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Efficient red-emitting cyclometalated iridium(III) complex and applications of organic light-emitting diode
Inorganica Chimica Acta 362 (2009) 5017–5022
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Efficient red-emitting cyclometalated iridium(III) complex and applications of organic light-emitting diode Ta-Hsien Chuang a, Cheng-Hsien Yang b,*, Po-Ching Kao c a
School of Pharmacy, China Medical University, Taichung 404, Taiwan NanoPowder and Thin Film Technology Center, ITRI South, Tainan 709, Taiwan c Department of Applied Physics, National Chiayi University, Chiayi, 600, Taiwan b
a r t i c l e
i n f o
Article history: Received 26 May 2009 Received in revised form 27 July 2009 Accepted 3 August 2009 Available online 8 August 2009 Keywords: Iridium Red emitter Organic light-emitting diodes
a b s t r a c t Novel red phosphorescent emitter bis(2,3-diphenylquinolinato-N,C2’) iridium(acetylacetonate) [(23dpq)2Ir(acac)] has been synthesised and fully characterized. A highly efficient red organic light-emitting diode was fabricated by using (23dpq)2Ir(acac) as an emitter, in which (23dpq)2Ir(acac) was synthesised from a well-designed ligand-2,3-diphenylquinoline. Electroluminescent device with a configuration of ITO/ 2-TNATA/NPB/BAlq:(23dpq)2Ir(acac)/AlQ3/LiF/Al was fabricated. The device using (23dpq)2Ir(acac) as a dopant showed pure-red emission with 1931 CIE (Commission International de L’Eclairage) chromaticity coordinates x = 0.66, y = 0.34. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction As a promising technology for flat panel displays, organic lightemitting diodes (OLEDs) have attracted a great deal of attention since 1987 [1,2]. In modern OLED research, cyclometalated iridium(III) complexes represent one of the most studied class of compounds for use as an emitter [3–5]. The strong spin–orbit coupling, caused by the heavy metal atom, makes the intersystem cross from the singlet to the triplet excited states more efficiently. To date, the development of materials for red OLEDs has gone through several important evolutional stages, including Ir(btp)2(acac) [Bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate) iridium(III)], Ir(piq)3 [Tris(1-phenylisoquinolinato-N,C2’) iridium(III)] and so on [6,7]. However, the highly efficient pure-red-emitting complexes are still scarce because their luminescent quantum yields tend to be intrinsically low, which makes them unsuitable for practical usage. Recently, we have reported a series of bi-substituted phenylisoquinoline iridium(III) complexes and 3’-substituted (F, CH3, OCH3 and CF3) phenylisoquinoline iridium(III) complexes [8,9]. These promising results stimulated our interest in new types of iridium complexes, and in this paper we report a new strategy for the synthesis of ligand, 2,3-diphenylquinoline (23dpq), bearing one novel iridium(III) complex with the synthesis and photo-physical/electrochemical characterization given. A high performance red OLED
* Corresponding author. Tel.: +886 6384 7289; fax: +886 6384 7439. E-mail address: [email protected] (C.-H. Yang). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.08.002
is achieved for the new red emitter, and this verifies our successful molecular design.
2. Experimental 1 H NMR and 13C NMR spectra were measured in CD2Cl2 solution on Bruker Avance-300 (300 MHz) or AMX-400 (400 MHz) NMR spectrometers with tetramethylsilane (TMS) as the internal standard. The EI-Mass spectra were recorded on a Bruker APEX II. The UV–Vis spectra were measured in CH2Cl2 solution on an Agilent 8453 spectrophotometer, and the photoluminescence spectra were recorded in CH2Cl2 solution with a HITACHI model F-2500 fluorescence spectrophotometer. HRMS spectra were obtained using a MAT-95XL high-resolution mass spectrometer. Elemental analyses have been carried out by using an Elementar vario EL III analyzer. 2,3-Diphenylquinoline was synthesised by using the process reported by Narasimhan et al. [10] and Shi et al. [11], respectively. Cyclometalated Ir(III) l-chloro-bridged dimers were synthesised by the method reported by Lamansky et al. [12]. The ligand-23dpq can be efficiently synthesised in six steps from a known starting material, 2-carboxybenzaldehyde (1) (as shown in Scheme 1). After the Wittig reaction, 2-styrylbenzoic acid (3) was readily obtained in a good yield. The 2-styrylbenzoyl azide (5) was obtained from 2-styrylbenzoic acid with oxalyl chloride followed by the addition of sodium azide. Without further purification, 3-phenyl-1H-quinolin-2-one (6) could be synthesised by a one-pot Curtius rearrangement and cyclization of the 2-styrylbenzoyl azide (5) under refluxing o-dichlorobenzene. According
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Scheme 1. Strategy for the synthesis of 2,3-diphenylquinoline.
to the procedures in the literature [13], quinolinone (6) reacted easily with phosphorus oxychloride and transformed into 2chloro-3-phenylquinoline (7). Via a crosscoupling reaction, 2,3diphenylquinoline (8) was synthesised from 2-chloro3-phenylquinoline and phenyl Grignard reagent.
s). 13C NMR (CDCl3, 75M Hz) d 127.1, 127.2, 127.4, 128.2, 129.5, 130.2, 134.6, 137.4, 138.7, 146.7, 149.4. HREIMS Calc. for C17H15N, 239.0502, found 239.0500.
2.1. 2-Styrylbenzoic acid (3)
Yield: 97%. 1H NMR (CDCl3, 300 MHz) d 7.22 (8H, m), 7.45 (2H, m), 7.50 (1H, t, J = 7.7 Hz), 7.68 (1H, 5, J = 7.7 Hz), 7.79 (1H, d, J = 8.3 Hz), 8.11 (1H, s), 8.20 (1H, d, J = 8.3 Hz). 13C NMR (CDCl3, 75 MHz) d 126.6, 127.0, 127.1, 127.3, 127.8, 127.9, 128.1, 129.3, 129.5, 129.6, 129.9, 134.4, 137.5, 139.8, 140.3, 147.1, 158.3. HREIMS Calc. for C21H15N, 281.1204, found 281.1208. The iridium complex was prepared from the bi-substituted 2,3diphenylquinoline ligands and iridium trichloride hydrate to form a dimer, [(23dpq)2Ir(l-Cl)2Ir(23dpq)2], followed by the reaction with acetylacetone in the presence of sodium carbonate [12] (as shown in Scheme 2). All procedures involving Ir(III) species were carried out under a nitrogen gas atmosphere. All intermediates and the final product were characterized unambiguously by 1H NMR, 13C NMR spectroscopy and mass spectroscopy.
Yield: 78%. 1H NMR (CDCl3, 300 MHz) d 6.94 (1H, d, J = 16.2 Hz), 7.19 (1H, t, J = 7.3 Hz), 7.28 (3H, m), 7.48 (3H, m), 7.65 (1H, d, J = 7.8 Hz), 7.99 (1H, d, J = 16.2 Hz), 8.01 (1H, d, J = 7.8 Hz). 13C NMR (CDCl3, 75M Hz) d 126.9, 127.2, 127.3, 127.5, 127.9, 128.7, 131.7, 131.8, 133.1, 137.3, 140.2, 173.1. Anal. Calc. for C15H12O2: C, 80.34; H, 5.39. Found: C, 80.33; H, 5.41%. 2.2. 2-Styrylbenzoyl azide (5) Yield: 99%. 1H NMR (acetone-d6, 300 MHz) d 6.99 (1H, d, J = 16.3 Hz), 7.25 (4H, m), 7.48 (3H, m), 7.64 (1H, d, J = 7.9 Hz), 7.81 (1H, d, J = 7.9 Hz), 8.03 (1H, d, J = 16.3 Hz). 13C NMR (acetone-d6, 75 MHz) d 127.4, 127.5, 127.8, 127.9, 128.6, 128.7, 129.3, 132.6, 134.2, 138.0, 140.4, 173.4. HREIMS calc. for C15H11N3O, 249.0902, found 249.0901. 2.3. 3-Phenyl-1H-quinolin-2-one (6) Yield: 97%. 1H NMR (CDCl3, 300 MHz) d 7.22 (1H, t, J = 7.3 Hz), 7.44 (5H, m), 7.61 (1H, d, J = 7.8 Hz), 7.82 (2H, d, J = 7.3 Hz), 7.92 (1H, s), 11.70 (1H, brs). 13C NMR (CDCl3, 75 MHz) d 115.6, 120.3, 122.7, 127.8, 128.2, 128.3, 128.9, 130.3, 132.4, 136.2, 138.0, 138.5, 163.1. Anal. Calc. for C15H11NO: C, 81.43; H, 5.01; N, 6.33. Found: C, 81.44; H, 5.05; N, 6.34%. 2.4. 2-Chloro-3-phenylquinoline (7) Yield: 99%. 1H NMR (CDCl3, 300 MHz) d 7.49 (6H, m), 7.70 (1H, t, J = 7.9 Hz), 7.78 (1H, d, J = 7.9 Hz), 8.06 (1H, d, J = 7.9 Hz), 7.07 (1H,
2.5. 2,3-Diphenylquinoline (8)
2.6. Bis(2,3-diphenylquinolinato-N,C2’) iridium(acetylacetonate) [(23dpq)2Ir(acac)] Yield: 45%. 1H NMR (CD2Cl2, 500 MHz) d 1.61 (6H, s), 4.68 (1H, s), 6.48 (2H, t, J = 7.0 Hz), 6.55 (4H, m), 6.92 (2H, d, J = 8.0 Hz), 7.50 (8H, m), 7.57 (2H, m), 7.78 (4H, m), 7.84 (2H, m), 8.08 (2H, s), 8.36 (2H, m). 13C NMR (CD2Cl2, 125 MHz) d 18.1, 89.3, 110.2, 116.0, 116.4, 116.7, 117.6, 117.8, 118.3, 120.3, 120.4, 124.1, 126.9, 129.6, 131.1, 138.3, 138.7, 143.0, 159.5, 175.7. EIMS m/z 852, [M+]. HREIMS Calc. for C47H35IrN2O2 852.2328, found 852.2322. Anal. Calc. for C47H35IrN2O2: C, 66.26; H, 4.14; N, 3.29. Found: C, 66.33; H, 4.10; N, 3.40%. Devices were fabricated by high-vacuum (106 torr) thermal evaporation on precleaned indium–tin oxide (ITO) glass substrates. The electroluminescent devices configurations were ITO/4, triphenylamine 40 ,400 -Tris(N-(naphthylen-2-yl)-N-phenylamine)
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Scheme 2. Synthesis of iridium complex-(23dpq)2Ir(acac).
(2TNATA) (5 nm)/N,N0 -Bis-(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) (40 nm)/Bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum (III) (BAlq):(23dpq)2Ir(acac) (60 nm, 5%)/ Tris(8-hydroxyquinoline) aluminum(III) (Alq3) (30 nm)/lithium fluoride (LiF) (1 nm)/Al (100 nm). In the devices, 2TNATA was used as the hole injection layer, NPB as the hole-transport layer, Alq3 as the electron-transport layer, BAlq as the host material, and LiF/Al layers served as the composite cathode to enhance the electron injection. 3. Results and discussion The crystallographic data for the structure reported here have been deposited in the Cambridge Database: CCDC #700734. The
single crystal structure of the iridium complex is represented with an ORTEP diagram in Fig. 1. It was obtained from solutions of dichlomethane-hexane (1:1). The monoclinic space group of complex (23dpq)2Ir(acac) is C2/c, and the structure reveals a distorted octahedral geometry around iridium, consisting of two cyclometalated quinoline ligands and one acac ligand. Due to the steric interactions, the two phenyl groups are not coplanar with the quinoline group, especially for the 3-phenyl group. Xray data show that the dihedral angles between the two planes (quinoline and 2-phenyl group) for (23dpq)2Ir(acac) are 9.8° and 11.3°, and another two planes (quinoline and 3-phenyl group) are 40.3° and 46.9°. The angles are much larger (9.8° and 11.3°) than that of non-substituted in 3-position complex (dpq)2Ir(acac) (1.5° and 3.4°) [14].
Fig. 1. ORTEP diagram of (23dpq)2Ir(acac) with the thermal ellipsoids at the 50% probability limit.
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Fig. 2. Absorption and photoluminescence spectra of iridium complexes in CH2Cl2.
The absorption and photoluminescence spectra of (23dpq)2Ir(acac) in CH2Cl2 solutions are depicted in Fig. 2. Highly intensive luminescence was observed for the iridium(III) complex in CH2Cl2, with kmax at 608 nm. With regard to (pq)2Ir(acac) [5], which emits orange light at 589 nm, it is clear that the incorporation of the 3phenyl group leads to a 19 nm bathochromic shift. The 3-phenyl group increases the probability of p–p conjugation of the cyclometalated ligand and lowers the triplet energy level, thus lowering the lowest unoccupied molecular orbital (LUMO) level. However, the PL emission data of (23dpq)2Ir(acac) agrees fairly well with the previously reported material, Ir(4-Me-2,3-dpq)2(acac), which shows an emission band at 604 nm in CH2Cl2 [15]. The strong absorption bands in the ultraviolet region at about 280–340 nm with distinct vibronic features are assigned to the spin-allowed intraligand 1p–p* transitions. Due to the perturbation from iridium
metal, these transitions have shifted with respect to that of the free ligand. The next lower energy in the visible region, weak absorption bands at about 450 nm, can be ascribed to the typical spin-allowed metal to ligand charge-transfer (1MLCT) transition. On the other hand, the weak shoulder extending into the visible region is believed to be associated with both spin–orbit coupling enhanced 3p–p* and 3MLCT (spin-forbidden metal to ligand chargetransfer) transitions. As shown in Fig. 3, cyclic voltammetry was conducted at a Pt disk electrode (BAS Co.) in a CH2Cl2 solution containing 0.001 M of the iridium(III) complex and 0.1 M tetra-n-butylammonium perchlorate as the supporting electrolyte. The data shows the complex undergoes a reversible one-electron oxidation with no reduction process observed within the solvent cathodic potential limit. Based on the literature [16,17], the HOMO value of (23dpq)2Ir(acac) is
Fig. 3. The cyclic voltammogram of (23dpq)2Ir(acac) at a Pt electrode. Scan rate: 50 mV/s, T = 25 °C.
T.-H. Chuang et al. / Inorganica Chimica Acta 362 (2009) 5017–5022
5021
Fig. 4. (a) Electroluminescence spectrum and (b) current density–luminance–voltage characteristics of OLEDs.
5.14 eV. From spectroscopy of the absorption edge and the theory presented in Burrows’s report [18], the energy band gap and the LUMO value of (23dpq)2Ir(acac) are 2.0 and 3.14 eV, respectively, which is in agreement with the iridium(III) complexes that were used for organic light-emitting diodes. Fig. 4a shows the EL spectra of the device at different applied voltages for the red emission device with 5% (23dpq)2Ir(acac) doped in BAlq. As can be seen, red emission at around 620 nm with a shoulder at about 668 nm is attributed to the phosphorescence of (23dpq)2Ir(acac). As compared to the PL spectra, the EL maximum is consistently red-shifted by about 16 nm. Two effects may be responsible for the EL spectra. One possible effect is the polarization effect caused by the electric field [19], and the other may be the dependence of the microcavity-induced wave-guiding effects on the thickness of organic films [20], which both effects leading to a red-shift of the spectra. However, the resemblance between the PL and EL spectra indicates that the origin of EL emission from the triplet excited states was confirmed. In addition, there was a blue emission centered at about 490 nm when the driving voltage was high; this peak originated from the host BAlq. As the driving voltage increases, more carriers can be injected and recombine in the doped BAlq layer. The saturation of the red emission will occur because of the low doping concentration, leading to the increase in the blue emission relative to the dopant emission. However, it is obvious that the emission spectra is changed a little at different driving voltages and the CIE coordinates are well within the red region, changing slightly from (0.66, 0.34) at 9 V to (0.65, 0.34) at 18 V, which is very close to the National Television System Committee (NTSC) standard red point (0.66, 0.33). Fig. 4b shows the
current density-luminance-voltage characteristics of OLEDs. A turn-on voltage of 6 V, maximum luminance of 22040 cd/m2 at 18 V, power efficiency of 2.4 lm/W, and current efficiency of 11.4 cd/A at 15 V were achieved, demonstrating that (23dpq)2Ir(acac) is an excellent dopant for OLEDs. 4. Conclusions In summary, we have developed a new strategy for the synthesis of substituted quinoline. The iridium(III) complex based on 2,3diphenylquinoline ligand exhibits excellent photo-physical properties. This would facilitate the designing of new ligands for lightemitting iridium complexes. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.08.002. References [1] [2] [3] [4]
C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. K.A. King, P.J. Spellane, R.J. Watts, J. Am. Chem. Soc. 107 (1985) 1431. M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [5] J.I. Kim, I.S. Shin, H. Kim, J.K. Lee, J. Am. Chem. Soc. 127 (2005) 1614. [6] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971. [7] C.H. Yang, C.C. Tai, I.W. Sun, J. Mater. Chem. 14 (2004) 947.
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[8] Y.T. Huang, T.H. Chuang, Y.L. Shu, Y.C. Kuo, P.L. Wu, C.H. Yang, I.W. Sun, Organometallics 24 (2005) 6230. [9] C.H. Yang, W.L. Su, K.H. Fang, S.P. Wang, I.W. Sun, Organometallics 25 (2006) 4514. [10] N.S. Narasimhan, R.S. Mali, B.K. Kulkarni, Indian J. Chem. 228 (1983) 850. [11] M. Shi, B. Xu, J. Org. Chem. 67 (2002) 294. [12] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. [13] S.C. Tucker, J.M. Brown, J. Oakes, D. Thornthwaite, Tetrahedron 57 (2001) 2545. [14] F.I. Wu, H.J. Su, C.F. Shu, L. Luo, W.G. Diau, C.H. Cheng, J.P. Duan, G.H. Lee, J. Mater. Chem. 15 (2005) 1035.
[15] G.Y. Park, Y. Ha, Synth. Met. 158 (2008) 120. [16] H.M. Koepp, H. Wendt, H. Strehlow, Z. Electrochem. 64 (1960) 483. [17] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bassler, M. Proseh, J. Daub, Adv. Mater. 7 (1995) 551. [18] P.E. Burrows, Z. Shen, V. Bulovic, D.M. McCarty, S.R. Forrest, J. Appl. Phys. 79 (1996) 7991. [19] Y.S. Yao, Q.X. Zhou, X.S. Wang, Y. Wang, B.W. Zhang, J. Mater. Chem. 16 (2006) 3512. [20] V. Bulovi, V.B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R. Forrest, Phys. Rev. B 58 (1998) 3730.
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Efficient red-emitting cyclometalated iridium(III) complex and applications of organic light-emitting diode Ta-Hsien Chuang a, Cheng-Hsien Yang b,*, Po-Ching Kao c a
School of Pharmacy, China Medical University, Taichung 404, Taiwan NanoPowder and Thin Film Technology Center, ITRI South, Tainan 709, Taiwan c Department of Applied Physics, National Chiayi University, Chiayi, 600, Taiwan b
a r t i c l e
i n f o
Article history: Received 26 May 2009 Received in revised form 27 July 2009 Accepted 3 August 2009 Available online 8 August 2009 Keywords: Iridium Red emitter Organic light-emitting diodes
a b s t r a c t Novel red phosphorescent emitter bis(2,3-diphenylquinolinato-N,C2’) iridium(acetylacetonate) [(23dpq)2Ir(acac)] has been synthesised and fully characterized. A highly efficient red organic light-emitting diode was fabricated by using (23dpq)2Ir(acac) as an emitter, in which (23dpq)2Ir(acac) was synthesised from a well-designed ligand-2,3-diphenylquinoline. Electroluminescent device with a configuration of ITO/ 2-TNATA/NPB/BAlq:(23dpq)2Ir(acac)/AlQ3/LiF/Al was fabricated. The device using (23dpq)2Ir(acac) as a dopant showed pure-red emission with 1931 CIE (Commission International de L’Eclairage) chromaticity coordinates x = 0.66, y = 0.34. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction As a promising technology for flat panel displays, organic lightemitting diodes (OLEDs) have attracted a great deal of attention since 1987 [1,2]. In modern OLED research, cyclometalated iridium(III) complexes represent one of the most studied class of compounds for use as an emitter [3–5]. The strong spin–orbit coupling, caused by the heavy metal atom, makes the intersystem cross from the singlet to the triplet excited states more efficiently. To date, the development of materials for red OLEDs has gone through several important evolutional stages, including Ir(btp)2(acac) [Bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate) iridium(III)], Ir(piq)3 [Tris(1-phenylisoquinolinato-N,C2’) iridium(III)] and so on [6,7]. However, the highly efficient pure-red-emitting complexes are still scarce because their luminescent quantum yields tend to be intrinsically low, which makes them unsuitable for practical usage. Recently, we have reported a series of bi-substituted phenylisoquinoline iridium(III) complexes and 3’-substituted (F, CH3, OCH3 and CF3) phenylisoquinoline iridium(III) complexes [8,9]. These promising results stimulated our interest in new types of iridium complexes, and in this paper we report a new strategy for the synthesis of ligand, 2,3-diphenylquinoline (23dpq), bearing one novel iridium(III) complex with the synthesis and photo-physical/electrochemical characterization given. A high performance red OLED
* Corresponding author. Tel.: +886 6384 7289; fax: +886 6384 7439. E-mail address: [email protected] (C.-H. Yang). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.08.002
is achieved for the new red emitter, and this verifies our successful molecular design.
2. Experimental 1 H NMR and 13C NMR spectra were measured in CD2Cl2 solution on Bruker Avance-300 (300 MHz) or AMX-400 (400 MHz) NMR spectrometers with tetramethylsilane (TMS) as the internal standard. The EI-Mass spectra were recorded on a Bruker APEX II. The UV–Vis spectra were measured in CH2Cl2 solution on an Agilent 8453 spectrophotometer, and the photoluminescence spectra were recorded in CH2Cl2 solution with a HITACHI model F-2500 fluorescence spectrophotometer. HRMS spectra were obtained using a MAT-95XL high-resolution mass spectrometer. Elemental analyses have been carried out by using an Elementar vario EL III analyzer. 2,3-Diphenylquinoline was synthesised by using the process reported by Narasimhan et al. [10] and Shi et al. [11], respectively. Cyclometalated Ir(III) l-chloro-bridged dimers were synthesised by the method reported by Lamansky et al. [12]. The ligand-23dpq can be efficiently synthesised in six steps from a known starting material, 2-carboxybenzaldehyde (1) (as shown in Scheme 1). After the Wittig reaction, 2-styrylbenzoic acid (3) was readily obtained in a good yield. The 2-styrylbenzoyl azide (5) was obtained from 2-styrylbenzoic acid with oxalyl chloride followed by the addition of sodium azide. Without further purification, 3-phenyl-1H-quinolin-2-one (6) could be synthesised by a one-pot Curtius rearrangement and cyclization of the 2-styrylbenzoyl azide (5) under refluxing o-dichlorobenzene. According
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Scheme 1. Strategy for the synthesis of 2,3-diphenylquinoline.
to the procedures in the literature [13], quinolinone (6) reacted easily with phosphorus oxychloride and transformed into 2chloro-3-phenylquinoline (7). Via a crosscoupling reaction, 2,3diphenylquinoline (8) was synthesised from 2-chloro3-phenylquinoline and phenyl Grignard reagent.
s). 13C NMR (CDCl3, 75M Hz) d 127.1, 127.2, 127.4, 128.2, 129.5, 130.2, 134.6, 137.4, 138.7, 146.7, 149.4. HREIMS Calc. for C17H15N, 239.0502, found 239.0500.
2.1. 2-Styrylbenzoic acid (3)
Yield: 97%. 1H NMR (CDCl3, 300 MHz) d 7.22 (8H, m), 7.45 (2H, m), 7.50 (1H, t, J = 7.7 Hz), 7.68 (1H, 5, J = 7.7 Hz), 7.79 (1H, d, J = 8.3 Hz), 8.11 (1H, s), 8.20 (1H, d, J = 8.3 Hz). 13C NMR (CDCl3, 75 MHz) d 126.6, 127.0, 127.1, 127.3, 127.8, 127.9, 128.1, 129.3, 129.5, 129.6, 129.9, 134.4, 137.5, 139.8, 140.3, 147.1, 158.3. HREIMS Calc. for C21H15N, 281.1204, found 281.1208. The iridium complex was prepared from the bi-substituted 2,3diphenylquinoline ligands and iridium trichloride hydrate to form a dimer, [(23dpq)2Ir(l-Cl)2Ir(23dpq)2], followed by the reaction with acetylacetone in the presence of sodium carbonate [12] (as shown in Scheme 2). All procedures involving Ir(III) species were carried out under a nitrogen gas atmosphere. All intermediates and the final product were characterized unambiguously by 1H NMR, 13C NMR spectroscopy and mass spectroscopy.
Yield: 78%. 1H NMR (CDCl3, 300 MHz) d 6.94 (1H, d, J = 16.2 Hz), 7.19 (1H, t, J = 7.3 Hz), 7.28 (3H, m), 7.48 (3H, m), 7.65 (1H, d, J = 7.8 Hz), 7.99 (1H, d, J = 16.2 Hz), 8.01 (1H, d, J = 7.8 Hz). 13C NMR (CDCl3, 75M Hz) d 126.9, 127.2, 127.3, 127.5, 127.9, 128.7, 131.7, 131.8, 133.1, 137.3, 140.2, 173.1. Anal. Calc. for C15H12O2: C, 80.34; H, 5.39. Found: C, 80.33; H, 5.41%. 2.2. 2-Styrylbenzoyl azide (5) Yield: 99%. 1H NMR (acetone-d6, 300 MHz) d 6.99 (1H, d, J = 16.3 Hz), 7.25 (4H, m), 7.48 (3H, m), 7.64 (1H, d, J = 7.9 Hz), 7.81 (1H, d, J = 7.9 Hz), 8.03 (1H, d, J = 16.3 Hz). 13C NMR (acetone-d6, 75 MHz) d 127.4, 127.5, 127.8, 127.9, 128.6, 128.7, 129.3, 132.6, 134.2, 138.0, 140.4, 173.4. HREIMS calc. for C15H11N3O, 249.0902, found 249.0901. 2.3. 3-Phenyl-1H-quinolin-2-one (6) Yield: 97%. 1H NMR (CDCl3, 300 MHz) d 7.22 (1H, t, J = 7.3 Hz), 7.44 (5H, m), 7.61 (1H, d, J = 7.8 Hz), 7.82 (2H, d, J = 7.3 Hz), 7.92 (1H, s), 11.70 (1H, brs). 13C NMR (CDCl3, 75 MHz) d 115.6, 120.3, 122.7, 127.8, 128.2, 128.3, 128.9, 130.3, 132.4, 136.2, 138.0, 138.5, 163.1. Anal. Calc. for C15H11NO: C, 81.43; H, 5.01; N, 6.33. Found: C, 81.44; H, 5.05; N, 6.34%. 2.4. 2-Chloro-3-phenylquinoline (7) Yield: 99%. 1H NMR (CDCl3, 300 MHz) d 7.49 (6H, m), 7.70 (1H, t, J = 7.9 Hz), 7.78 (1H, d, J = 7.9 Hz), 8.06 (1H, d, J = 7.9 Hz), 7.07 (1H,
2.5. 2,3-Diphenylquinoline (8)
2.6. Bis(2,3-diphenylquinolinato-N,C2’) iridium(acetylacetonate) [(23dpq)2Ir(acac)] Yield: 45%. 1H NMR (CD2Cl2, 500 MHz) d 1.61 (6H, s), 4.68 (1H, s), 6.48 (2H, t, J = 7.0 Hz), 6.55 (4H, m), 6.92 (2H, d, J = 8.0 Hz), 7.50 (8H, m), 7.57 (2H, m), 7.78 (4H, m), 7.84 (2H, m), 8.08 (2H, s), 8.36 (2H, m). 13C NMR (CD2Cl2, 125 MHz) d 18.1, 89.3, 110.2, 116.0, 116.4, 116.7, 117.6, 117.8, 118.3, 120.3, 120.4, 124.1, 126.9, 129.6, 131.1, 138.3, 138.7, 143.0, 159.5, 175.7. EIMS m/z 852, [M+]. HREIMS Calc. for C47H35IrN2O2 852.2328, found 852.2322. Anal. Calc. for C47H35IrN2O2: C, 66.26; H, 4.14; N, 3.29. Found: C, 66.33; H, 4.10; N, 3.40%. Devices were fabricated by high-vacuum (106 torr) thermal evaporation on precleaned indium–tin oxide (ITO) glass substrates. The electroluminescent devices configurations were ITO/4, triphenylamine 40 ,400 -Tris(N-(naphthylen-2-yl)-N-phenylamine)
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Scheme 2. Synthesis of iridium complex-(23dpq)2Ir(acac).
(2TNATA) (5 nm)/N,N0 -Bis-(naphthalen-1-yl)-N,N0 -bis(phenyl)benzidine (NPB) (40 nm)/Bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum (III) (BAlq):(23dpq)2Ir(acac) (60 nm, 5%)/ Tris(8-hydroxyquinoline) aluminum(III) (Alq3) (30 nm)/lithium fluoride (LiF) (1 nm)/Al (100 nm). In the devices, 2TNATA was used as the hole injection layer, NPB as the hole-transport layer, Alq3 as the electron-transport layer, BAlq as the host material, and LiF/Al layers served as the composite cathode to enhance the electron injection. 3. Results and discussion The crystallographic data for the structure reported here have been deposited in the Cambridge Database: CCDC #700734. The
single crystal structure of the iridium complex is represented with an ORTEP diagram in Fig. 1. It was obtained from solutions of dichlomethane-hexane (1:1). The monoclinic space group of complex (23dpq)2Ir(acac) is C2/c, and the structure reveals a distorted octahedral geometry around iridium, consisting of two cyclometalated quinoline ligands and one acac ligand. Due to the steric interactions, the two phenyl groups are not coplanar with the quinoline group, especially for the 3-phenyl group. Xray data show that the dihedral angles between the two planes (quinoline and 2-phenyl group) for (23dpq)2Ir(acac) are 9.8° and 11.3°, and another two planes (quinoline and 3-phenyl group) are 40.3° and 46.9°. The angles are much larger (9.8° and 11.3°) than that of non-substituted in 3-position complex (dpq)2Ir(acac) (1.5° and 3.4°) [14].
Fig. 1. ORTEP diagram of (23dpq)2Ir(acac) with the thermal ellipsoids at the 50% probability limit.
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Fig. 2. Absorption and photoluminescence spectra of iridium complexes in CH2Cl2.
The absorption and photoluminescence spectra of (23dpq)2Ir(acac) in CH2Cl2 solutions are depicted in Fig. 2. Highly intensive luminescence was observed for the iridium(III) complex in CH2Cl2, with kmax at 608 nm. With regard to (pq)2Ir(acac) [5], which emits orange light at 589 nm, it is clear that the incorporation of the 3phenyl group leads to a 19 nm bathochromic shift. The 3-phenyl group increases the probability of p–p conjugation of the cyclometalated ligand and lowers the triplet energy level, thus lowering the lowest unoccupied molecular orbital (LUMO) level. However, the PL emission data of (23dpq)2Ir(acac) agrees fairly well with the previously reported material, Ir(4-Me-2,3-dpq)2(acac), which shows an emission band at 604 nm in CH2Cl2 [15]. The strong absorption bands in the ultraviolet region at about 280–340 nm with distinct vibronic features are assigned to the spin-allowed intraligand 1p–p* transitions. Due to the perturbation from iridium
metal, these transitions have shifted with respect to that of the free ligand. The next lower energy in the visible region, weak absorption bands at about 450 nm, can be ascribed to the typical spin-allowed metal to ligand charge-transfer (1MLCT) transition. On the other hand, the weak shoulder extending into the visible region is believed to be associated with both spin–orbit coupling enhanced 3p–p* and 3MLCT (spin-forbidden metal to ligand chargetransfer) transitions. As shown in Fig. 3, cyclic voltammetry was conducted at a Pt disk electrode (BAS Co.) in a CH2Cl2 solution containing 0.001 M of the iridium(III) complex and 0.1 M tetra-n-butylammonium perchlorate as the supporting electrolyte. The data shows the complex undergoes a reversible one-electron oxidation with no reduction process observed within the solvent cathodic potential limit. Based on the literature [16,17], the HOMO value of (23dpq)2Ir(acac) is
Fig. 3. The cyclic voltammogram of (23dpq)2Ir(acac) at a Pt electrode. Scan rate: 50 mV/s, T = 25 °C.
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Fig. 4. (a) Electroluminescence spectrum and (b) current density–luminance–voltage characteristics of OLEDs.
5.14 eV. From spectroscopy of the absorption edge and the theory presented in Burrows’s report [18], the energy band gap and the LUMO value of (23dpq)2Ir(acac) are 2.0 and 3.14 eV, respectively, which is in agreement with the iridium(III) complexes that were used for organic light-emitting diodes. Fig. 4a shows the EL spectra of the device at different applied voltages for the red emission device with 5% (23dpq)2Ir(acac) doped in BAlq. As can be seen, red emission at around 620 nm with a shoulder at about 668 nm is attributed to the phosphorescence of (23dpq)2Ir(acac). As compared to the PL spectra, the EL maximum is consistently red-shifted by about 16 nm. Two effects may be responsible for the EL spectra. One possible effect is the polarization effect caused by the electric field [19], and the other may be the dependence of the microcavity-induced wave-guiding effects on the thickness of organic films [20], which both effects leading to a red-shift of the spectra. However, the resemblance between the PL and EL spectra indicates that the origin of EL emission from the triplet excited states was confirmed. In addition, there was a blue emission centered at about 490 nm when the driving voltage was high; this peak originated from the host BAlq. As the driving voltage increases, more carriers can be injected and recombine in the doped BAlq layer. The saturation of the red emission will occur because of the low doping concentration, leading to the increase in the blue emission relative to the dopant emission. However, it is obvious that the emission spectra is changed a little at different driving voltages and the CIE coordinates are well within the red region, changing slightly from (0.66, 0.34) at 9 V to (0.65, 0.34) at 18 V, which is very close to the National Television System Committee (NTSC) standard red point (0.66, 0.33). Fig. 4b shows the
current density-luminance-voltage characteristics of OLEDs. A turn-on voltage of 6 V, maximum luminance of 22040 cd/m2 at 18 V, power efficiency of 2.4 lm/W, and current efficiency of 11.4 cd/A at 15 V were achieved, demonstrating that (23dpq)2Ir(acac) is an excellent dopant for OLEDs. 4. Conclusions In summary, we have developed a new strategy for the synthesis of substituted quinoline. The iridium(III) complex based on 2,3diphenylquinoline ligand exhibits excellent photo-physical properties. This would facilitate the designing of new ligands for lightemitting iridium complexes. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.08.002. References [1] [2] [3] [4]
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