), 9-(2-naphthylanthracene)-10-boronic acid (3) (2.22 g, 6.372 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.34 g, 0.289 mmol) dissolved in THF (15 mL) was stirred in a two-necked flask under a argon atmosphere for 30 min. To the reaction mixture, potassium carbonate (16.58 g, 119.6 mmol) in distilled water (60 mL) was added dropwise over a period of 20 min. The resulting solution was refluxed overnight at 80 ◦ C. The reaction mixture was extracted with dichloromethane and the organic layer was separated. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography using n-hexane as the eluent to give a white crystalline solid. Yield 84%. Mp 367 ◦ C. Tg 173 ◦ C. 1 H NMR (200 MHz, CDCl3 ): ı 8.14–8.04 (m, 2H), 7.93–7.52 (m, 7H), 7.47–7.18 (m, 12H), 6.96–5.29 (m, 9H). MS (FAB) m/z 618 [(M+1)+]. 2.6. Device fabrication and measurements The device configuration used in this work was indium tin oxide (150 nm)/N,N -diphenyl-N,N-bis-[4-(phenyl-m-tolyl-amino)-phe-
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nyl]-biphenyl-4,4 -diamine (60 nm)/N, N -di(1-naphthyl)-N,N diphenylbenzidine (NPB, 30 nm)/host: diphenyl-[4-(2-[1,1 ;4 ,1 ] terphenyl-4-yl-vinyl)-phenyl]-amine(BD-1) (30 nm, x% doping)/ tris(8-hydroxyquinoline) aluminium (Alq3 , 20 nm)/LiF (1 nm)/Al (200 nm). Two host materials, SPAN1 and SPAN11, were used as the host materials in the emitting layer. The doping concentration of the BD-1 dopant was 7% [16]. Organic materials were deposited at a deposition rate of 0.1 nm/s and the deposition rate of the dopant was controlled according to the doping concentration. The blue devices were encapsulated with a glass lid and a CaO getter after device fabrication. The 1 H nuclear magnetic resonance spectrometer (NMR, Varian 200) and the low and high resolution mass spectrometer (JEOL, JMSAX505WA) were used to identify the synthesized compounds. The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (HITACHI, F-7000) and the ultraviolet–visible (UV–vis) spectra were obtained by means of a UV–vis spectrophotometer (Shimadzu, UV-2501PC). The differential scanning calorimeter (DSC) measurements were performed on a Mettler DSC 822e under nitrogen at a heating rate of 10 ◦ C/min. The energy levels were measured with a cyclic voltammetry. Current density–voltage–luminance relationship of the blue OLEDs were measured using Keithley 2400 source measurement unit and CS1000 spectroradiometer. Voltage sweep of the blue devices was carried out to obtain the device performances.
3. Results and discussion The 4-bromospirobifluorene was difficult to be prepared by the bromination of the spirobifluorene as the spirobifluorene is selectively brominated at 2-position of the spirobifluorene. Therefore, an alternate synthetic route which utilizes the reaction of the 2,2 -dibromobiphenyl with 9H-fluoren-9-one was developed and a synthetic yield of 50% was obtained [15]. The 4bromospirobifluorene and 2-bromospirobifluorene were attached to the 9-(2-naphthylanthracene)-10-boronic acid to synthesize blue host materials with the anthracene core and spirobifluorene side group. A highly pure blue fluorescent host material with a purity over 99% could be obtained after purification by column chromatography. The synthetic scheme of the SPAN1 and SPAN11 is shown in Scheme 1. The spirobifluorene is a rigid moiety with amorphous nature due to the twisted structure of the fluorene unit and it can improve the morphological stability of the host material at high temperature. Fig. 1 shows the geometrical structure of the SPAN1 and SPAN11. The anthracene core structure was not affected by the spirobifluorene in the SPAN1, but it was affected by the spirobifluorene in the SPAN11. The spirobifluorene in the SPAN1 can be freely rotated, but the rotation of the spirobifluorene in the SPAN11 is limited by the steric hindrance of the spirobifluorene with the anthracence. This leads to the distortion of the anthracene core structure and the conjugation of the anthracene core can be destroyed by the spirobifluorene group in the SPAN11. The molecular simulation of the SPAN1 and SPAN11 was carried out to compare the electron distribution of the SPAN1 and SPAN11. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the SPAN1 and SPAN11 is shown in Fig. 2. HOMO and LUMO orbitals of the two host materials were concentrated in the anthracene core of the host materials and the substitution position of the spirobifluorene did not affect the HOMO and LUMO distribution of the host materials. UV–vis and PL spectra of the SPAN1 and SPAN11 are shown in Fig. 3. The UV–vis absorption spectra of the SPAN1 and SPAN 11
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S.E. Jang et al. / Synthetic Metals 160 (2010) 1184–1188
Scheme 1. Synthetic scheme of the SPAN1 and SPAN11.
were similar except for the absorption edge of the UV–vis spectra. The same anthracene core and spirobifluorene side group were used in the molecular structure and similar absorption spectra were obtained. The difference of the absorption edge in the two host materials is due to the steric hindrance of the spirobifluorene group in the SPAN11. As seen in the molecular structure of the SPAN11, the anthracene core structure can be distorted by the spirobifluorene substituted at 4-position, leading to the reduction of the degree of conjugation of the anthracene. The distortion of the conjugated structure of the anthracene increases the bandgap of the host material, leading to the blue shift of the absorption edge of the UV–vis
spectra. The PL spectrum of the SPAN11 was also blue shifted due to the reduced conjugation length of the anthracene in the SPAN11. In particular, the solid PL spectrum of the SPAN11 was very narrow compared with that of the SPAN1. The SPAN11 is difficult to be crystallized due to the steric hindrance of the 4-spirobifluorene, while the SPAN1 can be easily crystallized compared with SPAN11. Therefore, excimer and closely packed molecular structure formation is suppressed in the SPAN11, resulting in narrow solid PL emission. The steric hindrance of the SPAN11 shifted the emission peak to short wavelength and reduced the full width at half maximum.
Fig. 1. Optimized geometrical structure of the SPAN1 and SPAN11.
S.E. Jang et al. / Synthetic Metals 160 (2010) 1184–1188
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Fig. 2. Molecular orbital distribution of the SPAN1 and SPAN11.
Fig. 3. Ultraviolet–visible and photoluminescence spectra of the SPAN1 and SPAN11.
The use of the spirobifluorene side group is advantageous for the morphological stability of the host material at high temperature, and the morphological stability of the vacuum deposited host film was investigated. The glass transition temperature of the SPAN1 and SPAN11 was over 170 ◦ C, indicating that the SPAN1 and SPAN11 can be morphologically stable at high temperature. Fig. 4 shows the AFM images of the evaporated SPAN1 and SPAN11 films. Thermal annealing of the evaporated film was carried out at 100 ◦ C for 10 min and the AFM data of the films before and after thermal annealing were compared. The SPAN1 and SPAN11 showed a surface roughness lower than 0.5 nm, indicating that the SPAN1 and SPAN11 form a stable morphology. The bulky and twisted structure of the spirobifluorene prohibited the crystallization of the SPAN1 and SPAN11, stabilizing the morphology of the host materials. The amorphous morphology of the SPAN11 was maintained even after thermal treatment at 100 ◦ C and an average surface roughness of less than 0.5 nm was obtained. However, the surface roughness of the SPAN1 was increased over 1 nm after thermal annealing. This result indicates that the SPAN11 is better than the SPAN1 in terms
Fig. 4. Atomic force microscopic images of the SPAN1 and SPAN11 before and after thermal annealing at 100 ◦ C for 10 min. Average surface roughness of the film is also shown.
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Fig. 5. Current density–voltage–luminance curves of the SPAN1 and SPAN11 devices.
Fig. 7. Electroluminescence spectra of the SPAN1 and SPAN11 devices.
device. Although holes and electrons are balanced at low luminance, more electron injection at high driving voltage disrupts the holes and electrons balance in the emitting layer due to the limited hole injection, leading to the low quantum efficiency at high luminance. Electroluminescence spectra of the blue OLEDs are shown in Fig. 7. The SPAN1 and SPAN11 devices showed the deep blue emission of the BD-1 dopant without the emission of the host material, implying an effective energy transfer from the SPAN1 and SPAN11 to the BD-1 dopant. 4. Conclusions
Fig. 6. Quantum efficiency–luminance curves of the SPAN1 and SPAN11 devices.
of thermal stability due to the suppressed crystallization by the 4-spirobifluorene. The SPAN1 and SPAN11 were evaluated as the host materials in the blue fluorescent OLEDs. The BD-1 was doped as a deep blue dopant in the SPAN1 and SPAN11 host materials and device performances of the blue OLEDs were studied. Fig. 5 shows the current density–voltage–luminance curves of the blue OLEDs with the SPAN1 and SPAN11 host materials. The device performances of the blue OLEDs were optimized at a doping concentration of 7% and the device performances of the SPAN1 and SPAN11 with a doping concentration of 7% were compared. The SPAN1 showed higher current density and luminance than the SPAN11 at the same driving voltage. The decrease of the current density and luminance in the SPAN11 device is originated from the wide bandgap of the SPAN11. The HOMO level of the SPAN11 was 6.11 eV compared with 6.05 eV of the SPAN1, while the LUMO level of the SPAN11 was the same as that of the SPAN1. The deep HOMO level of the SPAN11 hinders the hole injection from the NPB hole transport layer to the emitting layer due to the large energy barrier for hole injection, leading to low current density and low luminance. The quantum efficiency of the SPAN1 and SPAN11 devices is shown in Fig. 6. The maximum quantum efficiency of the SPAN1 and SPAN11 devices was similar, but the quantum efficiency decrease at high luminance was significant in the SPAN11 device. The decrease of the quantum efficiency at high luminance is related with the high-energy barrier for hole injection in the SPAN11
In conclusion, the substitution of the spirobifluorene from 4position to the anthracene core distorted the conjugated structure of the anthracene core and improved the thermal stability of the host materials. In addition, the bandgap of the anthracene based blue host materials was also widened. Therefore, the substitution position was critical to the device performances and this approach can be useful in the design of host and dopant materials to manage the bandgap and device performances. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Adv. Funct. Mater. 19 (2009) 3644. J. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2000) 3201. C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett. 67 (1995) 3853. S. Wang, W.J. Oldham Jr., R.A. Hudack Jr., G.C. Bazan, J. Am. Chem. Soc. 122 (2000) 5695. A. Saitoh, N. Yamada, M. Yashima, K. Okinaka, A. Senoo, K. Ueno, D. Tanaka, R. Yashiro, SID Dig. (2004) 150. C.C. Yeh, M.T. Lee, M.T.H.H. Chen, C.H. Chen, SID Dig. (2004) 788. R.C. Chiechi, R.J. Tseng, F. Marchioni, Y. Yang, F. Wudl, Adv. Mater. 18 (2006) 325. S.W. Wen, M.T. Lee, C.H. Chen, J. Display Technol. 1 (2005) 90. Y.H. Kim, H.C. Jeong, S.H. Kim, K. Yang, S.K. Kwon, Adv. Funct. Mater. 15 (2005) 1799. W.J. Shen, R. Dodda, C.C. Wu, F.L. Wu, T.H. Liu, H.H. Chen, C.H. Chen, C.F. Shu, Chem. Mater. 16 (2004) 930. M.T. Lee, H.H. Chen, C.H. Liao, C.H. Tsai, C.H. Chen, Appl. Phys. Lett. 85 (2004) 3301. K.S. Kim, Y.M. Jeon, J.W. Kim, C.W. Lee, M.S. Gong, Org. Electron. 9 (2008) 797. Y.-H. Kim, D.-C. Shin, S.-H. Kim, C.-H. Ko, H.-S. Yu, Y.-S. Chae, S.-K. Kwon, Adv. Mater. 13 (2001) 1690. S.E. Jang, C.W. Joo, S.O. Jeon, K.S. Yook, J.Y. Lee, Org. Electron. (2010) doi:10.1016/j.orgel.2010.03.005. Z.Q. Gao, B.X. Mi, C.H. Chen, K.W. Cheah, Y.K. Cheng, W.-S. Wen, Appl. Phys. Lett. 90 (2007) 123506.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Thermally stable fluorescent blue organic light-emitting diodes using spirobifluorene based anthracene host materials with different substitution position Sang Eok Jang a , Chul Woong Joo a , Kyoung Soo Yook a , Joon-Woo Kim b , Chil-Won Lee b , Jun Yeob Lee a,∗ a b
Department of Polymer Science and Engineering, Dankook University Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea OLED Team, Daejoo Electronic Materials, Siheung, Gyeonggi-do 429-848, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 February 2010 Accepted 8 March 2010 Available online 3 April 2010 Keywords: Blue organic light-emitting diode Spirobifluorene Anthracene High efficiency Thermal stability Bandgap
a b s t r a c t Thermally stable blue organic light-emitting diodes (OLEDs) were developed using anthracene based host materials with a spirobifluorene group. 4-Bromospirobifluorene and 2-bromospirobifluorene were attached to the anthracene core and the effect of the substitution position on the physical properties and device performances of the blue fluorescent OLEDs was investigated. The 4-spirobifluorene substitution was better than the 2-spirobifluorene substitution in terms of thermal stability and widened the bandgap of the anthracene based host material due to the geometrical structure of the material. However, the wide bandgap of the host material with 4-spirobifluorene had negative effect on the current density and efficiency of the blue devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have been developed for more than 20 years and there has been much progress in the device performances. A theoretical maximum internal quantum efficiency of 100% was already achieved in the red and green devices [1]. However, the efficiency of deep blue phosphorescent OLEDs is not good enough and the lifetime is also too short for commercialization [2]. Therefore, the blue fluorescent OLEDs have been actively studied in spite of low efficiency compared with blue phosphorescent devices. Many host and dopant materials for blue fluorescent devices were developed, which include diarylanthracene [3], di(styryl)arylene [4,5], fluorene [6], pyrene [7], and fluoranthene [8]. One of the most well known blue host material is 2-methyl9,10-di(2 -naphthyl)anthracene (MADN) with an anthracene core structure [9]. It showed good device performances as a host in the blue fluorescent OLEDs. Other than the MADN, many blue emitting materials with the anthracene core structure were developed and showed good device performances in the blue device [10–12]. The spirobifluorene is also a group which can be introduced in the molecular structure of the anthracene based blue emitting
∗ Corresponding author. Tel.: +82 31 8005 3585; fax: +82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.03.006
materials. A spirofluorenebenzofluorene was used as the substituent of the anthracene type blue material and high efficiency was reported [13]. A spirobifluorene attached anthracene host material was also reported to be effective as the host material in the fluorescent blue OLEDs [14]. The t-butyl substituted spirobifluorene unit was connected to the anthracene through the 2-position of the spirobifluorene. Other than the 2-spirobifluorene, no spirobifluorene based anthracene host material was synthesized due to the difficulty of the synthesis of 3-bromospirobifluorene and 4bromospirobifluorene. In this work, 4-bromospirobifluorene was synthesized and it was attached to the 1-naphthylanthracene. The effect of the substitution position of the spirobifluorene on the physical properties and device performances of the anthracene based host materials was investigated. 2. Experimental 2.1. Materials 9H-fluoren-9-one, n-butyllithium, triethyl borate, and tetrakis(triphenylphosphine)palladium(0) (Aldrich Chem. Co.) were used as received. 2,2 -Dibromobiphenyl (TCI Chem. Co.) and sodium hydrogen carbonate (Junsei Chem. Co.) were used without further purification. Tetrahydrofuran was distilled over sodium and calcium hydride
S.E. Jang et al. / Synthetic Metals 160 (2010) 1184–1188
2.2. Synthesis of 2-bromo-9,9 -spirobi[fluorene] (1) The 2-bromo-9,9 -spirobi[fluorene] and 4-bromo-9,9 -spirobi [fluorene] were synthesized according to the synthetic procedure reported earlier [15]. 2.3. Synthesis of 9-(2-naphthylanthracene)-10-boronic acid (3) Into a 250 mL, two-neck flask, was placed a 9-bromo-10(naphthalen-1-yl)anthracene (3.00 g, 7.82 mmol) in THF (60 mL). The reaction flask was cooled to −78 ◦ C and n-BuLi (10 M in hexane, 1.01 mL) was added dropwise slowly. The whole solution was stirred at this temperature for 1 h, followed by addition of triethyl borate (1.48 g, 16.7 mmol) under argon atmosphere. After stirring for another 12 h, the solution was warmed to room temperature slowly and the solution was then quenched with 2 N HCl. The mixture was extracted with ethyl acetate. The combined organic layers were dried over magnesium sulfate, filtered, and evaporated under reduced pressure. The resulting powdery product was purified by reprecipitation from ethyl acetate and n-hexane to give yellow powder. 2.4. Synthesis of 2-(10-(naphthalen-1-yl)anthracen-9-yl)-9,9 (SPAN1)
-spirobi[fluorene]
A solution of 2-bromo-9,9 -spirobi[fluorene] (1) (2.00 g, 5.059 mmol), 9-(2-naphthylanthracene)-10-boronic acid (3) (2.11 g, 6.070 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.29 g, 0.253 mmol) dissolved in THF (30 mL) was stirred in a two-necked flask under a argon atmosphere for 30 min. To the reaction mixture, potassium carbonate (8.29 g, 59.98 mmol) in distilled water (30 mL) was added dropwise over a period of 20 min. The resulting solution was refluxed overnight at 80 ◦ C. The reaction mixture was extracted with dichloromethane and the organic layer was separated. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography using n-hexane as the eluent to give a white crystalline solid. Yield 44%. Tg 180 ◦ C. 1 H NMR (200 MHz, CDCl3 ): ı 8.12–7.94 (m, 4H), 7.78–7.63 (m, 6H), 7.56–7.41 (m, 5H), 7.36–6.79 (m, 12H). MS (FAB) m/z 618 [(M+1)+]. 2.5. Synthesis of 4-(10-(naphthalen-1-yl)anthracen-9-yl)-9,9 (SPAN11)
-spirobi[fluorene]
A solution of 4-bromo-9,9 -spirobi[fluorene] (2) (2.29 g, 5.793 mmol), 9-(2-naphthylanthracene)-10-boronic acid (3) (2.22 g, 6.372 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.34 g, 0.289 mmol) dissolved in THF (15 mL) was stirred in a two-necked flask under a argon atmosphere for 30 min. To the reaction mixture, potassium carbonate (16.58 g, 119.6 mmol) in distilled water (60 mL) was added dropwise over a period of 20 min. The resulting solution was refluxed overnight at 80 ◦ C. The reaction mixture was extracted with dichloromethane and the organic layer was separated. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromatography using n-hexane as the eluent to give a white crystalline solid. Yield 84%. Mp 367 ◦ C. Tg 173 ◦ C. 1 H NMR (200 MHz, CDCl3 ): ı 8.14–8.04 (m, 2H), 7.93–7.52 (m, 7H), 7.47–7.18 (m, 12H), 6.96–5.29 (m, 9H). MS (FAB) m/z 618 [(M+1)+]. 2.6. Device fabrication and measurements The device configuration used in this work was indium tin oxide (150 nm)/N,N -diphenyl-N,N-bis-[4-(phenyl-m-tolyl-amino)-phe-
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nyl]-biphenyl-4,4 -diamine (60 nm)/N, N -di(1-naphthyl)-N,N diphenylbenzidine (NPB, 30 nm)/host: diphenyl-[4-(2-[1,1 ;4 ,1 ] terphenyl-4-yl-vinyl)-phenyl]-amine(BD-1) (30 nm, x% doping)/ tris(8-hydroxyquinoline) aluminium (Alq3 , 20 nm)/LiF (1 nm)/Al (200 nm). Two host materials, SPAN1 and SPAN11, were used as the host materials in the emitting layer. The doping concentration of the BD-1 dopant was 7% [16]. Organic materials were deposited at a deposition rate of 0.1 nm/s and the deposition rate of the dopant was controlled according to the doping concentration. The blue devices were encapsulated with a glass lid and a CaO getter after device fabrication. The 1 H nuclear magnetic resonance spectrometer (NMR, Varian 200) and the low and high resolution mass spectrometer (JEOL, JMSAX505WA) were used to identify the synthesized compounds. The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (HITACHI, F-7000) and the ultraviolet–visible (UV–vis) spectra were obtained by means of a UV–vis spectrophotometer (Shimadzu, UV-2501PC). The differential scanning calorimeter (DSC) measurements were performed on a Mettler DSC 822e under nitrogen at a heating rate of 10 ◦ C/min. The energy levels were measured with a cyclic voltammetry. Current density–voltage–luminance relationship of the blue OLEDs were measured using Keithley 2400 source measurement unit and CS1000 spectroradiometer. Voltage sweep of the blue devices was carried out to obtain the device performances.
3. Results and discussion The 4-bromospirobifluorene was difficult to be prepared by the bromination of the spirobifluorene as the spirobifluorene is selectively brominated at 2-position of the spirobifluorene. Therefore, an alternate synthetic route which utilizes the reaction of the 2,2 -dibromobiphenyl with 9H-fluoren-9-one was developed and a synthetic yield of 50% was obtained [15]. The 4bromospirobifluorene and 2-bromospirobifluorene were attached to the 9-(2-naphthylanthracene)-10-boronic acid to synthesize blue host materials with the anthracene core and spirobifluorene side group. A highly pure blue fluorescent host material with a purity over 99% could be obtained after purification by column chromatography. The synthetic scheme of the SPAN1 and SPAN11 is shown in Scheme 1. The spirobifluorene is a rigid moiety with amorphous nature due to the twisted structure of the fluorene unit and it can improve the morphological stability of the host material at high temperature. Fig. 1 shows the geometrical structure of the SPAN1 and SPAN11. The anthracene core structure was not affected by the spirobifluorene in the SPAN1, but it was affected by the spirobifluorene in the SPAN11. The spirobifluorene in the SPAN1 can be freely rotated, but the rotation of the spirobifluorene in the SPAN11 is limited by the steric hindrance of the spirobifluorene with the anthracence. This leads to the distortion of the anthracene core structure and the conjugation of the anthracene core can be destroyed by the spirobifluorene group in the SPAN11. The molecular simulation of the SPAN1 and SPAN11 was carried out to compare the electron distribution of the SPAN1 and SPAN11. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the SPAN1 and SPAN11 is shown in Fig. 2. HOMO and LUMO orbitals of the two host materials were concentrated in the anthracene core of the host materials and the substitution position of the spirobifluorene did not affect the HOMO and LUMO distribution of the host materials. UV–vis and PL spectra of the SPAN1 and SPAN11 are shown in Fig. 3. The UV–vis absorption spectra of the SPAN1 and SPAN 11
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Scheme 1. Synthetic scheme of the SPAN1 and SPAN11.
were similar except for the absorption edge of the UV–vis spectra. The same anthracene core and spirobifluorene side group were used in the molecular structure and similar absorption spectra were obtained. The difference of the absorption edge in the two host materials is due to the steric hindrance of the spirobifluorene group in the SPAN11. As seen in the molecular structure of the SPAN11, the anthracene core structure can be distorted by the spirobifluorene substituted at 4-position, leading to the reduction of the degree of conjugation of the anthracene. The distortion of the conjugated structure of the anthracene increases the bandgap of the host material, leading to the blue shift of the absorption edge of the UV–vis
spectra. The PL spectrum of the SPAN11 was also blue shifted due to the reduced conjugation length of the anthracene in the SPAN11. In particular, the solid PL spectrum of the SPAN11 was very narrow compared with that of the SPAN1. The SPAN11 is difficult to be crystallized due to the steric hindrance of the 4-spirobifluorene, while the SPAN1 can be easily crystallized compared with SPAN11. Therefore, excimer and closely packed molecular structure formation is suppressed in the SPAN11, resulting in narrow solid PL emission. The steric hindrance of the SPAN11 shifted the emission peak to short wavelength and reduced the full width at half maximum.
Fig. 1. Optimized geometrical structure of the SPAN1 and SPAN11.
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Fig. 2. Molecular orbital distribution of the SPAN1 and SPAN11.
Fig. 3. Ultraviolet–visible and photoluminescence spectra of the SPAN1 and SPAN11.
The use of the spirobifluorene side group is advantageous for the morphological stability of the host material at high temperature, and the morphological stability of the vacuum deposited host film was investigated. The glass transition temperature of the SPAN1 and SPAN11 was over 170 ◦ C, indicating that the SPAN1 and SPAN11 can be morphologically stable at high temperature. Fig. 4 shows the AFM images of the evaporated SPAN1 and SPAN11 films. Thermal annealing of the evaporated film was carried out at 100 ◦ C for 10 min and the AFM data of the films before and after thermal annealing were compared. The SPAN1 and SPAN11 showed a surface roughness lower than 0.5 nm, indicating that the SPAN1 and SPAN11 form a stable morphology. The bulky and twisted structure of the spirobifluorene prohibited the crystallization of the SPAN1 and SPAN11, stabilizing the morphology of the host materials. The amorphous morphology of the SPAN11 was maintained even after thermal treatment at 100 ◦ C and an average surface roughness of less than 0.5 nm was obtained. However, the surface roughness of the SPAN1 was increased over 1 nm after thermal annealing. This result indicates that the SPAN11 is better than the SPAN1 in terms
Fig. 4. Atomic force microscopic images of the SPAN1 and SPAN11 before and after thermal annealing at 100 ◦ C for 10 min. Average surface roughness of the film is also shown.
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Fig. 5. Current density–voltage–luminance curves of the SPAN1 and SPAN11 devices.
Fig. 7. Electroluminescence spectra of the SPAN1 and SPAN11 devices.
device. Although holes and electrons are balanced at low luminance, more electron injection at high driving voltage disrupts the holes and electrons balance in the emitting layer due to the limited hole injection, leading to the low quantum efficiency at high luminance. Electroluminescence spectra of the blue OLEDs are shown in Fig. 7. The SPAN1 and SPAN11 devices showed the deep blue emission of the BD-1 dopant without the emission of the host material, implying an effective energy transfer from the SPAN1 and SPAN11 to the BD-1 dopant. 4. Conclusions
Fig. 6. Quantum efficiency–luminance curves of the SPAN1 and SPAN11 devices.
of thermal stability due to the suppressed crystallization by the 4-spirobifluorene. The SPAN1 and SPAN11 were evaluated as the host materials in the blue fluorescent OLEDs. The BD-1 was doped as a deep blue dopant in the SPAN1 and SPAN11 host materials and device performances of the blue OLEDs were studied. Fig. 5 shows the current density–voltage–luminance curves of the blue OLEDs with the SPAN1 and SPAN11 host materials. The device performances of the blue OLEDs were optimized at a doping concentration of 7% and the device performances of the SPAN1 and SPAN11 with a doping concentration of 7% were compared. The SPAN1 showed higher current density and luminance than the SPAN11 at the same driving voltage. The decrease of the current density and luminance in the SPAN11 device is originated from the wide bandgap of the SPAN11. The HOMO level of the SPAN11 was 6.11 eV compared with 6.05 eV of the SPAN1, while the LUMO level of the SPAN11 was the same as that of the SPAN1. The deep HOMO level of the SPAN11 hinders the hole injection from the NPB hole transport layer to the emitting layer due to the large energy barrier for hole injection, leading to low current density and low luminance. The quantum efficiency of the SPAN1 and SPAN11 devices is shown in Fig. 6. The maximum quantum efficiency of the SPAN1 and SPAN11 devices was similar, but the quantum efficiency decrease at high luminance was significant in the SPAN11 device. The decrease of the quantum efficiency at high luminance is related with the high-energy barrier for hole injection in the SPAN11
In conclusion, the substitution of the spirobifluorene from 4position to the anthracene core distorted the conjugated structure of the anthracene core and improved the thermal stability of the host materials. In addition, the bandgap of the anthracene based blue host materials was also widened. Therefore, the substitution position was critical to the device performances and this approach can be useful in the design of host and dopant materials to manage the bandgap and device performances. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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