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Synthesis, structure, photophysical and electroluminescent properties of a blue-green self-host phosphorescent iridium(III) complex
Materials Chemistry and Physics 162 (2015) 392e399
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Synthesis, structure, photophysical and electroluminescent properties of a blue-green self-host phosphorescent iridium(III) complex Jing Sun a, Hua Wang a, Huixia Xu a, *, Jie Li a, Yuling Wu a, Xiaogang Du a, Bingshe Xu a, b, ** a b
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
h i g h l i g h t s A blue-green self-host phosphorescent iridium(III) complex was synthesized. The molecular structure, and photophysical properties were investigated. Electroluminescent performance in host-free devices were discussed. The maximum current efficiency 8.2 cd A1 and the maximum brightness 5420 cd m2 were achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2014 Received in revised form 29 April 2015 Accepted 1 June 2015 Available online 29 June 2015
A kind of blue-green self-host phosphorescent iridium(III) complex, (CzPhBI)2Ir(tfmptz) [CzPhBI ¼ 9-(6(2-phenyl-1-benzimidazolyl)hexyl)-9-carbazole; tfmptz ¼ 2-(5-trifluoromethyl-1,2,4-triazolyl)pyridine], was designed and synthesized. The synthesized iridium(III) complex was characterized by 1H NMR, 19F NMR, FT-IR, elemental analysis and X-ray single-crystal diffraction, respectively. Its thermal properties, optical properties and electrochemical properties were also investigated. The host-free organic electroluminescent devices with the configuration of ITO/MoO3 (3 nm)/NPB (30 nm)/TAPC (15 nm)/ (CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm) had been fabricated. The devices exhibited excellent performance indicating that (CzPhBI)2Ir(tfmptz) was a promising phosphorescent material. © 2015 Elsevier B.V. All rights reserved.
Keywords: Organometallic compounds Crystal structure Optical properties Vacuum deposition Luminescence
1. Introduction Iridium(III) complexes have attracted much attention because of their unique characteristics, particularly in harvesting both singlet and triplet exciton, leading to a potentially achievable 100% internal quantum efficiency [1], theoretically. However, the serious problem of concentration quenching at high current density limits its application. The iridium(III) complexes have to be utilized as guest in phosphorescent organic light emitting devices (PhOLEDs) [3]. Therefore, appropriate match of energy levels between host and
Abbreviations: NMR, nuclear magnetic resonance; FT-IR, Fourier transform infrared spectroscopy. * Corresponding author. ** Corresponding author. Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China. E-mail addresses: [email protected] (H. Xu), [email protected] (B. Xu). http://dx.doi.org/10.1016/j.matchemphys.2015.06.005 0254-0584/© 2015 Elsevier B.V. All rights reserved.
guest is necessary [4e7] in order to obtain efficient energy transfer and restrain energy transfer back from guest to host which would reduce the electroluminescent performance. Hence, developing a kind of high-effective self-host phosphorescent material is the most important for PhOLEDs [8]. Yue et al. [10] have reported a series of green iridium(III) complexes with hole-transporting carbazole group as its side chain that could act as single emitting layer. With the introduction of carbazole group, good electroluminescent performances have been achieved with a high luminance over 6000 cd m2. An ambipolar iridium(III) complex IrPBIC2O2 with electron-transporting 1,3,4oxadiazole (OXD) has also been developed [11], which has a maximum external quantum efficiency (E.Q.E.) of more than 4% and exhibits reduced efficiency “roll-off” to of less than 16% at 1000 cd m2. However, the preparation of self-host phosphorescent materials with excellent electroluminescent properties is still a challenge [12e16].
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
Therefore, a blue-green self-host phosphorescent complex of (CzPhBI)2Ir(tfmptz) [CzPhBI ¼ 9-(6-(2-phenyl-1-benzimidazolyl) hexyl)-9-carbazole; tfmptz ¼ 2-(5-trifluoromethyl-1,2,4-triazolyl) pyridine] has been designed and synthesized. The blue-green emitting benzimidazole iridium(III) complex acts as a luminescent center. The functional group of carbazole has been chosen due to the excellent hole-transporting property [17,18] and the flexible hexyl is used to prevent adjacent molecular aggregation. Meanwhile, the auxiliary ligand with 1,2,4-triazole group is selected based on its desirable electron-transporting property [19e21]. The structure, photophysical and electroluminescent properties of (CzPhBI)2Ir(tfmptz) have been investigated in detail. 2. Experimental section 2.1. General information All of the chemicals and reagents are used as purchased without further purification. Solvents for chemical synthesis are purified according to standard procedures. All chemical reactions were performed in a nitrogen atmosphere. 1 H NMR, 13C NMR and 19F NMR data were recorded using a Switzerland Bruker spectrometer relative to tetramethylsilane (TMS) as internal standard. The melting point were measured on the SGW X-4. Elemental analyses were performed using a Vario EL elemental analyzer. The FT-IR spectra were recorded on Bruker Tensor 27. Single crystals were grown in a mixture of CH2Cl2 and methanol. The final structure was confirmed by X-ray diffraction analysis (Bruker-Nonius SMART APEX II CCD), with graphitemonochromatic Mo Ka (l ¼ 0.71073 Å). UVevis absorption spectra were recorded on a Hitachi U-3900 spectrophotometer and the photoluminescence (PL) spectra were recorded using a Fluoromax-4 spectrophotometer in diluted CH2Cl2 solution. The PL lifetime of the iridium(Ⅲ) complex was measured by FLS 980 in the 105 mol L1 CH2Cl2 solution at room temperature. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 at a heating rate of 10 C min1 under nitrogen atmosphere. Cyclic voltammetry (CV) was performed using Autolab/PG STAT302 in a one-compartment electrolysis cell consisting of a platinum wire as working electrode, a platinum electrode as counter, and a calomel electrode as reference. Tetrabutylammonium perchlorate dissolved in acetonitrile was used as a supporting electrolyte (0.1 mol L1), and cyclic voltammetric runs were monitored at a scan rate of 50 mV s1. To investigate the electroluminescent properties of (CzPhBI)2Ir(tfmptz), non-doped electroluminescent devices were fabricated by high-vacuum (3 104 Pa) thermal evaporation onto a glass substrate pre-coated with indium tin oxide (ITO). The configuration of the devices was ITO/MoO3 (3 nm)/NPB (30 nm)/ TAPC (15 nm)/(CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm). MoO3 and N, N0 -bis-(1-naphthalenyl)-N, N0 bis-phenyl-(1, 10 -biphenyl)-4, 40 -diamine (NPB) were used as holeinjecting/transporting materials. 4, 40 -cyclohexylidenebis[N, Nbis(4-methylphenyl)aniline] (TAPC) was selected as the holetransporting and electron-blocking layer and (CzPhBI)2Ir(tfmptz) acted as the single emitting layer. 1,3,5-tris(1-phenyl-1H-benzo[d] imidazol-2-yl)benzene (TPBI) worked as electron-transporting and hole-blocking material, LiF was used as electron injection layer, and Al was evaporated as cathode. Luminanceevoltageecurrent density (LeVeJ) characteristics of the devices were recorded using Keithley 2400 Source Meter and L-2188 spot Brightness Meter. The active area of ITO was 3 mm 3 mm. Electroluminescent (EL) spectra were recorded on SpectraScan PR655. All of the experiments and measurements were conducted at room temperature.
393
2.2. Synthesis and characterization 2.2.1. Synthesis of 9-(6-brominehexyl)-carbazole (L1) Carbazole (5.00 g, 31 mmol) and 1,6-dibromohexane (70.00 g, 300 mmol) were dissolved in 80 ml toluene. Tertbutyl ammonium bromide (TBAB) (0.5 g, 15 mmol) was dissolved in 5 mL deionized water and 2 M K2CO3 solution, and then added to the mixture. The mixture was stirred at room temperature for 24 h and then washed with deionized water and dried. The precipitate was purified by column chromatography (silica gel, dichloromethane: petroleum ether ¼ 1: 20) to obtain a white crystal product (yield: 56%); m. p. 56e59 C. 1H NMR (CDCl3): d (ppm) ¼ 8.10 (ddd, J1 ¼ 7.8 Hz, J2 ¼ 1.2 Hz, J3 ¼ 0.6 Hz, 2H, AreH), 7.46 (dt, J1 ¼ 7.8 Hz, J2 ¼ 6.6 Hz, J3 ¼ 1.2 Hz, 2H, AreH), 7.40 (dt, J1 ¼ 7.8 Hz, J2 ¼ J3 ¼ 0.6 Hz, 2H, AreH), 7.23 (ddd, J1 ¼ 7.8 Hz, J2 ¼ 7.2 Hz, J3 ¼ 1.2 Hz, 2H, AreH), 4.32 (t, J ¼ 7.2 Hz, 2H, CH2), 3.36 (t, J ¼ 6.6 Hz, 2H, CH2), 1.82e1.90 (m, 4H, CH2), 1.41e1.48 (m, 4H, CH2). 13C NMR (CDCl3): d (ppm) ¼ 140.4, 125.6, 122.8, 120.3, 118.8, 108.6, 42.8, 33.6, 32.5, 28.8, 27.9, 26.4. FTIR (KBr, cm1): 3439, 3053, 2930, 2854, 1904, 1599, 1464, 1323, 1211, 1155, 748, 640. Anal. Calcd for C18H20BrN (%): C, 65.46; H, 6.10; N, 4.24; found: C, 64.83; H, 6.38; N, 4.02. 2.2.2. Synthesis of 2-phenyl-benzimidazole [2] (L2) Benzoic acid (6.10 g, 50 mmol) and o-phenylenediamine (5.50 g, 50 mmol) were added into a uniform mixture of poly-phosphoric acid with P2O5 (32.5 g, 100 mmol). The mixture was heated to 210 C for 4 h. After cooling to room temperature, the mixture was poured into water to obtain a suspension, and then the pH was adjusted to neutral. The precipitate was filtered and dried, and then burned in a vacuum to crystallize. Finally, canary yellow crystals were collected (yield: 84%); m. p. 297e302 C. 1H NMR ((CD3)2SO): d(ppm) ¼ 12.86 (brs, 1H, NeH), 8.17 (dt, J1 ¼ 8.4 Hz, J2 ¼ J3 ¼ 1.2 Hz, 2H, AreH), 7.65 (d, J ¼ 7.2 Hz, 1H, AreH), 7.56e7.45 (m, 4H, AreH), 7.23e7.12 (m, 2H, AreH). 13C NMR ((CD3)2OS): d(ppm) ¼ 151.3, 144.0, 135.2, 130.3, 130.0, 129.1, 126.5, 122.7, 121.8, 119.0, 111.6. FT-IR (KBr, cm1): 3439, 3055, 2837, 2650, 2300, 1549, 1456, 1281, 1217, 1120, 961, 746, 692. Anal. Calcd for C13H10N2 (%): C, 80.39; H, 5.19; N, 14.42; found: C, 79.83; H, 5.01; N, 15.12. 2.2.3. Synthesis of 1-[6-(9-carbazol)hexyl]-2-phenyl-benzimidazole (L3) L2 (1.92 g, 10 mmol), TBAB (0.32 g, 1 mmol), and 30 mL toluene were mixed and stirred. Then 2 M KOH solution was added under nitrogen atmosphere. The mixture was stirred at room temperature for 30 min. L1 (3.30 g, 10 mmol) was dissolved in 10 mL toluene, and then added slowly into the solution. The mixture solution was stirred at 100 C for 24 h. After cooling to room temperature, the mixture was poured into 100 mL deionized water. The organic layer was dried and evaporated, yielding a yellow oily liquid. The product was purified by column chromatography (silica gel, dichloromethane: ethyl acetate (EA) ¼ 1: 20) to obtain the white powder product (yield: 78%); m. p. 115e120 C. 1H NMR (CDCl3): d (ppm) ¼ 8.09 (d, J ¼ 7.8 Hz, 2H, AreH), 7.83e7.80 (m, 1H, AreH), 7.67e7.64 (m, 2H, AreH), 7.49e7.45 (m, 3H, AreH), 7.43 (dt, J1 ¼ 7.8 Hz, J2 ¼ 7.2 Hz, J3 ¼ 0.6 Hz, 2H, AreH), 7.33e7.30 (m, 3H, AreH), 7.28 (dd, J1 ¼ 5.4 Hz, J2 ¼ 1.8 Hz, 2H, AreH), 7.22 (dd, J1 ¼ 7.8 Hz, J2 ¼ 0.6 Hz, 2H, AreH), 4.23 (t, J ¼ 7.2 Hz, 2H, CH2), 4.18 (t, J ¼ 7.2 Hz, 2H, CH2), 1.82e1.72 (m, 4H, CH2), 1.30e1.19 (m, 4H, CH2). 13C NMR (CDCl3): d (ppm) ¼ 153.7, 143.2, 140.3, 135.4, 130.6, 129.6, 129.3, 128.7, 126.9, 125.7, 122.8, 122.7, 120.4, 120.0, 118.8, 110.1, 108.6, 44.3, 42.7, 29.5, 28.7, 26.6, 26.4. FT-IR (KBr, cm1): 3427, 3053, 2936, 2857, 2307, 1595, 1460, 1333, 1269, 1234, 1151, 1006, 916, 837, 743, 700. Anal. Calcd for C31H29N3 (%): C, 83.94; H, 6.59; N, 9.47; found: C, 83.78; H, 6.38; N, 9.76.
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2.2.4. Synthesis of (CzPhBI)2Ir(tfmptz) (a) In nitrogen atmosphere, iridium trichloride hydrate (267 mg, 1 mmol) was added to a mixture solution of the main ligand L3 (976 mg, 2.2 mmol) in 2-ethoxyethanol (18 mL) and water (6 mL). The mixture was heated to 120 C under constant stirring for 24 h, and then cooled to room temperature. The green amorphous precipitates were filtrated, washed with water and ethyl alcohol, and then dried. Finally, we obtained the crude dichloro-bridged iridium dimers. The mixture of the crude dimers (224 mg, 0.1 mmol), the auxiliary ligand (53.5 mg, 0.25 mmol), and Na2CO3 (212 mg, 20 mmol) in 2-ethoxyethanol (15 mL) were slowly heated to 130 C for 24 h and then cooled to room temperature. Excess water was added, and the mixed solution was extracted with CH2Cl2 (3 30 mL). The organic phase was dried with anhydrous MgSO4, and then the solvent was removed through evaporation. The residues were purified by column chromatography using the eluent CH2Cl2: acetonitrile: methanol (40:1:1) to obtain the blue-green solid (yield: 62%); m.p. >320 C. 1H NMR ((CD2)3OS): d(ppm) ¼ 8.13 (d, J ¼ 7.8 Hz, 4H, AreH), 8.09 (d, J ¼ 6.6 Hz, 1H, AreH), 7.97 (dt, J1 ¼ 7.8 Hz, J2 ¼ 1.2 Hz, 1H, AreH), 7.82 (d, J ¼ 7.2 Hz, 1H, AreH), 7.79e7.74 (m, 2H, AreH), 7.73 (d, J ¼ 8.4 Hz, 1H, AreH), 7.70 (d, J ¼ 8.4 Hz, 1H, AreH), 7.53 (d, J ¼ 7.8 Hz, 2H, AreH), 7.51 (d, J ¼ 7.8 Hz, 2H, AreH), 7.40 (t, J ¼ 7.8 Hz, 4H, AreH), 7.32 (t, J ¼ 7.8 Hz, 1H, AreH), 7.23 (t, J ¼ 8.4 Hz, 2H, AreH), 7.17 (t, J ¼ 7.2 Hz, 4H, AreH), 6.93e6.87 (m, 2H, AreH), 6.85 (t, J ¼ 8.4 Hz, 1H, AreH), 6.80 (t, J ¼ 8.4 Hz, 1H, AreH), 6.54 (t, J ¼ 8.4 Hz, 1H, AreH), 6.49 (t, J ¼ 8.4 Hz, 1H, AreH), 6.15 (dd, J1 ¼ 7.8 Hz, J2 ¼ 4.2 Hz, 2H, AreH), 5.75 (d, J ¼ 8.4 Hz, 1H, AreH), 5.62 (d, J ¼ 8.4 Hz, 1H, AreH), 4.88e4.67 (m, 4H, CH2), 4.37e4.27 (m, 4H, CH2), 1.84e1.62 (m, 8H, CH2), 1.42e1.21 (m, 8H, CH2). 19F NMR (CDCl3): d(ppm) ¼ -63.03
(3 F). FT-IR (KBr, cm1): 3435, 3036, 2930, 2860, 2305, 1462, 1330, 1269, 1234, 1196, 1159, 1120, 1020, 743. Anal. Calcd for C70H60F3IrN10 (%): C, 65.15; H, 4.69; N, 10.85; found: C, 65.03; H, 4.58; N, 10.66. The synthetic process is shown in Scheme 1. 3. Results and discussion 3.1. Molecular structure The faint yellow transparent crystals are obtained from the mixed solution of CH2Cl2 and methanol (1:1) [9]. The crystal structure of (CzPhBI)2Ir(tfmptz) are depicted in Fig. 1, and the lattice parameters are given in Table 1. A single crystal of (CzPhBI)2Ir(tfmptz) is monoclinic, space group P21/a. X-ray diffraction data [23e26] show that iridium atoms reside in the rhombic site of monoclinal crystals, and the iridium atoms display six-coordinate geometric structures with distorted prism geometries. Iridium atoms are coordinated with four nitrogen and two carbon atoms. The groups of carbazole linked to 2-phenylbenzimidazole by a hexyl chain provide steric protection against further iridium coordination. The IreN bond length of auxiliary ligand (Ir1eN8 ¼ 2.124 Å, Ir1eN7 ¼ 2.152 Å) is longer than the main ligand (Ir1eN4 ¼ 2.036 Å, Ir1eN1 ¼ 2.048 Å) by approximately 0.1 nm, because the 1,2,4-triazole ring in the auxiliary tfmptz ligand is an electron-rich group. Thus, their electron donation weakens the interaction of covalent bonds. The p-conjugation in the 1,2,4triazole ring has strong interaction with the electron-withdrawing trifluoromethyl group which results in shorter bonds (F1eC70 ¼ 1.320 Е, F2eC70 ¼ 1.342 Е, F3eC70 ¼ 1.330 Е). The alkyl chain ends of the main ligand are connected to a p-conjugation
Scheme 1. Synthetic route of (CzPhBI)2Ir(tfmptz).
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
395
Fig. 1. Single-crystal structure of (CzPhBI)2Ir(tfmptz) and selected geometric parameters (Ir1eC1 ¼ 2.016 (4) Å, Ir1eN1 ¼ 2.048 (4) Å, Ir1eC32 ¼ 2.012 (4) Å, Ir1eN4 ¼ 2.036 (3) Å, Ir1eN7 ¼ 2.152 (3) Å, Ir1eN8 ¼ 2.124 (3) Å; F1eC70 ¼ 1.320 Å, F2eC70 ¼ 1.342 Å, F3eC70 ¼ 1.330 Å; N8eIr1eN7 ¼ 76.43(11) , C1eIr1eN1 ¼ 79.78 (15) , C32eIr1eN4 ¼ 79.35 (15) , N1eIr1eN8 ¼ 88.02 (12) , N4eIr1eN7 ¼ 90.47 (12) , C32eIr1eC1 ¼ 87.20 (14) ).
ring, which induces electrons from alkyl C atom to act on the adjacent p-electron, forming the hyperconjugation that causes the CeC bond angles of the alkyl chain ends (C14eC15eC16 ¼ 113.4 , C17eC18eC19 ¼ 113.2 ) smaller than the CeC bond angles in the middle chains (C15eC16eC17 ¼ 115.9 , C16eC17eC18 ¼ 115.7 ). The single-crystal packing diagram is shown in Fig. 2. The carbazole groups are non-coplanar in a single molecule that increases the
steric hindrance, which broadens the exciton diffusion region. The adjacent carbazole groups in different molecules are unable to form pep accumulation with a distance of 15.59 Å, which indicates that the carbazole groups enlarge the distance between the adjacent molecules and restrain the tripletetriplet annihilation. The electron-rich ligand with 1,2,4-triazole and the hole-type carbazole are in an ordered arrangement that benefits carrier transport. 3.2. Thermal stability
Table 1 Crystallographic data for (CzPhBI)2Ir(tfmptz). Identifical code
(CzPhBI)2Ir(tfmptz)
Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å a/ b/ g/ Volume/Å3 Z r calcd mg/mm3 m/mm1 F(000) Crystal size/mm3 2Q range for data collection Index ranges Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 Final R indexes [I 2s (I)] Final R indexes [all data] Largest diff. peak/hole/e* Å3
C70H60N10IrF3 1290.48 115(20) Monoclinic P21/a 17.9287(4) 18.4660(5) 18.1472(4) 90 109.121(3) 90 5676.6(2) 4 1.51 2.416 2616 0.32 0.28 0.25 5.96e57.16 23 h 23, 23 k 23, 16 l 24 31,236 13003[R(int) ¼ 0.0556] 13003/0/757 1.037 R1 ¼ 0.0425, wR2 ¼ 0.0646 R1 ¼ 0.0646, wR2 ¼ 0.0743 1.67/-1.40
In the vacuum deposition process, the decomposition temperature of 5% weight loss should exceed 300 C that guarantee the
Fig. 2. Crystal packing diagram between adjacent molecules of (CzPhBI)2Ir(tfmptz).
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thermal stability of the OLED devices. The decomposition temperature of (CzPhBI)2Ir(tfmptz) is 447 C, which indicates high stability in the vacuum evaporation process. The TG and DTG curves are shown in Fig. 3. 3.3. Photophysical properties The photophysical properties are depicted in Fig. 4. The UVevis and PL spectra of the complex in CH2Cl2 solution (105 mol L1) were obtained at room temperature, and the phosphorescent spectrum of the complex in the 2-methyltetrahydrofuran solution (105 mol L1) was measured at 77 K. The photophysical data are shown in Table 2. The UVevis absorption spectrum in CH2Cl2 solution shows five main absorption bands at approximately 238, 263, 295, 346, and 380e490 nm. The bands at 237 and 294 nm are assigned to the ligand-centered singlet 1pep* transition of the
carbazole groups. The absorption band at 264 nm is assigned to the singlet pep* charge transition in the main ligand 2phenylbenzimidazole. The above absorption bands are similar to the main ligand CzPhBI [10]. The band at 380e490 nm is ascribed to the triplet exciton transition from the metal to ligand charge transition (3MLCT). Molar extinction coefficients (3 ¼ 2.18 105 L mol1) [13] is achieved by the maximum absorbance (2.18). The PL spectra of (CzPhBI)2Ir(tfmptz) show a maximum emission peak at 485 nm with a shoulder emission at 519 nm, belonging to the blue-green emission. Three F atoms with strong electronwithdrawing characteristics are introduced into the 1,2,4-triazole ring of the auxiliary ligand (tfmptz), which exhibits an obvious blue-shift compared with the Ir(CzPhBI)3 complex [10]. In addition, the quantum efficiency (FPL) of (CzPhBI)2Ir(tfmptz) (9.66%) is measured in diluted CH2Cl2 solution (106 mol L1). The maximum peak of the phosphorescent spectra is found at 483 nm. At low temperature, the electronic vibration and the intersystem crossing (ISC) are restrained that results in the blue-shift [19]. Compared with the PL spectra obtained in solution, the emission peak of film has a 7 nm red-shift because aggregation decreases the distance between adjacent molecules [12]. The shorter distance leads to orbital overlap, which consequently produces serious ISC phenomenon. The alkyl chain prevents intermolecular p-electron transition between the carbazole groups and iridium core through the non-conjugated bond that suppress the concentration quenching. Fig. 5 shows the PL lifetime decay curve of (CzPhBI)2Ir(tfmptz) at room temperature with a well-fitted biexponential decay equation:
RðtÞ ¼ B1 e
t t1
þ B2 e
t t2
Fig. 3. The TG and DTG curves of (CzPhBI)2Ir(tfmptz).
Fig. 4. UVevis absorption spectra and PL spectra of (CzPhBI)2Ir(tfmptz) in CH2Cl2 solution and films; the phosphorescent spectra in 2-methyltetrahydrofuran at 77 K (excited at 365 nm).
Fig. 5. The PL lifetime decay curve and the fit result of (CzPhBI)2Ir(tfmptz) excited at 375 nm in CH2Cl2 solution (105 mol L1) at room temperature.
Table 2 The photophysical properties of (CzPhBI)2Ir(tfmptz). Complex
(CzPhBI)2Ir(tfmptz) a
3
labs (log 3)a (nm)
238(5.34), 263(5.12), 295(5.03), 346(4.81), 450(3.15)
the absorption coefficient in the LamberteBeer equation.
lPL (nm) Solution
Film
77 K
485, 519
492, 525
483, 519
QY (%)
t1 (ns)
t2 (ns)
c2
9.66
234
778
1.29
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
EHOMO
397
. i h ox ¼ Eonset E1=2 Fc Fcþ þ 4:8
The HOMO level of (CzPhBI)2Ir(tfmptz) is estimated to be 5.58 eV, and the LUMO level can be calculated by the absorption band edge (l) and the HOMO level:
ELUMO ¼ EHOMO þ e
1240 l
The band edge is found at 490 nm, corresponding to the optical band gap 2.53 eV; therefore, the LUMO level is 3.05 eV. 3.5. Electroluminescence studies
Fig. 6. Cyclic voltammetry curve of (CzPhBI)2Ir(tfmptz).
3.4. Electrochemical properties The electrochemical property of the iridium complex is investigated in the mixture of CH2Cl2 solution (~104 mol L1) and supporting electrolyte solution (1:1) at room temperature. The recorded electrochemical behavior is shown in Fig. 6. In the measurement, Fc/Fcþ is used as calibration. The oxidation potential at 0.76 V and 1.09 V are attributed to the oxidation of acetonitrile from the supporting electrolyte. The starting reversible ox ) of (CzPhBI)2Ir(tfmptz) is located at oxidation potential (Eonset 1.23 V. HOMO level was calculated using the following equation [22]:
Non-doped electroluminescent devices have been fabricated with (CzPhBI)2Ir(tfmptz) as the single emitting layer. The configuration is ITO/MoO3 (3 nm)/NPB (30 nm)/TAPC (10 nm)/ (CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm). Device structure scheme and molecular structures of the compounds used in the devices are shown in Fig. 7. The LUMO level of TAPC is 2.0 eV higher than the iridium complex that could block the electron and make the exciton recombination at the emitting layer. The EL spectra at different applied voltages are shown in Fig. 8(a). The EL spectra are similar to their PL spectra, and the maximum emission peak is found at 488 nm, with a shoulder emission at 521 nm with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.264, 0.514). The shape of the EL spectra have no change and no additional emission is observed [13] when the applied voltages increase. However, the EL intensity enhances obviously, because high voltages are beneficial to charge mobility, thereby facilitating exciton recombination for emission. The LeVeJ curve and CE-J curve are displayed in Fig. 8(b) and the data are shown in Table 3. The non-doped device has low turn-on
Fig. 7. Device structure scheme and molecular structures of the compounds used in the devices.
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Fig. 8. The EL spectra (a) and the current density-voltage-luminance (JeVeL) curve with the current efficiency-current density (CE-J) (insert) curve (b) of the non-doped devices.
voltage of 3.2 V (the voltage corresponding luminescence at 1 cd m2). The maximum luminance is 5420 cd m2 at 6 V and the maximum current efficiency is 8.2 cd A1 at the current density of 16.8 mA cm2. The power efficiency achieves at 5.4 lm W1. Carbazole groups with good hole-transporting capability and the 1,2,4-triazole group with good electron-transporting capability are advantageous to balance the charge carriers. For the non-doped device, the phenomenon of the current efficiency “roll-off” caused by aggregation has not been restricted completely, which indicates that the carbazole groups linked by the alkyl chain could
prevent concentration quenching and tripletetriplet annihilation, but cannot completely restrain the efficiency “roll-off” [21]. 4. Conclusion In summary, a novel blue-green self-host phosphorescent iridium(III) complex has been designed and synthesized using a simple method. The introduction of carbazole groups and 1,2,4triazole groups not only enhances the thermal stability, but also balances carrier injection/transport in the emitting layer. The alkyl
Table 3 Device performance of (CzPhBI)2Ir(tfmptz). Device
lEL (nm)
Von (V)a
Lmax (cd m2)
C.E.max (cd A1)
P.E.max (lm W1)
CIE
(CzPhBI)2Ir(tfmptz)
488
3.2
5420
8.2
5.4
(0.264, 0.514)
a
The turn-on voltage of 1 cd m2.
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chains enlarge the steric hindrance by separating the carbazole groups and green iridium center that restricts the concentration quenching and tripletetriplet annihilation. Our results have provided information to reduce the complexity and cost of fabricating efficient organic light emitting devices for displays and lighting applications. Acknowledgments This work was financially supported by Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-13-0927); International Science & Technology Cooperation Program of China (2012DFR50460); National Natural Scientific Foundation of China (11004218, 21101111, 61274056, 61205179, 61307030, 61307029); Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.06.005. References [1] A.D. Becke, Phys. Rev. A 38 (1988) 3098e3100. [2] T. Hisano, M. Ichikawa, K. Tsumoto, M. Tasaki, Chem. Pharm. Bull. 30 (1982) 2996e3004. [3] H.F. Chen, L.C. Chi, W.Y. Hung, W.J. Chen, T.Y. Hwu, Y.H. Chen, S.H. Chou, E. Mondal, Y.H. Liu, K.T. Wong, Org. Electron. 13 (2012) 2671e2681. [4] H. Xu, Z.F. Xu, Z.Y. Yue, P.F. Yan, B. Wang, L.W. Jia, G.M. Li, W.B. Sun, J.W. Zhang, J. Phys. Chem. C 112 (2008) 15517e15525. [5] S.H. Chou, W.Y. Hung, C.M. Chen, Q.Y. Liu, Y.H. Liu, K.T. Wong, RSC Adv. 3 (2013) 13891e13900.
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Synthesis, structure, photophysical and electroluminescent properties of a blue-green self-host phosphorescent iridium(III) complex Jing Sun a, Hua Wang a, Huixia Xu a, *, Jie Li a, Yuling Wu a, Xiaogang Du a, Bingshe Xu a, b, ** a b
Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
h i g h l i g h t s A blue-green self-host phosphorescent iridium(III) complex was synthesized. The molecular structure, and photophysical properties were investigated. Electroluminescent performance in host-free devices were discussed. The maximum current efficiency 8.2 cd A1 and the maximum brightness 5420 cd m2 were achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2014 Received in revised form 29 April 2015 Accepted 1 June 2015 Available online 29 June 2015
A kind of blue-green self-host phosphorescent iridium(III) complex, (CzPhBI)2Ir(tfmptz) [CzPhBI ¼ 9-(6(2-phenyl-1-benzimidazolyl)hexyl)-9-carbazole; tfmptz ¼ 2-(5-trifluoromethyl-1,2,4-triazolyl)pyridine], was designed and synthesized. The synthesized iridium(III) complex was characterized by 1H NMR, 19F NMR, FT-IR, elemental analysis and X-ray single-crystal diffraction, respectively. Its thermal properties, optical properties and electrochemical properties were also investigated. The host-free organic electroluminescent devices with the configuration of ITO/MoO3 (3 nm)/NPB (30 nm)/TAPC (15 nm)/ (CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm) had been fabricated. The devices exhibited excellent performance indicating that (CzPhBI)2Ir(tfmptz) was a promising phosphorescent material. © 2015 Elsevier B.V. All rights reserved.
Keywords: Organometallic compounds Crystal structure Optical properties Vacuum deposition Luminescence
1. Introduction Iridium(III) complexes have attracted much attention because of their unique characteristics, particularly in harvesting both singlet and triplet exciton, leading to a potentially achievable 100% internal quantum efficiency [1], theoretically. However, the serious problem of concentration quenching at high current density limits its application. The iridium(III) complexes have to be utilized as guest in phosphorescent organic light emitting devices (PhOLEDs) [3]. Therefore, appropriate match of energy levels between host and
Abbreviations: NMR, nuclear magnetic resonance; FT-IR, Fourier transform infrared spectroscopy. * Corresponding author. ** Corresponding author. Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, China. E-mail addresses: [email protected] (H. Xu), [email protected] (B. Xu). http://dx.doi.org/10.1016/j.matchemphys.2015.06.005 0254-0584/© 2015 Elsevier B.V. All rights reserved.
guest is necessary [4e7] in order to obtain efficient energy transfer and restrain energy transfer back from guest to host which would reduce the electroluminescent performance. Hence, developing a kind of high-effective self-host phosphorescent material is the most important for PhOLEDs [8]. Yue et al. [10] have reported a series of green iridium(III) complexes with hole-transporting carbazole group as its side chain that could act as single emitting layer. With the introduction of carbazole group, good electroluminescent performances have been achieved with a high luminance over 6000 cd m2. An ambipolar iridium(III) complex IrPBIC2O2 with electron-transporting 1,3,4oxadiazole (OXD) has also been developed [11], which has a maximum external quantum efficiency (E.Q.E.) of more than 4% and exhibits reduced efficiency “roll-off” to of less than 16% at 1000 cd m2. However, the preparation of self-host phosphorescent materials with excellent electroluminescent properties is still a challenge [12e16].
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Therefore, a blue-green self-host phosphorescent complex of (CzPhBI)2Ir(tfmptz) [CzPhBI ¼ 9-(6-(2-phenyl-1-benzimidazolyl) hexyl)-9-carbazole; tfmptz ¼ 2-(5-trifluoromethyl-1,2,4-triazolyl) pyridine] has been designed and synthesized. The blue-green emitting benzimidazole iridium(III) complex acts as a luminescent center. The functional group of carbazole has been chosen due to the excellent hole-transporting property [17,18] and the flexible hexyl is used to prevent adjacent molecular aggregation. Meanwhile, the auxiliary ligand with 1,2,4-triazole group is selected based on its desirable electron-transporting property [19e21]. The structure, photophysical and electroluminescent properties of (CzPhBI)2Ir(tfmptz) have been investigated in detail. 2. Experimental section 2.1. General information All of the chemicals and reagents are used as purchased without further purification. Solvents for chemical synthesis are purified according to standard procedures. All chemical reactions were performed in a nitrogen atmosphere. 1 H NMR, 13C NMR and 19F NMR data were recorded using a Switzerland Bruker spectrometer relative to tetramethylsilane (TMS) as internal standard. The melting point were measured on the SGW X-4. Elemental analyses were performed using a Vario EL elemental analyzer. The FT-IR spectra were recorded on Bruker Tensor 27. Single crystals were grown in a mixture of CH2Cl2 and methanol. The final structure was confirmed by X-ray diffraction analysis (Bruker-Nonius SMART APEX II CCD), with graphitemonochromatic Mo Ka (l ¼ 0.71073 Å). UVevis absorption spectra were recorded on a Hitachi U-3900 spectrophotometer and the photoluminescence (PL) spectra were recorded using a Fluoromax-4 spectrophotometer in diluted CH2Cl2 solution. The PL lifetime of the iridium(Ⅲ) complex was measured by FLS 980 in the 105 mol L1 CH2Cl2 solution at room temperature. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209 at a heating rate of 10 C min1 under nitrogen atmosphere. Cyclic voltammetry (CV) was performed using Autolab/PG STAT302 in a one-compartment electrolysis cell consisting of a platinum wire as working electrode, a platinum electrode as counter, and a calomel electrode as reference. Tetrabutylammonium perchlorate dissolved in acetonitrile was used as a supporting electrolyte (0.1 mol L1), and cyclic voltammetric runs were monitored at a scan rate of 50 mV s1. To investigate the electroluminescent properties of (CzPhBI)2Ir(tfmptz), non-doped electroluminescent devices were fabricated by high-vacuum (3 104 Pa) thermal evaporation onto a glass substrate pre-coated with indium tin oxide (ITO). The configuration of the devices was ITO/MoO3 (3 nm)/NPB (30 nm)/ TAPC (15 nm)/(CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm). MoO3 and N, N0 -bis-(1-naphthalenyl)-N, N0 bis-phenyl-(1, 10 -biphenyl)-4, 40 -diamine (NPB) were used as holeinjecting/transporting materials. 4, 40 -cyclohexylidenebis[N, Nbis(4-methylphenyl)aniline] (TAPC) was selected as the holetransporting and electron-blocking layer and (CzPhBI)2Ir(tfmptz) acted as the single emitting layer. 1,3,5-tris(1-phenyl-1H-benzo[d] imidazol-2-yl)benzene (TPBI) worked as electron-transporting and hole-blocking material, LiF was used as electron injection layer, and Al was evaporated as cathode. Luminanceevoltageecurrent density (LeVeJ) characteristics of the devices were recorded using Keithley 2400 Source Meter and L-2188 spot Brightness Meter. The active area of ITO was 3 mm 3 mm. Electroluminescent (EL) spectra were recorded on SpectraScan PR655. All of the experiments and measurements were conducted at room temperature.
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2.2. Synthesis and characterization 2.2.1. Synthesis of 9-(6-brominehexyl)-carbazole (L1) Carbazole (5.00 g, 31 mmol) and 1,6-dibromohexane (70.00 g, 300 mmol) were dissolved in 80 ml toluene. Tertbutyl ammonium bromide (TBAB) (0.5 g, 15 mmol) was dissolved in 5 mL deionized water and 2 M K2CO3 solution, and then added to the mixture. The mixture was stirred at room temperature for 24 h and then washed with deionized water and dried. The precipitate was purified by column chromatography (silica gel, dichloromethane: petroleum ether ¼ 1: 20) to obtain a white crystal product (yield: 56%); m. p. 56e59 C. 1H NMR (CDCl3): d (ppm) ¼ 8.10 (ddd, J1 ¼ 7.8 Hz, J2 ¼ 1.2 Hz, J3 ¼ 0.6 Hz, 2H, AreH), 7.46 (dt, J1 ¼ 7.8 Hz, J2 ¼ 6.6 Hz, J3 ¼ 1.2 Hz, 2H, AreH), 7.40 (dt, J1 ¼ 7.8 Hz, J2 ¼ J3 ¼ 0.6 Hz, 2H, AreH), 7.23 (ddd, J1 ¼ 7.8 Hz, J2 ¼ 7.2 Hz, J3 ¼ 1.2 Hz, 2H, AreH), 4.32 (t, J ¼ 7.2 Hz, 2H, CH2), 3.36 (t, J ¼ 6.6 Hz, 2H, CH2), 1.82e1.90 (m, 4H, CH2), 1.41e1.48 (m, 4H, CH2). 13C NMR (CDCl3): d (ppm) ¼ 140.4, 125.6, 122.8, 120.3, 118.8, 108.6, 42.8, 33.6, 32.5, 28.8, 27.9, 26.4. FTIR (KBr, cm1): 3439, 3053, 2930, 2854, 1904, 1599, 1464, 1323, 1211, 1155, 748, 640. Anal. Calcd for C18H20BrN (%): C, 65.46; H, 6.10; N, 4.24; found: C, 64.83; H, 6.38; N, 4.02. 2.2.2. Synthesis of 2-phenyl-benzimidazole [2] (L2) Benzoic acid (6.10 g, 50 mmol) and o-phenylenediamine (5.50 g, 50 mmol) were added into a uniform mixture of poly-phosphoric acid with P2O5 (32.5 g, 100 mmol). The mixture was heated to 210 C for 4 h. After cooling to room temperature, the mixture was poured into water to obtain a suspension, and then the pH was adjusted to neutral. The precipitate was filtered and dried, and then burned in a vacuum to crystallize. Finally, canary yellow crystals were collected (yield: 84%); m. p. 297e302 C. 1H NMR ((CD3)2SO): d(ppm) ¼ 12.86 (brs, 1H, NeH), 8.17 (dt, J1 ¼ 8.4 Hz, J2 ¼ J3 ¼ 1.2 Hz, 2H, AreH), 7.65 (d, J ¼ 7.2 Hz, 1H, AreH), 7.56e7.45 (m, 4H, AreH), 7.23e7.12 (m, 2H, AreH). 13C NMR ((CD3)2OS): d(ppm) ¼ 151.3, 144.0, 135.2, 130.3, 130.0, 129.1, 126.5, 122.7, 121.8, 119.0, 111.6. FT-IR (KBr, cm1): 3439, 3055, 2837, 2650, 2300, 1549, 1456, 1281, 1217, 1120, 961, 746, 692. Anal. Calcd for C13H10N2 (%): C, 80.39; H, 5.19; N, 14.42; found: C, 79.83; H, 5.01; N, 15.12. 2.2.3. Synthesis of 1-[6-(9-carbazol)hexyl]-2-phenyl-benzimidazole (L3) L2 (1.92 g, 10 mmol), TBAB (0.32 g, 1 mmol), and 30 mL toluene were mixed and stirred. Then 2 M KOH solution was added under nitrogen atmosphere. The mixture was stirred at room temperature for 30 min. L1 (3.30 g, 10 mmol) was dissolved in 10 mL toluene, and then added slowly into the solution. The mixture solution was stirred at 100 C for 24 h. After cooling to room temperature, the mixture was poured into 100 mL deionized water. The organic layer was dried and evaporated, yielding a yellow oily liquid. The product was purified by column chromatography (silica gel, dichloromethane: ethyl acetate (EA) ¼ 1: 20) to obtain the white powder product (yield: 78%); m. p. 115e120 C. 1H NMR (CDCl3): d (ppm) ¼ 8.09 (d, J ¼ 7.8 Hz, 2H, AreH), 7.83e7.80 (m, 1H, AreH), 7.67e7.64 (m, 2H, AreH), 7.49e7.45 (m, 3H, AreH), 7.43 (dt, J1 ¼ 7.8 Hz, J2 ¼ 7.2 Hz, J3 ¼ 0.6 Hz, 2H, AreH), 7.33e7.30 (m, 3H, AreH), 7.28 (dd, J1 ¼ 5.4 Hz, J2 ¼ 1.8 Hz, 2H, AreH), 7.22 (dd, J1 ¼ 7.8 Hz, J2 ¼ 0.6 Hz, 2H, AreH), 4.23 (t, J ¼ 7.2 Hz, 2H, CH2), 4.18 (t, J ¼ 7.2 Hz, 2H, CH2), 1.82e1.72 (m, 4H, CH2), 1.30e1.19 (m, 4H, CH2). 13C NMR (CDCl3): d (ppm) ¼ 153.7, 143.2, 140.3, 135.4, 130.6, 129.6, 129.3, 128.7, 126.9, 125.7, 122.8, 122.7, 120.4, 120.0, 118.8, 110.1, 108.6, 44.3, 42.7, 29.5, 28.7, 26.6, 26.4. FT-IR (KBr, cm1): 3427, 3053, 2936, 2857, 2307, 1595, 1460, 1333, 1269, 1234, 1151, 1006, 916, 837, 743, 700. Anal. Calcd for C31H29N3 (%): C, 83.94; H, 6.59; N, 9.47; found: C, 83.78; H, 6.38; N, 9.76.
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2.2.4. Synthesis of (CzPhBI)2Ir(tfmptz) (a) In nitrogen atmosphere, iridium trichloride hydrate (267 mg, 1 mmol) was added to a mixture solution of the main ligand L3 (976 mg, 2.2 mmol) in 2-ethoxyethanol (18 mL) and water (6 mL). The mixture was heated to 120 C under constant stirring for 24 h, and then cooled to room temperature. The green amorphous precipitates were filtrated, washed with water and ethyl alcohol, and then dried. Finally, we obtained the crude dichloro-bridged iridium dimers. The mixture of the crude dimers (224 mg, 0.1 mmol), the auxiliary ligand (53.5 mg, 0.25 mmol), and Na2CO3 (212 mg, 20 mmol) in 2-ethoxyethanol (15 mL) were slowly heated to 130 C for 24 h and then cooled to room temperature. Excess water was added, and the mixed solution was extracted with CH2Cl2 (3 30 mL). The organic phase was dried with anhydrous MgSO4, and then the solvent was removed through evaporation. The residues were purified by column chromatography using the eluent CH2Cl2: acetonitrile: methanol (40:1:1) to obtain the blue-green solid (yield: 62%); m.p. >320 C. 1H NMR ((CD2)3OS): d(ppm) ¼ 8.13 (d, J ¼ 7.8 Hz, 4H, AreH), 8.09 (d, J ¼ 6.6 Hz, 1H, AreH), 7.97 (dt, J1 ¼ 7.8 Hz, J2 ¼ 1.2 Hz, 1H, AreH), 7.82 (d, J ¼ 7.2 Hz, 1H, AreH), 7.79e7.74 (m, 2H, AreH), 7.73 (d, J ¼ 8.4 Hz, 1H, AreH), 7.70 (d, J ¼ 8.4 Hz, 1H, AreH), 7.53 (d, J ¼ 7.8 Hz, 2H, AreH), 7.51 (d, J ¼ 7.8 Hz, 2H, AreH), 7.40 (t, J ¼ 7.8 Hz, 4H, AreH), 7.32 (t, J ¼ 7.8 Hz, 1H, AreH), 7.23 (t, J ¼ 8.4 Hz, 2H, AreH), 7.17 (t, J ¼ 7.2 Hz, 4H, AreH), 6.93e6.87 (m, 2H, AreH), 6.85 (t, J ¼ 8.4 Hz, 1H, AreH), 6.80 (t, J ¼ 8.4 Hz, 1H, AreH), 6.54 (t, J ¼ 8.4 Hz, 1H, AreH), 6.49 (t, J ¼ 8.4 Hz, 1H, AreH), 6.15 (dd, J1 ¼ 7.8 Hz, J2 ¼ 4.2 Hz, 2H, AreH), 5.75 (d, J ¼ 8.4 Hz, 1H, AreH), 5.62 (d, J ¼ 8.4 Hz, 1H, AreH), 4.88e4.67 (m, 4H, CH2), 4.37e4.27 (m, 4H, CH2), 1.84e1.62 (m, 8H, CH2), 1.42e1.21 (m, 8H, CH2). 19F NMR (CDCl3): d(ppm) ¼ -63.03
(3 F). FT-IR (KBr, cm1): 3435, 3036, 2930, 2860, 2305, 1462, 1330, 1269, 1234, 1196, 1159, 1120, 1020, 743. Anal. Calcd for C70H60F3IrN10 (%): C, 65.15; H, 4.69; N, 10.85; found: C, 65.03; H, 4.58; N, 10.66. The synthetic process is shown in Scheme 1. 3. Results and discussion 3.1. Molecular structure The faint yellow transparent crystals are obtained from the mixed solution of CH2Cl2 and methanol (1:1) [9]. The crystal structure of (CzPhBI)2Ir(tfmptz) are depicted in Fig. 1, and the lattice parameters are given in Table 1. A single crystal of (CzPhBI)2Ir(tfmptz) is monoclinic, space group P21/a. X-ray diffraction data [23e26] show that iridium atoms reside in the rhombic site of monoclinal crystals, and the iridium atoms display six-coordinate geometric structures with distorted prism geometries. Iridium atoms are coordinated with four nitrogen and two carbon atoms. The groups of carbazole linked to 2-phenylbenzimidazole by a hexyl chain provide steric protection against further iridium coordination. The IreN bond length of auxiliary ligand (Ir1eN8 ¼ 2.124 Å, Ir1eN7 ¼ 2.152 Å) is longer than the main ligand (Ir1eN4 ¼ 2.036 Å, Ir1eN1 ¼ 2.048 Å) by approximately 0.1 nm, because the 1,2,4-triazole ring in the auxiliary tfmptz ligand is an electron-rich group. Thus, their electron donation weakens the interaction of covalent bonds. The p-conjugation in the 1,2,4triazole ring has strong interaction with the electron-withdrawing trifluoromethyl group which results in shorter bonds (F1eC70 ¼ 1.320 Е, F2eC70 ¼ 1.342 Е, F3eC70 ¼ 1.330 Е). The alkyl chain ends of the main ligand are connected to a p-conjugation
Scheme 1. Synthetic route of (CzPhBI)2Ir(tfmptz).
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
395
Fig. 1. Single-crystal structure of (CzPhBI)2Ir(tfmptz) and selected geometric parameters (Ir1eC1 ¼ 2.016 (4) Å, Ir1eN1 ¼ 2.048 (4) Å, Ir1eC32 ¼ 2.012 (4) Å, Ir1eN4 ¼ 2.036 (3) Å, Ir1eN7 ¼ 2.152 (3) Å, Ir1eN8 ¼ 2.124 (3) Å; F1eC70 ¼ 1.320 Å, F2eC70 ¼ 1.342 Å, F3eC70 ¼ 1.330 Å; N8eIr1eN7 ¼ 76.43(11) , C1eIr1eN1 ¼ 79.78 (15) , C32eIr1eN4 ¼ 79.35 (15) , N1eIr1eN8 ¼ 88.02 (12) , N4eIr1eN7 ¼ 90.47 (12) , C32eIr1eC1 ¼ 87.20 (14) ).
ring, which induces electrons from alkyl C atom to act on the adjacent p-electron, forming the hyperconjugation that causes the CeC bond angles of the alkyl chain ends (C14eC15eC16 ¼ 113.4 , C17eC18eC19 ¼ 113.2 ) smaller than the CeC bond angles in the middle chains (C15eC16eC17 ¼ 115.9 , C16eC17eC18 ¼ 115.7 ). The single-crystal packing diagram is shown in Fig. 2. The carbazole groups are non-coplanar in a single molecule that increases the
steric hindrance, which broadens the exciton diffusion region. The adjacent carbazole groups in different molecules are unable to form pep accumulation with a distance of 15.59 Å, which indicates that the carbazole groups enlarge the distance between the adjacent molecules and restrain the tripletetriplet annihilation. The electron-rich ligand with 1,2,4-triazole and the hole-type carbazole are in an ordered arrangement that benefits carrier transport. 3.2. Thermal stability
Table 1 Crystallographic data for (CzPhBI)2Ir(tfmptz). Identifical code
(CzPhBI)2Ir(tfmptz)
Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å a/ b/ g/ Volume/Å3 Z r calcd mg/mm3 m/mm1 F(000) Crystal size/mm3 2Q range for data collection Index ranges Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 Final R indexes [I 2s (I)] Final R indexes [all data] Largest diff. peak/hole/e* Å3
C70H60N10IrF3 1290.48 115(20) Monoclinic P21/a 17.9287(4) 18.4660(5) 18.1472(4) 90 109.121(3) 90 5676.6(2) 4 1.51 2.416 2616 0.32 0.28 0.25 5.96e57.16 23 h 23, 23 k 23, 16 l 24 31,236 13003[R(int) ¼ 0.0556] 13003/0/757 1.037 R1 ¼ 0.0425, wR2 ¼ 0.0646 R1 ¼ 0.0646, wR2 ¼ 0.0743 1.67/-1.40
In the vacuum deposition process, the decomposition temperature of 5% weight loss should exceed 300 C that guarantee the
Fig. 2. Crystal packing diagram between adjacent molecules of (CzPhBI)2Ir(tfmptz).
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J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
thermal stability of the OLED devices. The decomposition temperature of (CzPhBI)2Ir(tfmptz) is 447 C, which indicates high stability in the vacuum evaporation process. The TG and DTG curves are shown in Fig. 3. 3.3. Photophysical properties The photophysical properties are depicted in Fig. 4. The UVevis and PL spectra of the complex in CH2Cl2 solution (105 mol L1) were obtained at room temperature, and the phosphorescent spectrum of the complex in the 2-methyltetrahydrofuran solution (105 mol L1) was measured at 77 K. The photophysical data are shown in Table 2. The UVevis absorption spectrum in CH2Cl2 solution shows five main absorption bands at approximately 238, 263, 295, 346, and 380e490 nm. The bands at 237 and 294 nm are assigned to the ligand-centered singlet 1pep* transition of the
carbazole groups. The absorption band at 264 nm is assigned to the singlet pep* charge transition in the main ligand 2phenylbenzimidazole. The above absorption bands are similar to the main ligand CzPhBI [10]. The band at 380e490 nm is ascribed to the triplet exciton transition from the metal to ligand charge transition (3MLCT). Molar extinction coefficients (3 ¼ 2.18 105 L mol1) [13] is achieved by the maximum absorbance (2.18). The PL spectra of (CzPhBI)2Ir(tfmptz) show a maximum emission peak at 485 nm with a shoulder emission at 519 nm, belonging to the blue-green emission. Three F atoms with strong electronwithdrawing characteristics are introduced into the 1,2,4-triazole ring of the auxiliary ligand (tfmptz), which exhibits an obvious blue-shift compared with the Ir(CzPhBI)3 complex [10]. In addition, the quantum efficiency (FPL) of (CzPhBI)2Ir(tfmptz) (9.66%) is measured in diluted CH2Cl2 solution (106 mol L1). The maximum peak of the phosphorescent spectra is found at 483 nm. At low temperature, the electronic vibration and the intersystem crossing (ISC) are restrained that results in the blue-shift [19]. Compared with the PL spectra obtained in solution, the emission peak of film has a 7 nm red-shift because aggregation decreases the distance between adjacent molecules [12]. The shorter distance leads to orbital overlap, which consequently produces serious ISC phenomenon. The alkyl chain prevents intermolecular p-electron transition between the carbazole groups and iridium core through the non-conjugated bond that suppress the concentration quenching. Fig. 5 shows the PL lifetime decay curve of (CzPhBI)2Ir(tfmptz) at room temperature with a well-fitted biexponential decay equation:
RðtÞ ¼ B1 e
t t1
þ B2 e
t t2
Fig. 3. The TG and DTG curves of (CzPhBI)2Ir(tfmptz).
Fig. 4. UVevis absorption spectra and PL spectra of (CzPhBI)2Ir(tfmptz) in CH2Cl2 solution and films; the phosphorescent spectra in 2-methyltetrahydrofuran at 77 K (excited at 365 nm).
Fig. 5. The PL lifetime decay curve and the fit result of (CzPhBI)2Ir(tfmptz) excited at 375 nm in CH2Cl2 solution (105 mol L1) at room temperature.
Table 2 The photophysical properties of (CzPhBI)2Ir(tfmptz). Complex
(CzPhBI)2Ir(tfmptz) a
3
labs (log 3)a (nm)
238(5.34), 263(5.12), 295(5.03), 346(4.81), 450(3.15)
the absorption coefficient in the LamberteBeer equation.
lPL (nm) Solution
Film
77 K
485, 519
492, 525
483, 519
QY (%)
t1 (ns)
t2 (ns)
c2
9.66
234
778
1.29
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
EHOMO
397
. i h ox ¼ Eonset E1=2 Fc Fcþ þ 4:8
The HOMO level of (CzPhBI)2Ir(tfmptz) is estimated to be 5.58 eV, and the LUMO level can be calculated by the absorption band edge (l) and the HOMO level:
ELUMO ¼ EHOMO þ e
1240 l
The band edge is found at 490 nm, corresponding to the optical band gap 2.53 eV; therefore, the LUMO level is 3.05 eV. 3.5. Electroluminescence studies
Fig. 6. Cyclic voltammetry curve of (CzPhBI)2Ir(tfmptz).
3.4. Electrochemical properties The electrochemical property of the iridium complex is investigated in the mixture of CH2Cl2 solution (~104 mol L1) and supporting electrolyte solution (1:1) at room temperature. The recorded electrochemical behavior is shown in Fig. 6. In the measurement, Fc/Fcþ is used as calibration. The oxidation potential at 0.76 V and 1.09 V are attributed to the oxidation of acetonitrile from the supporting electrolyte. The starting reversible ox ) of (CzPhBI)2Ir(tfmptz) is located at oxidation potential (Eonset 1.23 V. HOMO level was calculated using the following equation [22]:
Non-doped electroluminescent devices have been fabricated with (CzPhBI)2Ir(tfmptz) as the single emitting layer. The configuration is ITO/MoO3 (3 nm)/NPB (30 nm)/TAPC (10 nm)/ (CzPhBI)2Ir(tfmptz) (30 nm)/TBPI (30 nm)/LiF (1 nm)/Al (100 nm). Device structure scheme and molecular structures of the compounds used in the devices are shown in Fig. 7. The LUMO level of TAPC is 2.0 eV higher than the iridium complex that could block the electron and make the exciton recombination at the emitting layer. The EL spectra at different applied voltages are shown in Fig. 8(a). The EL spectra are similar to their PL spectra, and the maximum emission peak is found at 488 nm, with a shoulder emission at 521 nm with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.264, 0.514). The shape of the EL spectra have no change and no additional emission is observed [13] when the applied voltages increase. However, the EL intensity enhances obviously, because high voltages are beneficial to charge mobility, thereby facilitating exciton recombination for emission. The LeVeJ curve and CE-J curve are displayed in Fig. 8(b) and the data are shown in Table 3. The non-doped device has low turn-on
Fig. 7. Device structure scheme and molecular structures of the compounds used in the devices.
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J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
Fig. 8. The EL spectra (a) and the current density-voltage-luminance (JeVeL) curve with the current efficiency-current density (CE-J) (insert) curve (b) of the non-doped devices.
voltage of 3.2 V (the voltage corresponding luminescence at 1 cd m2). The maximum luminance is 5420 cd m2 at 6 V and the maximum current efficiency is 8.2 cd A1 at the current density of 16.8 mA cm2. The power efficiency achieves at 5.4 lm W1. Carbazole groups with good hole-transporting capability and the 1,2,4-triazole group with good electron-transporting capability are advantageous to balance the charge carriers. For the non-doped device, the phenomenon of the current efficiency “roll-off” caused by aggregation has not been restricted completely, which indicates that the carbazole groups linked by the alkyl chain could
prevent concentration quenching and tripletetriplet annihilation, but cannot completely restrain the efficiency “roll-off” [21]. 4. Conclusion In summary, a novel blue-green self-host phosphorescent iridium(III) complex has been designed and synthesized using a simple method. The introduction of carbazole groups and 1,2,4triazole groups not only enhances the thermal stability, but also balances carrier injection/transport in the emitting layer. The alkyl
Table 3 Device performance of (CzPhBI)2Ir(tfmptz). Device
lEL (nm)
Von (V)a
Lmax (cd m2)
C.E.max (cd A1)
P.E.max (lm W1)
CIE
(CzPhBI)2Ir(tfmptz)
488
3.2
5420
8.2
5.4
(0.264, 0.514)
a
The turn-on voltage of 1 cd m2.
J. Sun et al. / Materials Chemistry and Physics 162 (2015) 392e399
chains enlarge the steric hindrance by separating the carbazole groups and green iridium center that restricts the concentration quenching and tripletetriplet annihilation. Our results have provided information to reduce the complexity and cost of fabricating efficient organic light emitting devices for displays and lighting applications. Acknowledgments This work was financially supported by Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-13-0927); International Science & Technology Cooperation Program of China (2012DFR50460); National Natural Scientific Foundation of China (11004218, 21101111, 61274056, 61205179, 61307030, 61307029); Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.06.005. References [1] A.D. Becke, Phys. Rev. A 38 (1988) 3098e3100. [2] T. Hisano, M. Ichikawa, K. Tsumoto, M. Tasaki, Chem. Pharm. Bull. 30 (1982) 2996e3004. [3] H.F. Chen, L.C. Chi, W.Y. Hung, W.J. Chen, T.Y. Hwu, Y.H. Chen, S.H. Chou, E. Mondal, Y.H. Liu, K.T. Wong, Org. Electron. 13 (2012) 2671e2681. [4] H. Xu, Z.F. Xu, Z.Y. Yue, P.F. Yan, B. Wang, L.W. Jia, G.M. Li, W.B. Sun, J.W. Zhang, J. Phys. Chem. C 112 (2008) 15517e15525. [5] S.H. Chou, W.Y. Hung, C.M. Chen, Q.Y. Liu, Y.H. Liu, K.T. Wong, RSC Adv. 3 (2013) 13891e13900.
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