Highly Efficient Red Phosphorescent Osmium(II)
Complexes for OLED Applications
Yung-Liang Tung,
†Pei-Chi Wu,
†Chao-Shiuan Liu,
†Yun Chi,*
,†Jen-Kan Yu,
‡Ya-Hui Hu,
‡Pi-Tai Chou,*
,‡Shie-Ming Peng,
‡Gene-Hsiang Lee,
‡Ye Tao,*
,§Arthur J. Carty,
§Ching-Fong Shu,*
,|and Fang-Iy Wu
|Department of Chemistry, National Tsing Hua University, 300 Hsinchu, Taiwan, Department of Chemistry and Instrumentation Center, National Taiwan University, 106 Taipei, Taiwan, Institute for Microstructural Sciences and Steacie Institute for Molecular Sciences, National Research Council of Canada, Ontario K1A 0R6, Canada, and Department
of Applied Chemistry, National Chiao Tung University, 300 Hsinchu, Taiwan Received March 11, 2004
Summary: The nonionic, red-emitting complexes [Os-(fppz)2L2] (L ) PPh2Me (1), PPhMe2 (2)) and [Os-(bptz)2L2] (L ) PPh2Me (3)) were synthesized, showing highly intense red phosphorescent emission in CH2Cl2 solution at λmax617, 632, and 649 nm, respectively. The electroluminescent properties of these compounds on OLEDs showed promising device efficiencies required for future OLED applications.
Organometallic complexes containing third-row tran-sition-metal elements are crucial for the fabrication of organic light-emitting diodes (OLEDs).1 The strong spin-orbit coupling induced by these heavy-metal ions promotes an efficient intersystem crossing from the singlet to the triplet state, which then facilitates high internal quantum efficiencies (ηint) for the OLEDs through utilization of both singlet and triplet excitons. As a result, these phosphorescent molecules become in-creasingly important. With regard to the goal of achiev-ing three primary colors, the design and synthesis of red emitters presents a particular challenge, because their luminescence quantum yield tends to be low due to the smaller energy gap.2Despite this limitation, how-ever, success has been achieved in the preparation of Ir(III)3 and Pt(II)4 red emitters. Recently, some red-emitting Os(II) phosphors have begun to receive inten-sive study.5Unfortunately, because of their ionic nature, these OLED devices suffered inferior performance com-pared with the neutral Ir(III) counterparts. This is, in
part, attributed to the lack of strong bonding between the cationic emitters and their counteranions within the host matrix. Accordingly, it is proposed that only with the utilization of neutral Os(II) emitters can the goal of practical OLED applications be achieved. In this paper, a new series of such Os(II) emitting complexes are prepared, the molecular structure of which consists of two chelates containing either 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole (fppzH) or the corresponding 3-tert-butyl triazole (bptzH), to balance the 2+ charge on the Os(II) cation, and two phosphine donors located at the trans positions to complete the symmetrical octahedral coordination requirement.
Preparation of these Os(II) complexes requires the exploitation of the recently described blue-emitting pyr-idyl pyrazolate complex [Os(fppz)2(CO)2] or its relevant triazolate analogue [Os(bptz)2(CO)]2.6The desired syn-thesis was first initiated by the treatment of Me3NO to eliminate the CO ligands, followed by addition of corresponding phosphine ligands. The synthetic scheme
has led to the successful isolation of the red-emitting
†National Tsing Hua University. ‡National Taiwan University. §National Research Council of Canada. |National Chiao Tung University.
(1) (a) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.; Zuppiroli, L.; Graetzel, M. J. Am. Chem. Soc. 2003, 125, 8790. (b) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377. (c) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C.
Chem. Commun. 2002, 784. (d) Lamansky, S.; Djurovich, P.; Murphy,
D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (e) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 1999, 71, 2095.
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Sandrini, D.; Maestri, M.; Ciano, M. Inorg. Chem. 1990, 29, 2792. (3) (a) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (b) Duan, J.-P.; Sun, P.-P.; Cheng, C.-H. Adv. Mater. 2003, 15, 224. (c) Su, Y.-J.; Huang, H.-L.; Li, C.-L.; Chien, C.-H.; Tao, Y.-T.; Chou, P.-T.; Datta, S.; Liu, R.-S. Adv. Mater. 2003, 15, 884. (d) Gong, X.; Lim, S.-H.; Ostrowski, J. C.; Moses, D.; Bardeen, C. J.; Bazan, G. C. J. Appl.
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Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-T.; Chien, C.-H.; Carty, A. J. Adv. Functional Mater., in press.
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complexes [Os(fppz)2L2] (L ) PPh2Me (1), PPhMe2(2)) and [Os(bptz)2L2] (L ) PPh2Me (3)) in moderate yields (40-72%). These Os complexes were fully characterized using routine spectroscopic methods, while complex 1 was further examined by single-crystal X-ray diffraction analysis.
As depicted in Figure 1, the Os atom of 1 is located at a crystallographic center of inversion. The molecular frame reveals an octahedral configuration where two chelating fppz ligands establish a nearly planar OsN4 basal arrangement, together with two PPh2Me ligands located at the axial dispositions. The planar ligand arrangement is analogous to that of the porphinato ligand in metalloporphyrins such as [Os(TTP)(PPh3)2] (TTP ) meso-tetraphenylporphinate) and [Os(TPP)(CO)-(Im)] (Im ) 1-methylimidazole).7 The measured Os-N(pz) distances of 2.073(2) Å in 1 are slightly shorter than the respective Os-N(py) bonds of 2.090(2) Å; both lengths fall in the range expected for a typical Nf OsII dative bond. Of particular interest are the relatively weak nonbonding contacts (N3A‚‚‚C1 ) 3.305 Å and N3A‚‚‚H1≈ 2.50 Å) observed between the ortho hydro-gen atom of the pyridyl moiety and the N atom of the nearby pyrazolate fragment. In good agreement with this observation, the 1H NMR spectrum revealed a significantly downfield signal at δ 10.40, giving an additional indication of the deshielding effect exerted by the N atom. It is speculated that this H bonding, to a certain extent, is akin to that observed in the co-baloxime complexes.8
The absorption and luminescence spectra of com-plexes 1-3 in CH2Cl2are shown in Figure 2. The strong absorption bands in the UV region are assigned to the
spin-allowed1ππ* transition of the fppz (or bptz) ligands, owing to their spectral similarity to the free fppz (or bptz) anion. The next lower energy absorption can be ascribed to a typical spin-allowed metal to ligand charge transfer (1MLCT) transition, while two absorption bands extending into the visible region are associated with the spin-orbit coupling enhanced3ππ* and3MLCT transi-tions. Further luminescence properties (vide infra) support3MLCT to be in the lowest triplet state with peak wavelengths at 542 nm ( ) 1300 M-1cm-1), 553 nm ( ) 1600 M-1cm-1), and 560 nm ( ) 950 M-1cm-1) for complexes 1-3, respectively. It is notable that substitution with strong electron donors such as PPh2 -Me and PPh-Me2ligands not only increases the entire transition dipole moment but also causes a significant red shift due to the enhancement of dative interactions with Os(II), hence raising the d orbital energy level of the Os metal center. A similar mechanism has been proposed to delineate the electron-donating effect for the Os(II) polypyridyl complexes.9
Highly intense luminescence was observed for 1-3 with λmaxlocated at 617, 632, and 649 nm, respectively. The entire emission band originating from a triplet state manifold was ascertained by the O2 quenching rate constant of∼1.78 × 109M-1s-1for 1-3 in CH
2Cl2. The significant overlap of the 0-0 onsets between emission and the lowest energy absorption band, in combination with a broad, structureless spectral feature, leads us to conclude that the phosphorescence originates primarily from the3MLCT state. In comparison to 2 coordinated with PPhMe2 ligands, complex 1 bearing the PPh2Me group reveals a∼15 nm hypsochromic shift in λmaxand can qualitatively be rationalized by a decrease of Os-(II) d orbital energy level due to the greater electron-withdrawing strength of an additional phenyl substi-tution. Table 1 lists the corresponding photophysical data for the studied complexes in both solution and solid phases. The observed lifetimes of ca. 0.6-0.9 µs in degassed CH2Cl2 are considerably shorter than those of most reported red-emitting Ir(III) complexes.3aIn the solid state, the emission maximum for these osmium phosphors shifts to the red, possibly due to molecular packing, and the lifetime falls within the range of 0.4-0.6 µs (Table 1). The emission quantum efficiencies of 1-3 lie within the range 0.19-0.50 in CH2Cl2and 0.1-(6) Wu, P.-C.; Yu, J.-K.; Song, Y.-H.; Chi, Y.; Chou, P.-T.; Peng,
S.-M.; Lee, G.-H. Organometallics 2003, 22, 4938.
(7) (a) Che, C. M.; Lai, T. F.; Chung, W. C.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1987, 26, 3907. (b) Salzmann, R.; Ziegler, C. J.; Godbout, N.; McMahon, M. T.; Suslick, K. S.; Oldfield, E. J. Am. Chem.
Soc. 1998, 120, 11323.
(8) Gupta, B. D.; Singh, V.; Yamuna, R.; Barclay, T.; Cordes, W.
Organometallics 2003, 22, 2670.
(9) Kober, E. M.; Sullivan, B. P.; Dressick, W. J.; Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 7383.
Figure 1. ORTEP diagram of 1. Selected distances (Å):
Os-P(1) ) 2.3616(5), Os-N(1) ) 2.090(2), Os-N(2) ) 2.073(2), N(2)-N(3) ) 1.349(2), N(2)-C(6) ) 1.352(3), N(3)-C(8) ) 1.355(3). Selected angles (deg): N(1)-Os-N(2) ) 76.48(7), N(1)-Os-N(2A) ) 103.52(7).
Figure 2. UV-vis absorption and normalized emission
spectra of 1 (9), 2 (2), and 3 (O) in CH2Cl2 at room
temperature.
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0.3 in the solid state. The results correlate well with unusually large extinction coefficients measured for the 3MLCT bands and thus are very desirable for OLED-related applications.
The electrochemical behavior of these Os complexes was investigated by cyclic voltammetry using ferrocene as the internal standard. During the anodic scan in CH2 -Cl2, all Os complexes exhibited reversible oxidation with potential in the region -0.15 to -0.25 V (Table 1). As the oxidation mainly occurs at the Os metal site,10the formal oxidation potential is strongly dependent on the coordination environment of the osmium; i.e., good σ donor strength of the ancillary ligand would shift the oxidative potential to a more negative value. This was demonstrated by the fact that replacement of PPh2Me ligands by the more electron-donating PPhMe2 de-creased the oxidative potential from -0.15 V of 1 to -0.22 V of 2, while substitution of the fppz ligands with less electron deficient bptz ligands led to a cathodic shift to -0.25 V in 3.11It is noteworthy that the oxidation potentials of these Os complexes are significantly less than those of ionic Os(II) complexes such as [Os(bpy)2 L]-(PF6)2(where L is cis-1,2-bis(diphenylphosphino)ethyl-ene), which has E1/2ox) 0.87 V.12The largely cathodic shift of Os complexes 1-3 is apparently due to their neutral characteristics.
The electroluminescence (EL) spectra of OLEDs based on 1-3 are shown in Figure S1 of the Supporting Information. The energy transfer from the host material PVK to the Os(II) emitters is very efficient, as supported by the lack of any PVK emission in the EL spectra. The EL spectra remained unchanged over a wide range of bias voltages. The OLEDs with 1 reached a maximum efficiency (ηmax) of 3.9 cd/A at a driving voltage of 13 V with a luminance of 412 cd/m2. The η
max of 2 reached 3.0 cd/A at 21 V with a luminance of 615 cd/m2, while that of 3 was 1.0 cd/A at 20 V and 271 cd/m2. Although ηmaxand maximum luminance values of 3 appeared to be lower (Table 1), the quantum efficiencies and optical power output of these three compounds are similar,
considering the fact that the normalized photopic vision functions V(λ) at 630, 640, and 660 nm are 0.265, 0.175, and 0.061, respectively. We believe that the turn-on voltages can be significantly reduced by using either a host material with better charge carrier transport properties or a lower work-function cathode, such as Ca or Mg:Ag alloy.
In summary, we have demonstrated the synthesis, spectral and electrochemical properties, and OLED applications of a series of neutral red-emitting Os(II) complexes. In contrast to most previously reported Os-(II) red emitters with ionic character, in which the diffusion of the counteranions might seriously deterio-rate the device performance, this new series of com-plexes comprises discrete, neutral molecules that would remain at their fixed positions upon electron and hole injection. Moreover, the higher HOMO level at the Os-(II) center, which is induced by the superior donor ability of anionic fppz (or bptz), may also contribute to more efficient energy transfer and carrier trapping,13 giving additional advantages to the overall device ef-ficiency for complexes 1-3. Future focuses on fabricat-ing the OLED devices via direct deposition as well as functionalizing Os(II) complexes to fine-tune both device color and efficiency should stimulate a broad spectrum of interests in the field of OLED.
Experimental Section
General Experiments. All reactions were performed under nitrogen. Solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification unless otherwise stated. All reactions were monitored by TLC with Merck precoated glass plates (0.20 mm with fluorescent indicator UV254). Flash
column chromatography was carried out using silica gel from Merck (230-400 mesh). Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) mode or fast atom bombardment (FAB) mode.1H and13C NMR
spectra were recorded on Varian Mercury-400 and INOVA-500 instruments; chemical shifts are quoted with respect to the internal standard tetramethylsilane for1H and13C NMR
data.
Preparation of 1. Freshly sublimed Me3NO (90 mg, 1.19
mmol) was first dissolved in acetonitrile (5 mL), and the resulting solution was added dropwise to a stirred suspension of [Os(fppz)2(CO)2] (200 mg, 0.25 mmol) in toluene (30 mL),
giving a clear, yellow orange solution after stirring for 2 min at room temperature. The phosphine ligand PPh2Me (567 µL,
3.0 mmol) was then added, and the mixture was brought to reflux for 3 h, during which time the color was found to change to bright red. The reaction was then stopped, toluene solvent and excess phosphine ligand were removed under vacuum, the solid residue was dissolved in 50 mL of ethyl acetate, and this solution was washed with distilled water (30 mL × 2) to remove the remaining Me3NO. The organic phase was dried
over Na2SO4, and the solvent was removed in vacuo to yield
red-orange crude product. Further purification was conducted by silica gel column chromatography using a 1:1 mixture of ethyl acetate and hexane, followed by recrystallization from CH2Cl2and hexane, giving a red-orange crystalline solid (180
mg, 0.18 mmol); yield 72%. Complexes 2 and 3 were prepared in an analogous manner.
Spectral data of 1 are as follows. MS (EI, 192Os; observed
m/z [assignment]): 1015 [M+], 814 [M+- PPh
2Me], 614 [M+
- 2PPh2Me].1H NMR (400 MHz, d6-acetone): δ 10.40 (d, 2H,
(10) Abrun˜ a, H. D. J. Electrochem. Soc. 1985, 132, 842.
(11) The two heterocyclic chelates fppz and bptz exhibit irreversible reduction peak potentials at -2.70 and -2.73 V, respectively, relative to the ferrocene standard.
(12) The oxidation potential of [Os(bpy)2L](PF6)2was +1.38 V vs
SSCE as reported in refs 5b and 10, which can be converted to a value of +0.87 V relative to the ferrocene standard.
(13) Jiang, X.; Jen, A. K. Y.; Carlson, B.; Dalton, L. R. Appl. Phys.
Lett. 2002, 81, 3125.
Table 1. Photophysical, Electrochemical, and Device Properties for 1-3
1 2 3 UV/vis abs (nm ())a 405 (14 700) 411 (15 500) 406 (11 100) 454 (2400) 456 (2600) 466 (2000) 542 (1300) 553 (1600) 560 (950) PL λmax(nm)a 617 (618) 632 (655) 649 (670) Φb 0.50 (0.21) 0.19 (0.29) 0.25 (0.10) τobs(ns)b 855 (631) 725 (610) 634 (440) E1/2°x(V)d -0.15 -0.22 -0.25 EL λmax(nm)c 626 640 658 Lmax(cd/m2)c 3276 2155 774 ηmax(cd/A)c ∼4.0 ∼3.0 ∼1.0 V @ 1 cd/m2 5.5 8.5 8.5
aSamples were degassed and recorded in CH
2Cl2 at room
temperature with in M-1 cm-1.bData in parentheses are
measured in solid state at room temperature.cOLED device
structure: ITO/PVK:Os/PBD(200 Å)/LiF(10 Å)/Al(1500 Å).d
Revers-ible oxidation potential referenced to Fc/Fc+.
Notes Organometallics, Vol. 23, No. 15, 2004 3747
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JHH) 6.0 Hz, Hpy), 7.32 (ddd, 2H, JHH) 7.6, 6.0, 1.2 Hz, Hpy),
7.15-6.84 (m, 20H, Hpy, HPh), 6.73 (s, 2H, Hpz), 6.66-6.14 (m,
4H, HPh), 1.16 (t, 6H, JHP) 3.2 Hz, Me).19F NMR (470 MHz,
d6-acetone): δ -59.8 (s).31P NMR (202 MHz, d6-acetone): δ
-17.4 (s). Anal. Calcd for C44H36F6N6P2Os: C, 52.07; N, 8.28;
H, 3.58. Found: C, 51.99; N, 8.17; H, 3.78.
Spectral data of 2 are as follows. MS (FAB,192Os; observed
m/z [assignment]): 892 [M+], 754 [M+- PPhMe2], 616 [M+ -2PPhMe2].1H NMR (400 MHz, d6-acetone): δ 10.31 (dd, 2H, JHH) 6.6, 0.8 Hz, Hpy), 7.56-7.48 (m, 4H, Hpy), 7.05 (ddd, 2H, JHH) 6.8, 6.6, 0.8 Hz, Hpy), 6.94-6.87 (m, 8H, HPh, Hpz), 6.42-6.38 (m, 4H, HPh), 0.80 (t, 6H, JHP) 3.6 Hz, Me), 0.60 (t, 6H, JHP) 3.6 Hz, Me).19F NMR (470 MHz, d6-acetone): δ -59.5 (s).31P NMR (202 MHz, d
6-acetone): δ -19.6 (s). Anal. Calcd
for C34H32F6N6P2Os: C, 45.84; N, 9.43; H, 3.62. Found: C,
46.00; N, 9.32; H, 3.81.
Spectral data of 3 are as follows. MS (FAB,192Os; observed
m/z [assignment]): 994 [M+], 794 [M+- PPh2Me], 594 [M+ -2PPh2Me].1H NMR (500 MHz, d4-methanol): δ 10.37 (d, 2H, JHH) 4.8 Hz, Hpy), 7.34 (d, 4H, JHH) 4.8 Hz, Hpy), 7.09-6.86 (m, 18H, Hpy,HPh), 6.62-6.59 (m, 4H, HPh), 1.63 (s, 18H,tBu), 0.90 (s, br, 6H, Me).31P NMR (202 MHz, d 4-methanol): δ -19.6
(s). Anal. Calcd for C48H52N8P2Os: C, 58.05; N, 11.28; H, 5.28.
Found: C, 57.71; N, 11.43; H, 5.40.
X-ray Structural Analysis. Single-crystal X-ray diffraction data of 1 were measured on a Bruker SMART CCD diffrac-tometer using λ(Mo KR) radiation (λ ) 0.710 73 Å). The data
collection was executed using the SMART program. Cell refinement and data reduction were carried out with the SAINT program. The structure was determined using the SHELXTL/PC program and refined using full-matrix least squares. All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed at calculated positions and included in the final stage of refinements with fixed parameters.
Selected crystal data of 1: C44H36F6N6OsP2, Mr) 1014.93,
triclinic, space group P1h, a ) 10.4469(5) Å, b ) 10.5233(6) Å, c ) 10.6829(6) Å, R ) 71.968(1)°, β ) 62.053(1)°, γ ) 82.167(1)°, V ) 986.46(9) Å3, Z ) 1, F
calcd) 1.708 g cm-3, F(000) ) 502,
crystal size 0.35× 0.30 × 0.25 mm, λ(Mo KR) ) 0.7107 Å, T ) 295 K, µ ) 3.383 mm-1, 4516 reflections collected (Rint )
0.0253), final R1(I > 2σ(I)) ) 0.0182, wR2(all data) ) 0.0438. Acknowledgment. We thank the National Science Council of Taiwan for financial support (Grant Nos. NSC 91-2119-M-002-016 and NSC 91-2113-M-007-006). Supporting Information Available: Text giving experi-mental procedures involving photophysical and electrochemical data and OLED device fabrication and a CIF file giving X-ray data for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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