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Optical and electronic properties of phosphorescent iridium(III) complexes with phenylpyrazole and ancillary ligands
Synthetic Metals 159 (2009) 113–118
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Optical and electronic properties of phosphorescent iridium(III) complexes with phenylpyrazole and ancillary ligands Teng Fei, Xin Gu, Ming Zhang, Chunlei Wang, Muddasir Hanif, Houyu Zhang ∗ , Yuguang Ma ∗ State Key Laboratory for Supramolecular Structure and Materials, Jilin University, Changchun, 130012, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 11 August 2008 Accepted 14 August 2008 Available online 30 September 2008 Keywords: DFT Phosphorescent Iridium Ancillary ligand Cyclometalating ligand
a b s t r a c t Phosphorescent Ir(III) complexes Ir(ppz)3 , Ir(ppz)2 (acac), Ir(ppz)2 (pic) and Ir(ppz)2 (dbm) (here ppz = phenylpyrazole, acac = acetylacetonate, dbm = dibenzoylmethane, and pic = picolinate) have been synthesized and investigated by optical spectroscopy, electrochemistry as well as density functional theory (DFT) calculations. These complexes show either no emission or medium intensity emission in solution or solid state at room temperature, but exhibit very strong emission from blue (422 nm) to orange-red (587 nm) at low temperature (77 K). Combined experimental and theoretical study, we reveal that replacing one of ppz ligand by ancillary ligand acac, dbm, and pic is a feasible way to alter the electronic structures of complexes resulting in changing emission colors. Both the cyclic voltammetry and DFT study testify that ancillary ligands have little influence on the highest occupied molecular orbital (HOMO) but great effect on lowest unoccupied molecular orbital (LUMO) by lowering the LUMO levels dramatically. The decreased LUMO level induced by the ancillary ligand makes the excited state of the complex far from the non-radiative states, which might enhance the quantum efficiency. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Phosphorescent transition metal complexes such as Re(I) [1], Ru(II) [2], Os(II) [3] and Ir(III) [4] complexes have attracted considerable attention due to their applications in organic light-emitting diodes (OLEDs). These complexes can capture both singlet and triplet excitons and exhibit higher internal quantum efficiency up to 100% in principle [3a,5]. Among these complexes, Ir(III) complexes are regarded as the most effective materials in OLEDs due to their high phosphorescence efficiency. It is previously found that the emission color of Ir(III) complexes is strongly governed by the nature of cyclometalating ligand and is little influenced by ancillary ligands [4c,4f,4g,6]. But such a standpoint is changed since the emission colors of Ir(dfppy)2 (LX) (LX = ancillary ligand) complexes are tuned from blue to red by changing ancillary ligands [7]. Modified complex structures based on ancillary ligands are reported in succession [8]. Phenylpyrazole (ppz) and its derivatives are typical of strongfield cyclometalating ligands which can be introduced in the coordination of Ir(III) complexes to achieve high energy emission [6c]. Complex Ir(ppz)3 shows rather strong deep blue emission at 77 K in CH2 Cl2 glassy matrix, but quite poor emission at room temperature. It is considered that non-radiative decay of Ir(ppz)3 at
ambient temperature are possible through higher thermally activated metal-to-ligand charge transfer (MLCT) states or MLCT to metal dd state conversion [9,10]. Thus a feasible approach to achieve highly efficient emission from ppz-based Ir(III) complexes is to lower the energy level of radiative state by using ancillary ligands and preventing quenching by thermally accessible non-radiative states at room temperature. Spectroscopic and quantum chemical study could provide the detailed information on how ancillary ligands affect the optical properties of Ir(III) complexes. In this work, a series of Ir(III) complexes with cyclometalating ligand ppz and ancillary ligands are synthesized. The chemical structures are shown in Fig. 1. By using different ancillary ligands, the emission colors change from blue (422 nm) to orange-red (587 nm) at low temperature (77 K). Furthermore, the complex Ir(ppz)2 (pic) shows relatively stronger green emission (526 nm) at room temperature. Herein, we show a demonstration on how ancillary ligands affect the optical properties. Combined experimental and theoretical study, we are aiming at the deep understanding of the mechanism of the color tuning and relationship between the electronic structures and optical properties. 2. Experimental details 2.1. General information
∗ Corresponding author. E-mail addresses: [email protected] (H. Zhang), [email protected] (Y. Ma). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.08.004
The 1 H NMR spectra were measured on AVANCZ 500 spectrometers at 298 K using DMSO as solvent and tetramethylsilicane (TMS)
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Fig. 1. The structures of four Ir(III) complexes.
as standard. Elemental analyses of carbon, hydrogen, and nitrogen were performed by Flash EA 1112, CHNS-O elemental analysis instrument. Thermal gravimetric analysis (TGA) were undertaken on a Perkin-Elmer thermal analysis system under a heating rate of 20 ◦ C/min and a nitrogen flow rate of 80 ml/min. Differential scanning calorimetry (DSC) were performed on a NETZSCH (DSC-204) unit at a heating rate of 10 ◦ C/min under nitrogen. UV–vis absorption spectra were recorded on a UV-3100 spectrophotometer. Photoluminescent (PL) spectra were carried out with a RF-5301PC fluorometer. The electrochemical properties of the Ir(III) complexes were examined by cyclic voltammetry (CV) and performed by using a standard one-compartment, three-electrode electrochemical cell given by a BAS 100B/W electrochemical analyzer. The potentials were measured against an Ag/Ag+ (0.1 M AgNO3 ) reference electrode with ferrocene as the internal standard. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. 2.2. Synthesis and characterization of the materials All synthetic procedures were carried out under nitrogen atmosphere. Cyclometalated chloride-bridge dimer (ppz)2 Ir(Cl)2 Ir(ppz)2 was synthesized according to the procedure described in the literature [11], by refluxing IrCl3· nH2 O with 2–2.5 equiv. of cyclometalating ligand in a 3:1 mixture of 2-ethoxyethanol and water. Ir(ppz)3 was prepared following the description in the literature [6c]. The chloro-bridged dimer complex (0.11 mmol), 2.5 equiv. of phenylpyrazole (0.27 mmol), and 8 equiv of potassium carbonate (0.88 mmol) were heated to 200 ◦ C in 12 ml of glycerol for 24 h. The reaction mixture was cooled to room temperature and 24 ml deionized water was added to precipitate the product. The resulting precipitate was filtered and then flash chromatographed on a silica column eluting with dichloromethane to provide purified fac-Ir(ppz)3 . For preparation of mixed ligand complex Ir(ppz)2 (acac), chloro-bridged dimer complex (0.05 mmol), acetylacetonate (0.15 mmol), and sodium carbonate (0.50 mmol) were refluxed in 2-ethoxyethanol for 15 h [4c]. After cooling to room temperature, the precipitate was filtered and washed with water and ether. The crude product was applied to column chromatography on silica gel to provide the desired product. The similar procedure was adopted to synthesize Ir(ppz)2 pic and Ir(ppz)2 dbm. Iridium tris(1-phenylpyrazolato-N,C2 ) (Ir(ppz)3 ): Yield: 46.7%. ◦ ◦ 1 mp: 370 C. TGA(5%) : 225 C. H NMR (500 MHz, DMSO-d6): ı (ppm) 8.65 (d, J = 2.1 Hz, 1H),7.49 (d, J = 7.9 Hz, 1H), 7.00 (s, 1H), 6.84 (m, 1H), 6.64 (d, J = 4.1 Hz, 2H), 6.56 (s, 1H). Anal. Calcd for C27 H21 N6 Ir: C, 52.16; H, 3.40; N, 13.52. Found: C, 52.26; H, 3.53; N, 13.32. Iridium bis(1-phenylpyrazolato-N,C2 ) (acetylacetonate) (Ir(ppz)2 ◦ (acac)): Yield: 83.7%. mp: 316 C. TGA(5%) : 300 ◦ C. 1 H NMR (500 MHz, DMSO-d6): ı (ppm) 8.76 (d, J = 2.7 Hz, 2H), 7.62 (d, J = 1.8 Hz, 2H), 7.47 (d, J = 7.8 Hz, 2H), 6.80 (m, 4H), 6.55 (t, J = 7.4 Hz,
2H), 6.04 (d, J = 7.4 Hz, 2H), 5.21 (s, 1H), 1.72 (s, 6H). Anal. Calcd for C23 H21 N4 O2 Ir: C, 47.82; H, 3.66; N, 9.70. Found: C, 47.59; H, 3.44; N, 9.88. Iridium bis(1-phenylpyrazolato-N,C2 ) (picolinate) (Ir(ppz)2 (pic)): ◦ ◦ Yield: 82.0%. mp: 375 C. TGA(5%) : 364 C. 1 H NMR (500 MHz, DMSOd6): ı (ppm) 8.85 (d, J = 2.8 Hz, 1H), 8.83 (d, J = 2.8 Hz, 1H), 8.09 (m, 2H), 7.79 (d, J = 5.3 Hz, 1H), 7.63 (d, J = 2.2 Hz, 1H), 7.60 (m, 3H), 7.05 (d, J = 2.2 Hz, 1H), 6.93 (m, 1H), 6.89 (m, 1H), 6.77 (t, J = 2.5 Hz, 1H), 6.72 (m, 2H), 6.67 (t, J = 7.4 Hz, 1H), 6.20 (dd, J = 7.5, 1.1 Hz, 1H), 6.08 (dd, J = 7.5, 1.1 Hz, 1H). Anal. Calcd for C24 H18 N5 O2 Ir: C, 47.99; H, 3.02; N, 11.66. Found: C, 48.04; H, 3.00; N, 11.69. (dibenzoylmethane) Iridium bis(1-phenylpyrazolato-N,C2 ) ◦ (Ir(ppz)2 (dbm)): Yield: 30.8%. mp: 320 C. TGA(5%) : 310 ◦ C. 1 H NMR (500 MHz, DMSO-d6): ı (ppm) 8.82 (d, J = 2.8 Hz, 2H), 7.79 (d, J = 7.2 Hz, 4H), 7.63 (d, J = 2.2 Hz, 2H), 7.54 (d, J = 7.2 Hz, 2H), 7.48 (t, J = 7.3 Hz, 2H), 7.37 (t, J = 7.9 Hz, 4H), 6.84 (t, J = 7.4 Hz, 2H), 6.79 (t, J = 6.5 Hz, 2H), 6.63 (t, J = 7.4 Hz, 2H), 6.57 (s, 1H), 6.15 (dd, J = 7.5, 1.1 Hz, 2H). Anal. Calcd for C33 H25 N4 O2 Ir: C, 56.48; H, 3.59; N, 7.98. Found: C, 56.44; H, 3.35; N, 8.03. 2.3. Details of the calculations Calculations on the electronic ground state of the complexes were carried out using B3LYP density functional theory [12]. “Double-” quality basis sets were employed for ligands (6-31G*) [13] and the Ir (LANL2DZ). A relativistic effective core potential (ECP) [14] on Ir replaced the inner core electrons leaving the outer core [(5s)2 (5p)6 ] electrons and the (5d6 ) valence electrons of Ir(III). The ground-state B3LYP calculations were carried out using Gaussian03 package [15]. 3. Results and discussion 3.1. Optical properties Fig. 2 shows the absorption spectra of four complexes in CH2 Cl2 solution. As can be seen, four complexes show similar absorption spectra. The dominant absorption bands at short wavelength (<320 nm) are attributed to the spin-allowed ligand-centered absorption. The weak and broad absorptions with lower energy extending into the visible region from 320 to 420 nm can be considered to be metal-to-ligand charge transfer (MLCT) transitions [6a]. The PL spectra of the complexes were measured at 77 K in CH2 Cl2 glassy matrix (see Fig. 3). The spectra exhibit a large bathochromic shift (165 nm range) from blue (422 nm) to orange-red (587 nm), depending on ancillary ligands. Ir(ppz)3 , Ir(ppz)2 (acac) and Ir(ppz)2 (dbm) show very weak emission in solution or in solid state at room temperature but exhibit strong emission at low temperature of 77 K. Evidently, the absence of luminescence at room temperature indicates that the rate of non-
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Fig. 2. The UV-absorption spectra of four Ir(III) complexes in CH2 Cl2 solution. Fig. 4. Cyclic voltammograms of four ligands in acetonitrile.
radiative decay is significantly greater than the radiative rate. It is noteworthy that complex Ir(ppz)2 (pic) using pic as the ancillary ligand shows strong green emission both in the solid state and solution at room temperature, the PL quantum efficiency of Ir(ppz)2 (pic) in a solid-state film is about 19% [16]. It can be found from both absorption and emission spectra that ancillary ligands have prominent effects on the photophysical properties of Ir(ppz)2 (LX) complexes. Compared with the traditional method of altering the structure of the cyclometalating ligand to tune the emission of Ir(III) complexes, changing the ancillary ligands is more simple and convenient to develop new materials. The role of the ancillary ligands in the electronic properties of heteroleptic Ir(III) complexes deserves to be well explored by all means. 3.2. Cyclic voltammetry The electrochemical properties of ligands and Ir(III) complexes were examined by cyclic voltammetry. The voltammograms of ppz, acac, pic and dbm are shown in Fig. 4. The initial oxidation potentials of ppz, acac and dbm are 1.42, 1.60 and 1.42 V, respectively. For pic, the clear oxidation peak was not observable up to 3.2 V. The reduction of acac, dbm and pic start with potentials at −2.26, −1.66 and −1.36 V, respectively. As for ppz, the clear reduction peak does not show up till −3.0 V. Because of the absence of oxidation of pic and
Fig. 3. The PL spectra of four Ir(III) complexes at 77 K in CH2 Cl2 glassy matrix.
reduction of ppz, the energy gaps of ppz and pic are estimated from their respective onset wavelengths of absorption spectra in THF solution. The HOMO and LUMO levels can be calculated by the forOx Red + 4.66) eV. mula EHOMO = −(Eonset + 4.66) eV and ELUMO = −(Eonset Fig. 5 displays the calculated levels of HOMO and LUMO of the ligands. The HOMO–LUMO energy gaps of ppz, pic, and acac are similar (about 4.1 eV), and much bigger than that of dbm (2.78 eV). Except that pic has much lower HOMO level, the other three ligands have similar HOMO level. But for LUMO, they have different LUMO levels which would affect the reduction potentials of the complexes. The voltammograms of the four complexes are shown in Fig. 6. Redox potentials and calculated HOMO and LUMO levels are summarized in Table 1. All complexes show a reversible one-electron oxidation process. The onset oxidation potentials of the four complexes are 0.47, 0.55, 0.70 and 0.55 V, with oxidation peaks at 0.57, 0.66, 0.80 and 0.66 V, respectively. Ir(ppz)2 (pic) shows the highest oxidation potential. The similar oxidation potentials of the complexes indicate that HOMO is little affected by the ancillary ligands and oxidation process mainly occurs in the metal and ppz ligand. However, great differences for the reductions are observed for Ir(ppz)2 (acac), Ir(ppz)2 (pic) and Ir(ppz)2 (dbm). For Ir(ppz)3 , no reduction is observed up to −3.0 V. The absence of reduction process for Ir(ppz)3 indicates ppz is more difficult to be reduced than acac, pic and dbm, as shown in Fig. 4. Ir(ppz)2 (acac) shows a
Fig. 5. Schematic drawing of HOMO and LUMO energy levels of four ligands.
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T. Fei et al. / Synthetic Metals 159 (2009) 113–118 Table 2 Frontier orbitals compositions of four Ir(III) complexes from DFT
Fig. 6. Cyclic voltammograms of four Ir(III) complexes in acetonitrile.
quasi-reversible reduction starting at −2.73 V with reduction peak potential at −2.90 V, Ir(ppz)2 (pic) has a reversible reduction process starting at −2.18 V with reduction peak potential at −2.30 V, while Ir(ppz)2 (dbm) has two reversible reduction processes starting at −1.70 V with reduction peak potentials at −1.82 and −2.16 V. The reduction potentials of Ir(ppz)2 (pic) and Ir(ppz)2 (dbm) decrease due to relative lower LUMO levels of pic and dbm. Because of the addition of acac ligand, Ir(ppz)2 (acac) presents a reduction process at a still high potential (−2.90 V). It is strange that Ir(ppz)2 (acac) has similar HOMO and LUMO energies to those of Ir(ppz)3, but its triplet energy is much lower than that of Ir(ppz)3 (Table 1), it might be due to a low energy “Ir-acac centered” excited state [17]. 3.3. DFT calculations To explore the nature of effect of ancillary ligands on the complexes, density functional theory calculations were performed for complexes. The compositions of HOMO and LUMO of the four complexes are analyzed and listed in Table 2. The energy levels and contour plot of the HOMO and LUMO are shown in Fig. 7. The electronic densities of the complexes for HOMOs are mainly based on the d-orbital of the metal and the phenyl part of cyclometalating ligand, which is consistent with previous results [18,19]. The contributions from metallic d-orbital for the HOMOs are about 50%, while ancillary ligands have little contribution to HOMOs. The HOMO energy levels of the four complexes are around −5.0 eV, which is easy to understand from the similar HOMO compositions for the complexes. Ir(ppz)2 (pic) has a lower HOMO due to 7% contribution from pic, whose HOMO level is lowest in four ligands (Fig. 5). For Ir(ppz)3 and Ir(ppz)2 (acac), LUMO is mostly contributed by ppz (more than 94%) and LUMO levels of Ir(ppz)3 and Ir(ppz)2 (acac) are −0.55 and −0.69 eV, respectively. This means acac has little influence on LUMO level. But for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic), LUMO is mostly contributed by dbm and pic (more than 93%), respectively. The LUMO levels decrease for Ir(ppz)2 (pic) (−1.45 eV)
MOs
Ir(ppz)3
Ir(ppz)2 (acac)
Ir(ppz)2 (pic)
Ir(ppz)2 (dbm)
LUMO
1% Ir 99% ppz
4% Ir 94% ppz 2% acac
3% Ir 2% ppz 95% pic
2% Ir 5% ppz 93% dbm
HOMO
52% Ir 48% ppz
52% Ir 42% ppz 6% acac
48% Ir 45% ppz 7% pic
51% Ir 42% ppz 7% dbm
and Ir(ppz)2 (dbm) (−1.42 eV) compared with that of Ir(ppz)3 and Ir(ppz)2 (acac). Based on the results given by DFT, the ancillary ligands influence the electronic properties of Ir(ppz)2 (LX) complexes mostly by contributing to LUMO, while leaving HOMO almost unchanged. This conclusion is consistent with the CV measurements of the complexes in which all of the complexes have similar oxidation processes while reduction processes change greatly as ancillary ligands varied. From HOMO and LUMO compositions, we can understand that oxidation process is a metal-aryl centered process for each complex, while reduction happens on ppz for Ir(ppz)3 and Ir(ppz)2 (acac) (high reduction potentials), and reduction occurs on dbm and pic for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic) (low reduction potentials), respectively. From the discussion above, it is clear that complexes Ir(ppz)3 and Ir(ppz)2 (acac) having very high LUMO levels can be used as electron blocking layer in OLEDs [17]. The excited states of complexes Ir(ppz)3 and Ir(ppz)2 (acac) are less stable and could decay non-radiatively through higher thermally activated MLCT states or MLCT to dd state conversion at room temperature [9,10], then the emission is quenched almost completely. The LUMO levels decrease much for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic) compared with that of Ir(ppz)3 , but only Ir(ppz)2 (pic) shows relatively stronger green emission. It might be due to the nature of the lower LUMO in pic
Fig. 7. The energy levels and contour plot of the HOMO and LUMO of four Ir(III) complexes from DFT calculations.
Table 1 Electrochemical and photophysical properties of four Ir(III) complexes Complexes Ir(ppz)3 Ir(ppz)2 (acac) Ir(ppz)2 (pic) Ir(ppz)2 (dbm) a b c
Ox Eonset (V)
HOMOa (eV)
0.47 0.55 0.70 0.55
−5.13 −5.21 −5.36 −5.21
Red Eonset (V)
LUMOa (eV)
max,em (nm)
T1 b (eV)
– −2.73 −2.18 −1.82
−1.93 −1.93 −2.48 −2.84
422 514 507 587
2.94 2.41 2.45 2.11
c
Ox Red The HOMO and LUMO energy levels were determined using following formula: EHOMO = −(Eonset + 4.66) eV ELUMO = −(Eonset + 4.66) eV. The triplet energy was estimated by the max of the phosphorescence spectra. Estimated using the optical energy gap and HOMO value.
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and its aromatic structure, which is responsible for the qualitative change in phosphorescence properties. When LUMO is mostly contributed by dbm, the efficiency of Ir(ppz)2 (dbm) is still not high, which may be due to its poor conjugation, the vibration of two benzenes can quench the energy of the excited state. 3.4. Thermal analysis of complexes and electroluminescent properties of Ir(ppz)2 (pic) The TGA indicates that all complexes exhibit good thermal stability with 5% weight loss temperatures ranging from 225 to 364 ◦ C. DSC analysis revealed that the melting point (mp) of the complexes range from 316 to 375 ◦ C. The thermal stability of complexes ensures that those complexes can be used as emissive layers in light-emitting devices. Ir(ppz)2 (pic) shows bright emission at room temperature under air atmosphere, therefore, we fabricated electroluminescence (EL) devices using Ir(ppz)2 (pic) as dopant in the emitting layer. The multiplayer device structure investigated here was ITO/m-MTDATA:2 wt.% F4 -TCNQ (100 nm)/NPB (10 nm)/mcp:8 wt.% Ir(ppz)2 (pic) (30 nm)/TPBI (40 nm)/LiF (0.6 nm)/Al (100 nm)(Fig. 8). 4,4 ,4 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) doped with 2 wt.% tetrafluoro-tetracyanoquinodimethane (F4 -TCNQ) was used as the hole-injecting layer, 4,4 -bis[N-(1naphthyl)-N-phenylamino]biphenyl (NPB) was used as the holetransporting layer, N,N -dicarbazolyl-3,5-benzene (mcp) doped with 8 wt.% Ir(ppz)2 (pic) acted as the emitting layer, 2,2 ,2 (1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBI) was the electron-transporting layer and a thin LiF served as an electron injection layer at the Al cathode interface. The EL spectra of the device exhibited green emission with a peak at max = 520 nm (Fig. 8), similar to that of the complex in CH2 Cl2 glassy matrix. The current density–voltage–luminance (J–V–L) characteristics of the device are shown in Fig. 9. A turn on voltage of 4.0 V and a maximum brightness of 1217 cd/m2 at 11.5 V are obtained. The device gives a maximum current efficiency of 5.9 cd/A and a maximum power efficiency up to 4.6 lm/W at a current density of 0.07 mA/cm2 , corresponding to an external quantum efficiency of 2.0%. The quantum efficiency of Ir(ppz)2 (pic) is not as high as the green emission material Ir(ppy)3 , but in some sense, it motivates us to use ancillary ligands to modify the metal complex and improve quantum efficiency. Altering the ancillary ligands is a possible way from structural design to getting matched metal to mixed lig-
Fig. 9. Current density–voltage–luminance (J–V–L) characteristics of the device.
ands with promising properties, which deserves in-depth study in future. 4. Conclusion We have synthesized four Ir(III) complexes with broad range color emission by using different ancillary ligands. Ir(ppz)2 (pic) emits strongly at room temperature and shows good electroluminescent behavior. According to the study of the energy levels of ancillary ligands, we found that the ancillary ligands influence the electronic properties of Ir(ppz)2 (LX) complexes by stabilizing LUMO, while leaving HOMO almost unchanged. When LUMO level of ancillary ligand is lower than that of ppz, it results in lowering LUMO for the complex, which makes the excited state far from the non-radiative states, leading to enhanced efficiency. The results indicate that the ancillary ligands can be used to tune the emission color and luminescence efficiency of Ir(III) complexes, the importance of ancillary ligands should be noted in designing new Ir(III) complexes. Acknowledgments We thank the financial support from National Science Foundation of China (grant numbers 20573040, 20474024, 20704015, 90501001, 50303007), Ministry of Science and Technology of China (grant number 2002CB6134003) and PCSIRT. References
Fig. 8. Electroluminescence spectrum of the device at 6 V (inset: configuration of the device).
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[11] M. Nonoyama, Bull. Chem. Soc. Jpn. 47 (1974) 767. [12] (a) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1998) 785; (b) A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [13] A.E. Frisch, M.J. Frisch, Gaussian 98 User’s Reference; Gaussian, Inc., Pittsburgh, PA, 1998 (see also references therein). [14] W. Koch, M.C. Holthausen, A Chemist’s Guide to Density Functional Theory, Wiley-VCH, Weinheim, Germany, 2000. [15] M.J. Frisch, et al., Gaussian 03, Revision B. 05, Gaussian, Inc., Wallingford, CT, 2004. [16] Ir(ppz)2 (pic) was dispersed in inert host polymethylmethacrylate at 8.0 wt.%, the PL of the film was measured by an integrating sphere, using tris(8-quinolinolato)aluminum(III) as the standard fluorescent emitter. Tris(8quinolinolato)aluminum(III) showed an PL of 20 (±1)% in our system. [17] V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W. D’Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [18] P.J. Hay, J. Phys. Chem. A 106 (2002) 1634. [19] N.G. Park, G.C. Choi, J.E. Lee, Y.S. Kim, Curr. Appl. Phys. 5 (2005) 79.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Optical and electronic properties of phosphorescent iridium(III) complexes with phenylpyrazole and ancillary ligands Teng Fei, Xin Gu, Ming Zhang, Chunlei Wang, Muddasir Hanif, Houyu Zhang ∗ , Yuguang Ma ∗ State Key Laboratory for Supramolecular Structure and Materials, Jilin University, Changchun, 130012, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 11 August 2008 Accepted 14 August 2008 Available online 30 September 2008 Keywords: DFT Phosphorescent Iridium Ancillary ligand Cyclometalating ligand
a b s t r a c t Phosphorescent Ir(III) complexes Ir(ppz)3 , Ir(ppz)2 (acac), Ir(ppz)2 (pic) and Ir(ppz)2 (dbm) (here ppz = phenylpyrazole, acac = acetylacetonate, dbm = dibenzoylmethane, and pic = picolinate) have been synthesized and investigated by optical spectroscopy, electrochemistry as well as density functional theory (DFT) calculations. These complexes show either no emission or medium intensity emission in solution or solid state at room temperature, but exhibit very strong emission from blue (422 nm) to orange-red (587 nm) at low temperature (77 K). Combined experimental and theoretical study, we reveal that replacing one of ppz ligand by ancillary ligand acac, dbm, and pic is a feasible way to alter the electronic structures of complexes resulting in changing emission colors. Both the cyclic voltammetry and DFT study testify that ancillary ligands have little influence on the highest occupied molecular orbital (HOMO) but great effect on lowest unoccupied molecular orbital (LUMO) by lowering the LUMO levels dramatically. The decreased LUMO level induced by the ancillary ligand makes the excited state of the complex far from the non-radiative states, which might enhance the quantum efficiency. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Phosphorescent transition metal complexes such as Re(I) [1], Ru(II) [2], Os(II) [3] and Ir(III) [4] complexes have attracted considerable attention due to their applications in organic light-emitting diodes (OLEDs). These complexes can capture both singlet and triplet excitons and exhibit higher internal quantum efficiency up to 100% in principle [3a,5]. Among these complexes, Ir(III) complexes are regarded as the most effective materials in OLEDs due to their high phosphorescence efficiency. It is previously found that the emission color of Ir(III) complexes is strongly governed by the nature of cyclometalating ligand and is little influenced by ancillary ligands [4c,4f,4g,6]. But such a standpoint is changed since the emission colors of Ir(dfppy)2 (LX) (LX = ancillary ligand) complexes are tuned from blue to red by changing ancillary ligands [7]. Modified complex structures based on ancillary ligands are reported in succession [8]. Phenylpyrazole (ppz) and its derivatives are typical of strongfield cyclometalating ligands which can be introduced in the coordination of Ir(III) complexes to achieve high energy emission [6c]. Complex Ir(ppz)3 shows rather strong deep blue emission at 77 K in CH2 Cl2 glassy matrix, but quite poor emission at room temperature. It is considered that non-radiative decay of Ir(ppz)3 at
ambient temperature are possible through higher thermally activated metal-to-ligand charge transfer (MLCT) states or MLCT to metal dd state conversion [9,10]. Thus a feasible approach to achieve highly efficient emission from ppz-based Ir(III) complexes is to lower the energy level of radiative state by using ancillary ligands and preventing quenching by thermally accessible non-radiative states at room temperature. Spectroscopic and quantum chemical study could provide the detailed information on how ancillary ligands affect the optical properties of Ir(III) complexes. In this work, a series of Ir(III) complexes with cyclometalating ligand ppz and ancillary ligands are synthesized. The chemical structures are shown in Fig. 1. By using different ancillary ligands, the emission colors change from blue (422 nm) to orange-red (587 nm) at low temperature (77 K). Furthermore, the complex Ir(ppz)2 (pic) shows relatively stronger green emission (526 nm) at room temperature. Herein, we show a demonstration on how ancillary ligands affect the optical properties. Combined experimental and theoretical study, we are aiming at the deep understanding of the mechanism of the color tuning and relationship between the electronic structures and optical properties. 2. Experimental details 2.1. General information
∗ Corresponding author. E-mail addresses: [email protected] (H. Zhang), [email protected] (Y. Ma). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.08.004
The 1 H NMR spectra were measured on AVANCZ 500 spectrometers at 298 K using DMSO as solvent and tetramethylsilicane (TMS)
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Fig. 1. The structures of four Ir(III) complexes.
as standard. Elemental analyses of carbon, hydrogen, and nitrogen were performed by Flash EA 1112, CHNS-O elemental analysis instrument. Thermal gravimetric analysis (TGA) were undertaken on a Perkin-Elmer thermal analysis system under a heating rate of 20 ◦ C/min and a nitrogen flow rate of 80 ml/min. Differential scanning calorimetry (DSC) were performed on a NETZSCH (DSC-204) unit at a heating rate of 10 ◦ C/min under nitrogen. UV–vis absorption spectra were recorded on a UV-3100 spectrophotometer. Photoluminescent (PL) spectra were carried out with a RF-5301PC fluorometer. The electrochemical properties of the Ir(III) complexes were examined by cyclic voltammetry (CV) and performed by using a standard one-compartment, three-electrode electrochemical cell given by a BAS 100B/W electrochemical analyzer. The potentials were measured against an Ag/Ag+ (0.1 M AgNO3 ) reference electrode with ferrocene as the internal standard. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. 2.2. Synthesis and characterization of the materials All synthetic procedures were carried out under nitrogen atmosphere. Cyclometalated chloride-bridge dimer (ppz)2 Ir(Cl)2 Ir(ppz)2 was synthesized according to the procedure described in the literature [11], by refluxing IrCl3· nH2 O with 2–2.5 equiv. of cyclometalating ligand in a 3:1 mixture of 2-ethoxyethanol and water. Ir(ppz)3 was prepared following the description in the literature [6c]. The chloro-bridged dimer complex (0.11 mmol), 2.5 equiv. of phenylpyrazole (0.27 mmol), and 8 equiv of potassium carbonate (0.88 mmol) were heated to 200 ◦ C in 12 ml of glycerol for 24 h. The reaction mixture was cooled to room temperature and 24 ml deionized water was added to precipitate the product. The resulting precipitate was filtered and then flash chromatographed on a silica column eluting with dichloromethane to provide purified fac-Ir(ppz)3 . For preparation of mixed ligand complex Ir(ppz)2 (acac), chloro-bridged dimer complex (0.05 mmol), acetylacetonate (0.15 mmol), and sodium carbonate (0.50 mmol) were refluxed in 2-ethoxyethanol for 15 h [4c]. After cooling to room temperature, the precipitate was filtered and washed with water and ether. The crude product was applied to column chromatography on silica gel to provide the desired product. The similar procedure was adopted to synthesize Ir(ppz)2 pic and Ir(ppz)2 dbm. Iridium tris(1-phenylpyrazolato-N,C2 ) (Ir(ppz)3 ): Yield: 46.7%. ◦ ◦ 1 mp: 370 C. TGA(5%) : 225 C. H NMR (500 MHz, DMSO-d6): ı (ppm) 8.65 (d, J = 2.1 Hz, 1H),7.49 (d, J = 7.9 Hz, 1H), 7.00 (s, 1H), 6.84 (m, 1H), 6.64 (d, J = 4.1 Hz, 2H), 6.56 (s, 1H). Anal. Calcd for C27 H21 N6 Ir: C, 52.16; H, 3.40; N, 13.52. Found: C, 52.26; H, 3.53; N, 13.32. Iridium bis(1-phenylpyrazolato-N,C2 ) (acetylacetonate) (Ir(ppz)2 ◦ (acac)): Yield: 83.7%. mp: 316 C. TGA(5%) : 300 ◦ C. 1 H NMR (500 MHz, DMSO-d6): ı (ppm) 8.76 (d, J = 2.7 Hz, 2H), 7.62 (d, J = 1.8 Hz, 2H), 7.47 (d, J = 7.8 Hz, 2H), 6.80 (m, 4H), 6.55 (t, J = 7.4 Hz,
2H), 6.04 (d, J = 7.4 Hz, 2H), 5.21 (s, 1H), 1.72 (s, 6H). Anal. Calcd for C23 H21 N4 O2 Ir: C, 47.82; H, 3.66; N, 9.70. Found: C, 47.59; H, 3.44; N, 9.88. Iridium bis(1-phenylpyrazolato-N,C2 ) (picolinate) (Ir(ppz)2 (pic)): ◦ ◦ Yield: 82.0%. mp: 375 C. TGA(5%) : 364 C. 1 H NMR (500 MHz, DMSOd6): ı (ppm) 8.85 (d, J = 2.8 Hz, 1H), 8.83 (d, J = 2.8 Hz, 1H), 8.09 (m, 2H), 7.79 (d, J = 5.3 Hz, 1H), 7.63 (d, J = 2.2 Hz, 1H), 7.60 (m, 3H), 7.05 (d, J = 2.2 Hz, 1H), 6.93 (m, 1H), 6.89 (m, 1H), 6.77 (t, J = 2.5 Hz, 1H), 6.72 (m, 2H), 6.67 (t, J = 7.4 Hz, 1H), 6.20 (dd, J = 7.5, 1.1 Hz, 1H), 6.08 (dd, J = 7.5, 1.1 Hz, 1H). Anal. Calcd for C24 H18 N5 O2 Ir: C, 47.99; H, 3.02; N, 11.66. Found: C, 48.04; H, 3.00; N, 11.69. (dibenzoylmethane) Iridium bis(1-phenylpyrazolato-N,C2 ) ◦ (Ir(ppz)2 (dbm)): Yield: 30.8%. mp: 320 C. TGA(5%) : 310 ◦ C. 1 H NMR (500 MHz, DMSO-d6): ı (ppm) 8.82 (d, J = 2.8 Hz, 2H), 7.79 (d, J = 7.2 Hz, 4H), 7.63 (d, J = 2.2 Hz, 2H), 7.54 (d, J = 7.2 Hz, 2H), 7.48 (t, J = 7.3 Hz, 2H), 7.37 (t, J = 7.9 Hz, 4H), 6.84 (t, J = 7.4 Hz, 2H), 6.79 (t, J = 6.5 Hz, 2H), 6.63 (t, J = 7.4 Hz, 2H), 6.57 (s, 1H), 6.15 (dd, J = 7.5, 1.1 Hz, 2H). Anal. Calcd for C33 H25 N4 O2 Ir: C, 56.48; H, 3.59; N, 7.98. Found: C, 56.44; H, 3.35; N, 8.03. 2.3. Details of the calculations Calculations on the electronic ground state of the complexes were carried out using B3LYP density functional theory [12]. “Double-” quality basis sets were employed for ligands (6-31G*) [13] and the Ir (LANL2DZ). A relativistic effective core potential (ECP) [14] on Ir replaced the inner core electrons leaving the outer core [(5s)2 (5p)6 ] electrons and the (5d6 ) valence electrons of Ir(III). The ground-state B3LYP calculations were carried out using Gaussian03 package [15]. 3. Results and discussion 3.1. Optical properties Fig. 2 shows the absorption spectra of four complexes in CH2 Cl2 solution. As can be seen, four complexes show similar absorption spectra. The dominant absorption bands at short wavelength (<320 nm) are attributed to the spin-allowed ligand-centered absorption. The weak and broad absorptions with lower energy extending into the visible region from 320 to 420 nm can be considered to be metal-to-ligand charge transfer (MLCT) transitions [6a]. The PL spectra of the complexes were measured at 77 K in CH2 Cl2 glassy matrix (see Fig. 3). The spectra exhibit a large bathochromic shift (165 nm range) from blue (422 nm) to orange-red (587 nm), depending on ancillary ligands. Ir(ppz)3 , Ir(ppz)2 (acac) and Ir(ppz)2 (dbm) show very weak emission in solution or in solid state at room temperature but exhibit strong emission at low temperature of 77 K. Evidently, the absence of luminescence at room temperature indicates that the rate of non-
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Fig. 2. The UV-absorption spectra of four Ir(III) complexes in CH2 Cl2 solution. Fig. 4. Cyclic voltammograms of four ligands in acetonitrile.
radiative decay is significantly greater than the radiative rate. It is noteworthy that complex Ir(ppz)2 (pic) using pic as the ancillary ligand shows strong green emission both in the solid state and solution at room temperature, the PL quantum efficiency of Ir(ppz)2 (pic) in a solid-state film is about 19% [16]. It can be found from both absorption and emission spectra that ancillary ligands have prominent effects on the photophysical properties of Ir(ppz)2 (LX) complexes. Compared with the traditional method of altering the structure of the cyclometalating ligand to tune the emission of Ir(III) complexes, changing the ancillary ligands is more simple and convenient to develop new materials. The role of the ancillary ligands in the electronic properties of heteroleptic Ir(III) complexes deserves to be well explored by all means. 3.2. Cyclic voltammetry The electrochemical properties of ligands and Ir(III) complexes were examined by cyclic voltammetry. The voltammograms of ppz, acac, pic and dbm are shown in Fig. 4. The initial oxidation potentials of ppz, acac and dbm are 1.42, 1.60 and 1.42 V, respectively. For pic, the clear oxidation peak was not observable up to 3.2 V. The reduction of acac, dbm and pic start with potentials at −2.26, −1.66 and −1.36 V, respectively. As for ppz, the clear reduction peak does not show up till −3.0 V. Because of the absence of oxidation of pic and
Fig. 3. The PL spectra of four Ir(III) complexes at 77 K in CH2 Cl2 glassy matrix.
reduction of ppz, the energy gaps of ppz and pic are estimated from their respective onset wavelengths of absorption spectra in THF solution. The HOMO and LUMO levels can be calculated by the forOx Red + 4.66) eV. mula EHOMO = −(Eonset + 4.66) eV and ELUMO = −(Eonset Fig. 5 displays the calculated levels of HOMO and LUMO of the ligands. The HOMO–LUMO energy gaps of ppz, pic, and acac are similar (about 4.1 eV), and much bigger than that of dbm (2.78 eV). Except that pic has much lower HOMO level, the other three ligands have similar HOMO level. But for LUMO, they have different LUMO levels which would affect the reduction potentials of the complexes. The voltammograms of the four complexes are shown in Fig. 6. Redox potentials and calculated HOMO and LUMO levels are summarized in Table 1. All complexes show a reversible one-electron oxidation process. The onset oxidation potentials of the four complexes are 0.47, 0.55, 0.70 and 0.55 V, with oxidation peaks at 0.57, 0.66, 0.80 and 0.66 V, respectively. Ir(ppz)2 (pic) shows the highest oxidation potential. The similar oxidation potentials of the complexes indicate that HOMO is little affected by the ancillary ligands and oxidation process mainly occurs in the metal and ppz ligand. However, great differences for the reductions are observed for Ir(ppz)2 (acac), Ir(ppz)2 (pic) and Ir(ppz)2 (dbm). For Ir(ppz)3 , no reduction is observed up to −3.0 V. The absence of reduction process for Ir(ppz)3 indicates ppz is more difficult to be reduced than acac, pic and dbm, as shown in Fig. 4. Ir(ppz)2 (acac) shows a
Fig. 5. Schematic drawing of HOMO and LUMO energy levels of four ligands.
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T. Fei et al. / Synthetic Metals 159 (2009) 113–118 Table 2 Frontier orbitals compositions of four Ir(III) complexes from DFT
Fig. 6. Cyclic voltammograms of four Ir(III) complexes in acetonitrile.
quasi-reversible reduction starting at −2.73 V with reduction peak potential at −2.90 V, Ir(ppz)2 (pic) has a reversible reduction process starting at −2.18 V with reduction peak potential at −2.30 V, while Ir(ppz)2 (dbm) has two reversible reduction processes starting at −1.70 V with reduction peak potentials at −1.82 and −2.16 V. The reduction potentials of Ir(ppz)2 (pic) and Ir(ppz)2 (dbm) decrease due to relative lower LUMO levels of pic and dbm. Because of the addition of acac ligand, Ir(ppz)2 (acac) presents a reduction process at a still high potential (−2.90 V). It is strange that Ir(ppz)2 (acac) has similar HOMO and LUMO energies to those of Ir(ppz)3, but its triplet energy is much lower than that of Ir(ppz)3 (Table 1), it might be due to a low energy “Ir-acac centered” excited state [17]. 3.3. DFT calculations To explore the nature of effect of ancillary ligands on the complexes, density functional theory calculations were performed for complexes. The compositions of HOMO and LUMO of the four complexes are analyzed and listed in Table 2. The energy levels and contour plot of the HOMO and LUMO are shown in Fig. 7. The electronic densities of the complexes for HOMOs are mainly based on the d-orbital of the metal and the phenyl part of cyclometalating ligand, which is consistent with previous results [18,19]. The contributions from metallic d-orbital for the HOMOs are about 50%, while ancillary ligands have little contribution to HOMOs. The HOMO energy levels of the four complexes are around −5.0 eV, which is easy to understand from the similar HOMO compositions for the complexes. Ir(ppz)2 (pic) has a lower HOMO due to 7% contribution from pic, whose HOMO level is lowest in four ligands (Fig. 5). For Ir(ppz)3 and Ir(ppz)2 (acac), LUMO is mostly contributed by ppz (more than 94%) and LUMO levels of Ir(ppz)3 and Ir(ppz)2 (acac) are −0.55 and −0.69 eV, respectively. This means acac has little influence on LUMO level. But for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic), LUMO is mostly contributed by dbm and pic (more than 93%), respectively. The LUMO levels decrease for Ir(ppz)2 (pic) (−1.45 eV)
MOs
Ir(ppz)3
Ir(ppz)2 (acac)
Ir(ppz)2 (pic)
Ir(ppz)2 (dbm)
LUMO
1% Ir 99% ppz
4% Ir 94% ppz 2% acac
3% Ir 2% ppz 95% pic
2% Ir 5% ppz 93% dbm
HOMO
52% Ir 48% ppz
52% Ir 42% ppz 6% acac
48% Ir 45% ppz 7% pic
51% Ir 42% ppz 7% dbm
and Ir(ppz)2 (dbm) (−1.42 eV) compared with that of Ir(ppz)3 and Ir(ppz)2 (acac). Based on the results given by DFT, the ancillary ligands influence the electronic properties of Ir(ppz)2 (LX) complexes mostly by contributing to LUMO, while leaving HOMO almost unchanged. This conclusion is consistent with the CV measurements of the complexes in which all of the complexes have similar oxidation processes while reduction processes change greatly as ancillary ligands varied. From HOMO and LUMO compositions, we can understand that oxidation process is a metal-aryl centered process for each complex, while reduction happens on ppz for Ir(ppz)3 and Ir(ppz)2 (acac) (high reduction potentials), and reduction occurs on dbm and pic for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic) (low reduction potentials), respectively. From the discussion above, it is clear that complexes Ir(ppz)3 and Ir(ppz)2 (acac) having very high LUMO levels can be used as electron blocking layer in OLEDs [17]. The excited states of complexes Ir(ppz)3 and Ir(ppz)2 (acac) are less stable and could decay non-radiatively through higher thermally activated MLCT states or MLCT to dd state conversion at room temperature [9,10], then the emission is quenched almost completely. The LUMO levels decrease much for Ir(ppz)2 (dbm) and Ir(ppz)2 (pic) compared with that of Ir(ppz)3 , but only Ir(ppz)2 (pic) shows relatively stronger green emission. It might be due to the nature of the lower LUMO in pic
Fig. 7. The energy levels and contour plot of the HOMO and LUMO of four Ir(III) complexes from DFT calculations.
Table 1 Electrochemical and photophysical properties of four Ir(III) complexes Complexes Ir(ppz)3 Ir(ppz)2 (acac) Ir(ppz)2 (pic) Ir(ppz)2 (dbm) a b c
Ox Eonset (V)
HOMOa (eV)
0.47 0.55 0.70 0.55
−5.13 −5.21 −5.36 −5.21
Red Eonset (V)
LUMOa (eV)
max,em (nm)
T1 b (eV)
– −2.73 −2.18 −1.82
−1.93 −1.93 −2.48 −2.84
422 514 507 587
2.94 2.41 2.45 2.11
c
Ox Red The HOMO and LUMO energy levels were determined using following formula: EHOMO = −(Eonset + 4.66) eV ELUMO = −(Eonset + 4.66) eV. The triplet energy was estimated by the max of the phosphorescence spectra. Estimated using the optical energy gap and HOMO value.
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and its aromatic structure, which is responsible for the qualitative change in phosphorescence properties. When LUMO is mostly contributed by dbm, the efficiency of Ir(ppz)2 (dbm) is still not high, which may be due to its poor conjugation, the vibration of two benzenes can quench the energy of the excited state. 3.4. Thermal analysis of complexes and electroluminescent properties of Ir(ppz)2 (pic) The TGA indicates that all complexes exhibit good thermal stability with 5% weight loss temperatures ranging from 225 to 364 ◦ C. DSC analysis revealed that the melting point (mp) of the complexes range from 316 to 375 ◦ C. The thermal stability of complexes ensures that those complexes can be used as emissive layers in light-emitting devices. Ir(ppz)2 (pic) shows bright emission at room temperature under air atmosphere, therefore, we fabricated electroluminescence (EL) devices using Ir(ppz)2 (pic) as dopant in the emitting layer. The multiplayer device structure investigated here was ITO/m-MTDATA:2 wt.% F4 -TCNQ (100 nm)/NPB (10 nm)/mcp:8 wt.% Ir(ppz)2 (pic) (30 nm)/TPBI (40 nm)/LiF (0.6 nm)/Al (100 nm)(Fig. 8). 4,4 ,4 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) doped with 2 wt.% tetrafluoro-tetracyanoquinodimethane (F4 -TCNQ) was used as the hole-injecting layer, 4,4 -bis[N-(1naphthyl)-N-phenylamino]biphenyl (NPB) was used as the holetransporting layer, N,N -dicarbazolyl-3,5-benzene (mcp) doped with 8 wt.% Ir(ppz)2 (pic) acted as the emitting layer, 2,2 ,2 (1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBI) was the electron-transporting layer and a thin LiF served as an electron injection layer at the Al cathode interface. The EL spectra of the device exhibited green emission with a peak at max = 520 nm (Fig. 8), similar to that of the complex in CH2 Cl2 glassy matrix. The current density–voltage–luminance (J–V–L) characteristics of the device are shown in Fig. 9. A turn on voltage of 4.0 V and a maximum brightness of 1217 cd/m2 at 11.5 V are obtained. The device gives a maximum current efficiency of 5.9 cd/A and a maximum power efficiency up to 4.6 lm/W at a current density of 0.07 mA/cm2 , corresponding to an external quantum efficiency of 2.0%. The quantum efficiency of Ir(ppz)2 (pic) is not as high as the green emission material Ir(ppy)3 , but in some sense, it motivates us to use ancillary ligands to modify the metal complex and improve quantum efficiency. Altering the ancillary ligands is a possible way from structural design to getting matched metal to mixed lig-
Fig. 9. Current density–voltage–luminance (J–V–L) characteristics of the device.
ands with promising properties, which deserves in-depth study in future. 4. Conclusion We have synthesized four Ir(III) complexes with broad range color emission by using different ancillary ligands. Ir(ppz)2 (pic) emits strongly at room temperature and shows good electroluminescent behavior. According to the study of the energy levels of ancillary ligands, we found that the ancillary ligands influence the electronic properties of Ir(ppz)2 (LX) complexes by stabilizing LUMO, while leaving HOMO almost unchanged. When LUMO level of ancillary ligand is lower than that of ppz, it results in lowering LUMO for the complex, which makes the excited state far from the non-radiative states, leading to enhanced efficiency. The results indicate that the ancillary ligands can be used to tune the emission color and luminescence efficiency of Ir(III) complexes, the importance of ancillary ligands should be noted in designing new Ir(III) complexes. Acknowledgments We thank the financial support from National Science Foundation of China (grant numbers 20573040, 20474024, 20704015, 90501001, 50303007), Ministry of Science and Technology of China (grant number 2002CB6134003) and PCSIRT. References
Fig. 8. Electroluminescence spectrum of the device at 6 V (inset: configuration of the device).
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