Synthesis, Structure, Photoluminescence, and Electroluminescence

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J. Phys. Chem. C 2007, 111, 2295-2300

2295

Synthesis, Structure, Photoluminescence, and Electroluminescence Properties of a New Dysprosium Complex Zhe-Feng Li, Liang Zhou, Jiang-Bo Yu,* Hong-Jie Zhang,* Rui-Ping Deng, Ze-Ping Peng, and Zhi-Yong Guo Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences and the Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: July 25, 2006; In Final Form: September 29, 2006

A new dysprosium complex Dy(PM)3(TP)2 [where PM ) 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone and TP ) triphenyl phosphine oxide] was synthesized, and its single-crystal structure was also studied. Its photophysical properties were studied by absorption spectra, emission spectra, fluorescence quantum efficiency, and decay time of the f-f transition of the Dy3+ ion. In addition, the antenna effect was introduced to discuss the energy transfer mechanism between the ligand and the central Dy3+ ion. Finally, a series of devices with various structures was fabricated to investigate the electroluminescence (EL) performances of Dy(PM)3(TP)2. The best device with the structure ITO/CuPc 15 nm/Dy complex 70 nm/BCP 20 nm/AlQ 30 nm/LiF 1 nm/Al 100 nm exhibits a maximum brightness of 524 cd/m2, a current efficiency of 0.73 cd/A, and a power efficiency of 0.16 lm/W, which means that a great improvement in the performances of the device was obtained as compared to the results reported in published literature. Being identical to the PL spectrum, the EL spectrum of the complex also shows characteristic emissions of the Dy3+ ion, which consist of a yellow band at 572 nm and a blue emission band at 480 nm corresponding to the 4F9/2-6H13/2 and 4F9/2-6H15/2 transition of the Dy3+ ion, respectively. Consequently, an appropriate tuning of the blue/yellow intensity ratio can be presumed to accomplish a white luminescent emission.

1. Introduction There is a worldwide interest in complexes of lanthanide ions due to their applications in fluoroimmunoassays,1 spectroscopic structural probes in biologically important systems,2,3 laser systems,4 optical amplification,5 and organic light-emitting diodes.6 The unique luminescence properties of lanthanide compounds and their wide range of emission bands from the UV to NIR regions (380-2500 nm) make them very interesting for such purposes. Among all these lanthanide ions, the complexes of the Eu3+ and Tb3+ ions are the most widely studied since they have narrow emission bands located in the pure red and green region, respectively; furthermore, their emissions have a theoretical upper limit of an inner quantum efficiency near 100%. In contrast, other lanthanide ion complexes are rarely studied, especially in the research field of organic light-emitting diodes. Recently, other luminescent lanthanide ions have attracted incremental interest due to their emission bands in the blue (Tm3+ and Dy3+),7 red-orange (Pr3+ and Sm3+),8,9 near white (Dy3+)10, and near-infrared regions (Pr3+, Nd3+, Ho3+, Er3+, Dy3+, and Yb3+).11 Therefore, by selecting an appropriate organolanthanide complex as the emitter, one can achieve electroluminescence (EL) covering the spectrum from the blue to infrared region. It is well-known that lanthanide ions have very low absorption coefficients ( e 1 M-1 cm-1).12 To increase the light absorption cross-section, an antenna effect mechanism in some systems where the lanthanide ion is coordinated with a suitable organic chromophore was applied. Thus, the lanthanide gives its * Corresponding author. Phone: +86-431-5262127. Fax: +86-4315698041. E-mail: [email protected] (H.-J.Z.), [email protected] (J.-B.Y.).

characteristic emission with high efficiency via the energy transfer process from the ligands as antenna to harvest the light in the UV-vis region.13 The β-diketone ligand is one of the best ligands that can enhance the photoluminescence (PL) and EL properties of some lanthanide ions. The β-diketone- and pyrazolone-based ligands have been proved to be the best chromophores to benefit the Tb3+ ion to produce the excellent PL and EL properties, and triphenyl phosphine oxide (TP) is a good neutral ligand to shield the Tb3+ ion from quenchers,14,15 such as hydroxyl groups or oxygen, by leading to a stable coordination environment. To our best knowledge, the lowest excited energy level (4F9/2) of the Dy3+ ion (20830 cm-1 ) 480 nm) is similar to that (5D4) of the Tb3+ ion (20430 cm-1 ) 489 nm), and dysprosium(III) complexes exhibit characteristic 4F -6H 4 6 9/2 13/2 and F9/2- H15/2 transitions leading to blue (around 480 nm) and yellow (around 572 nm) emissions, respectively. The Commission Internationale de L’Eclairage (CIE) coordinates of the photoluminescence of the Dy complex are calculated as x ) 0.35 and y ) 0.40, which are located in the white region. Therefore, an appropriate tuning of the blue/yellow intensity ratio can be presumed to accomplish a white luminescent emission. In this work, 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone (PM) was synthesized, which as the primary ligand and TP as the neutral ligand were coordinated to the Dy3+ ion to obtain the new dysprosium complex [Dy(PM)3(TP)2]. By investigating the PL and EL properties of this complex, we have found that the PM donor can match the level (4F9/2) of the Dy3+ ion well, thus improving the quantum yield of the Dy3+ ion and extending the excited-state lifetime of the Dy3+ ion. The EL performance of devices with Dy(PM)3(TP)2 as the emitting layer has also

10.1021/jp064749t CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007

2296 J. Phys. Chem. C, Vol. 111, No. 5, 2007 been studied. The results show that this complex has good carrier (electron and hole) transport properties.

Li et al. metric analysis (TGA) was made using a SDT 2960 Simultaneous DSC-TGA of TA instruments, and the heating rate was 10 °C/min under N2.

2. Experimental Procedures 2.1. Materials. 1-Phenyl-3-methylpyrazolin-5-one was purchased from ABCR, and isobutyl chloride was obtained from ACROS. The hole-transport material N,N′-diphenyl-N,N′- bis(3-methylphenyl)-1,10-biphenyl-4,40-diamine (TPD, 99%) and the hole-blocking material 2,9-dimethyl-4,7-diphenyl-1,10-phenylanthroline (BCP, 98%) were purchased from ACROS. The electron transport material tris(8-hydroxyquinolinato) aluminum (AlQ) and the hole-injecting material copper phthalocyanine (CuPc, 99%) were obtained from the eLight Corporation. 2.2. Synthesis of the Ligand PM. The pyrazolone ligand was synthesized using conventional techniques.16,17 2.3. Synthesis of Dy(PM)3(TP)2. A mixture of PM (30 mmol), a solution of DyCl3 (10 mmol) in ethanol and TP (20 mmol) in ethanol (200 mL), was stirred for several minutes until the solids were completely dissolved. This solution was treated with sodium hydroxide (30 mL of a 1 mol/L solution), which resulted in the immediate formation of a pale precipitate, and refluxed for 5 h. The pale precipitate was then collected by vacuum filtration, washed with water, and dried at 80 °C to give the product as pale solid (11.6 g, 80%). Calcd for C78H75N6O8P2Dy: C, 64.68%; H, 5.18%; N, 5.80%. Found: C, 64.45%; H, 5.29%; N, 5.87%. 2.4. Preparation of EL Devices. All the organic layers were evaporated onto a pre-cleaned ITO (with a sheet resistance of 10 Ω/sq) glass substrate with a speed of 0.05 nm/s under high vacuum (e3.0 × 10-5 Pa). LiF and Al were evaporated in another vacuum chamber with a different speed of 0.01 and 0.5 nm/s without being exposed to the atmosphere, respectively. The thicknesses of the deposited layers and the evaporation speed of the individual materials were monitored in vacuo with quartz crystal monitors. 2.5. Crystallography. X-ray data for the selected crystal were recorded at room temperature (298 K) on a Bruker-AXS Smart CCD diffractometer equipped with a normal-focus, 2.4 kW sealed tube X-ray source (graphite-monochromated Mo KR radiation with λ ) 0.71073 Å) operating at 50 kV and 40 mA. Intensity data were collected in 1271 frames with increasing ω (width of 0.3° and exposure time of 30 s per frame). An absorption correction was made using the SADABS program (Tmax ) 0.3282 and Tmin ) 0.1718). The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares techniques against F2 using the SHELXL-97 program package. Non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were generated manually based on idealized geometries. 2.6. Apparatus. The photoluminescence and electroluminescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer, which was equipped with a Xenon lamp (150 W) as the excitation source of PL. The luminescence decay curve of the emitting level of Dy(PM)3(TP)2 was recorded under excitation at 355 nm with the third harmonic of a Spectraphysics Nd:YAG laser (using a 5 ns pulse width and 5 mJ of energy per pulse) as the exciting source. The luminescence lifetime was calculated by the Origin 7.0 software package. The UV-vis absorption spectra were obtained with a TU-1901 spectrophotometer. Current-luminance-voltage properties were measured by using a Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode. Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario-EL analyzer. Thermogravi-

3. Results and Discussion 3.1. Crystal Structure of Dy(PM)3(TP)2. The single crystal of the Dy(PM)3(TP)2 complex was obtained by recrystallization from its ethanol solution. The crystal structure of Dy(PM)3(TP)2 with the numbering scheme is displayed in Figure 1a. Important experimental parameters for the structure determinations are tabulated in Table 1. The central Dy3+ ion is coordinated by eight oxygen atoms from three PM ligands and two TP ligands. This coordination character is in accordance with the chemical structure of the complex (Figure 1c). Thus, the Dy3+ ion exhibits a coordination number of eight. The coordination geometry of the central Dy3+ ion may be described as a distorted bicapped trigonal prism from the coordination site angles (shown in Figure 1b). The PM oxygen atoms [O(1), O(2), O(4), and O(5)] and two TP oxygen atoms [O(7) and O(8)] form the trigonal prism. Another two oxygen atoms [O(6) and O(3)] cap the two quadrilateral faces O(8)-O(4)-O(7)-O(5) and O(4)-O(2)O(1)-O(7), respectively. The average Dy-O distance is 2.358 Å [2.261(3)-2.492(3) Å]. 3.2. Photoluminescence Properties. The room-temperature absorption spectra of PM and TP, the excitation and the emission spectra of Dy(PM)3(TP)2 are shown in Figure 2. The absorption spectra of PM and TP were measured with spin-coating them onto quartz substrate. From Figure 2, it can be found that the absorption band of PM and the excitation band (monitored at 572 nm) of Dy(PM)3(TP)2 are well-overlapping; also, the overlap between the absorption of PM and the excitation band is much larger than that between the absorption of TP and the excitation band, which suggests that the antenna effect of PM is more efficient than that of TP. Therefore, we conclude that intramolecular energy transfer in the Dy(PM)3(TP)2 complex mainly occurs between the PM ligand and the Dy3+ ion.9 However, TP as the natural ligand is still beneficial to energy absorption and transfer. With direct excitation (λex ) 327 nm) of the ligand, we obtain the emission spectrum of the Dy(PM)3(TP)2 complex (Figure 2d). The spectrum mainly shows two sharp emission bands (572 and 480 nm) attributed to the transitions from the excited-state 4F9/2 to the 6H13/2 and 6H15/2 states of the Dy3+ ion, respectively. The Commission Internationale de L’Eclairage (CIE) coordinates of the photoluminescence of the Dy complex are also calculated as x ) 0.35 and y ) 0.40, which are located in the edge of the white region. Therefore, Dy(PM)3(TP)2 appears to be a promising candidate for application as white luminescent material. To characterize the PL properties of Dy(PM)3(TP)2, the luminescence lifetime at room temperature was investigated. The decay curve shown in Figure 3 is a single exponential (eq 1) with a time constant of 7.24 µs at the emission wavelength of 572 nm.

I(t) ) A + I0 exp(-t/τ)

(1)

The overall emission quantum efficiency (ηS) of the Dy(PM)3(TP)2 complex at room temperature was also measured based on eq 2 using Eu(TTA)3phen as a reference.

ηS ) ηR

ARISnS2 ASIRnR2

(2)

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Figure 1. (a) ORTEP plot for Dy(PM)3(TP)2 with ellipsoids drawn at the 30% probability level. Hydrogen atoms were omitted for clarity. (b) Coordination polyhedron of the dysprosium (III) ion. (c) Chemical structure of the complex.

TABLE 1: Crystallographic Data for Dy(PM)3(TP)2 empirical formula Fw T (K) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z calcd density (mg/m3) wavelength (Å) abs coeff (mm-1) GOF on F2 R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data)

C78H75N6O8P2Dy 1448.88 293(2) monoclinic P2(1)/n 13.364(3) 23.167(5) 23.115(5) 91.87(3) 7153(2) 4 1.345 0.71073 1.149 1.039 0.0515 0.1145 0.1128 0.1498

Here, ηR ) 0.365 is the quantum efficiency of Eu(TTA)3phen in a 10-3 mol/L DMF solution.18,19 The n, A, and I denote the refractive index of solvent, the area of the emission spectrum, and the absorbance at the excitation wavelength, respectively. Both absorbance and emission spectra of Dy(PM)3(TP)2 and the reference were measured in a 10-3 mol/L DMF solution. Therefore, the refractive indeces nS and nR are equal to that of DMF. The overall fluorescent quantum yield (ηS) of Dy(PM)3(TP)2 was calculated to be 0.035 (estimated error (10%), which is a higher value in comparison with other Dy complexes reported in published works.11,30 This result indicates that there is an efficient energy transfer between the ligands and the central Dy3+ ion, which is a possible reason that Dy(PM)3(TP)2 has a relatively high overall fluorescent quantum yield. Another reason

Figure 2. (a and b) UV-vis absorption spectrum of ligand TP and PM. Photoluminescence spectra include excitation (c) and emission (d) of the complex Dy(PM)3(TP)2. All spectra have been normalized.

to the arising quantum yield is that there is a decrease in the nonradiative rate ascribed to the 4F9/2 emitting level of the Dy3+ ion. 3.3. Energy Transfer Mechanism. Mostly, in the luminescence process of the lanthanide complex, the direct excitation of the lanthanide ion is very inefficient because the optical transition within the 4f subshells of lanthanide ions is parity forbidden. The indirect excitation by energy transfer from an organic antenna chromophore not only circumvents this excitation problem but also allows excitation at wavelengths where the lanthanide ion does not display a significant absorption. To obtain an excellent luminescent material, one has to take advantage of the high absorbance of organic ligands and the

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Li et al. SCHEME 1: Energy Levels of the Dy3+ Ion in the Dy(PM)3(TP)2 Complex

Figure 3. Lifetime decay curve of the Dy3+ excited state (4F9/2) monitored within the 4F9/2 f 6H13/2 transition.

Figure 4. Low-temperature (77 K) phosphorescence of Gd(PM)3(TP)2 in DMF solution (5 × 10-4 mol/L).

possibility of transferring the excitation energy from the triplet states of the ligands to the lanthanide ion. Therefore, the tripletstate energy level of the ligand PM was examined from the phosphorescence spectrum of the Gd(PM)3(TP)2 complex in a 5 × 10-4 mol/L DMF solution at 77 K (shown in Figure 4), which was used to elucidate the energy transfer mechanism in the PL process of Dy(PM)3(TP)2. The lowest excited state of the Gd3+ ion (6P7/2) is located at about 32 000 cm-1, which is much higher than the energy of the triplet state of the ligand. Therefore, energy transfer from the ligand to the Gd3+ ion is impossible. Thus, the phosphorescence spectrum of Gd(PM)3(TP)2 is due to the emission of the ligand, and the emission band at the shortest wavelength peak (466 nm) is assumed to be a 0-0 transition.20-22 From Figure 4, the triplet-state level of the ligand PM can be easily calculated as 21 460 cm-1. Thus, a model for the indirect excitation mechanism is suggested and shown as a schematic energy diagram in Scheme 1. From Scheme 1, it can be seen that the electrons of the ligands of the lanthanide complex are excited from the singlet ground state (S0) to the singlet excited state (S1) by absorbing the energy. The energy of S1 is then transferred to the triplet excited state (T1) of the ligands through the intersystem crossing. Subsequently, the excitation energy of T1 is transferred to the 4f states of the Dy3+ ion via the internal conversion, ultimately resulting

in sensitized Dy3+ ion emission.23 As described previously, we obtained the characteristic Dy3+ ion emission with a high PL quantum efficiency upon excitation at the absorption wavelength of the organic ligands in Dy(PM)3(TP)2. This indicates (1) that ligands (PM and TP) are able to transfer the absorbed energy to the central metal Dy3+ ion, an antenna effect, which is in agreement with the excitation results mentioned previously (Figure 2) and (2) that the ligands shield the Dy3+ ion well from its surroundings and efficiently transfer energy from their triplet states to the Dy3+ ion. According to Dexter’s theory,24 the suitability of the energy difference between the resonance level of the Ln3+ ion and the triplet state of the ligand is a critical factor for efficient energy transfer. If the energy difference is too large, then the energy transfer rate constant decreases due to the diminution in the overlap between the donor and the acceptor. On the contrary, if the energy difference is too small, then the energy back-transfer can occur from the Ln3+ ion to the resonance level of the triplet state of the ligand. According to previous reports, for energy transfer to occur efficiently at room temperature, the energy gap (∆E) between the lowest triplet excited ligand and the excited state of the central lanthanide ion must be at least 500 cm-1 for the Eu3+ ion [∆E(T1 - 5D1)] and 2000 cm-1 for the Tb3+ ion [∆E(T1 - 5D4)], respectively.25,26 However, there is no report to discuss the ideal energy gap for the Dy3+ ion that ensures the intramolecular energy transfer to be efficient. For Dy(PM)3(TP)2, the T1 energy level of PM (21 460 cm-1) is 630 cm-1 higher than the resonance excited 4f level (4F9/2, 20 830 cm-1)27 of the Dy3+ ion. Although this complex has a higher PL quantum efficiency as compared with other Dy3+ complexes, its quantum efficiency is lower than that of the Tb complex with the same ligand.15 This can be attributed to that the ∆E (T1 - 4F9/2) for this complex is not large enough to effectively preclude back-energy transfer from 4F9/2 to T1 at room temperature. It is also noteworthy that TP as the natural ligand was introduced in Dy(PM)3(TP)2 to better sensitize Dy3+ ion luminescence by replacing the H2O molecules around the Dy3+ ion that can quench the luminescence of the Dy3+ ion. 3.4. EL Properties. The TGA-DTA analyses show that the melting point of Dy(PM)3(TP)2 is 190 °C (TGA-DTA curve is shown in Supporting Information). The results indicate that the complex has good thermal stability and can be sublimed at low temperature.

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Figure 5. EL spectra of devices B-D under the operating voltage of 18 V.

To investigate EL properties of Dy(PM)3(TP)2, several devices based on this Dy complex as an emitter were fabricated. First, the single layer device A: ITO/Dy(PM)3(TP)2 (80 nm)/ LiF (1 nm)/Al (100 nm) was fabricated, but no emission was detected from it. Then, two devices with structures of B: ITO/ TPD (50 nm)/Dy(PM)3(TP)2 (50 nm)/LiF (1 nm)/Al (100 nm) and C: ITO/Dy(PM)3(TP)2 (50 nm)/AlQ (20 nm)/LiF (1 nm)/ Al (100 nm) were fabricated. Device B shows the highest luminance of 98 cd/m2 (264 mA/cm2) and a turn-on voltage of 11.9 V. The EL spectra (shown in Figure 5) reveal that for device B, three emission bands are observed from TPD (410 nm) and Dy(PM)3(TP)2 (572 and 480 nm). The appearance of TPD emission implies that electrons from the cathode can be transported through the Dy complex and recombined with holes from the anode at the TPD/Dy(PM)3(TP)2 interface. This indicates that Dy(PM)3(TP)2 has good electron-transport property. For device C, the emission bands are detected from Dy(PM)3(TP)2 (572 and 480 nm) and AlQ, which suggests that the holes and electrons are recombined in an interface of Dy(PM)3(TP)2 and AlQ layers, so we can safely say that Dy(PM)3(TP)2 also has hole-transport property. Another device structure D: ITO/TPD (50 nm)/Dy-complex (50 nm)/AlQ (20 nm) /LiF (1 nm) /Al (100 nm) was selected to optimize the EL device of Dy(PM)3(TP)2, where the holetransport layer TPD and the electron transport layer AlQ were both introduced. However, in the EL spectra of this device, the main emission consists of a broadband at 520 nm from AlQ and a narrow band at 572 nm from Dy(PM)3(TP)2. The band at 480 nm of Dy3+ ion may be shielded by the broad AlQ emission band. At the same time, a weak emission band from TPD peaks at 410 nm. That is to say, there are many holes transported to AlQ layer, and they recombine with electrons. Then, the 20 nm thick BCP as the hole-block layer was introduced into device E: ITO/TPD (20 nm)/Dy(PM)3(TP)2 (50 nm)/BCP (20 nm)/ AlQ (30 nm)/LiF (1 nm)/Al (100 nm). The electroluminescent luminance and efficiency achieved from this device are 424 cd/ m2 and 1.2 cd/A, respectively. Besides the characteristic emission of the Dy3+ion, the emission band of BCP at 500 nm can be observed in the EL spectra (Figure 6) at low operating voltages. With the operation voltage increased, the emission from BCP disappears gradually followed by the emission from TPD becoming stronger. This phenomenon is due to the shift of the electron-hole recombination zone. At low operating voltages, the mobility rate of the hole is higher than that of electrons, which makes some remnant holes proceeding to the hole-block layer and coupling with the electrons there. Thus, it results in showing a weak emission from BCP. Generally, the

Figure 6. Normalized EL spectrum of device E: ITO/TPD (20 nm)/ Dy-complex (50 nm)/BCP (20 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm) at different operating voltages.

Figure 7. Photoluminescence spectrum of Dy(PM)3(TP)2 at room temperature (dotted line) and electroluminescence of device F: ITO/ CuPc (15 nm)/Dy(PM)3(TP)2 (50 nm)/BCP (20 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm) at 20 V (solid line).

carrier mobility rate increases with increasing the electric field.28 It has been reported that the electron mobility rate increases more quickly with an increasing applied field than the holemobility rate.29 With the operating voltage increased, the recombination zone shifts gradually to the interface of the TPD layer and Dy(PM)3(TP)2 layer. Consequently, the emissions mainly originate from Dy3+ and TPD. To obtain the pure emission from the Dy3+ ion, device F was designed with the structure of ITO/CuPc (15 nm)/Dy(PM)3(TP)2 (50 nm)/BCP (20 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm). Its EL spectrum shows the pure characteristic emission from the central Dy3+ ion of the Dy(PM)3(TP)2 complex without other emission bands from other layers (Figure 7). It is well-known that the HOMO value of CuPc (5.3 eV) is lower than that of TPD (5.6 eV). This leads to the holes transported in CuPc having more difficultly than in TPD. Herewith, the probability of holes transported to BCP becomes smaller. In other words, by introducing CuPc as the hole-inject and -transport layer, carriers are well-confined in the emitting layer and become more balanced. The highest brightness and current efficiency achieved from this device are 277 cd/m2 at 19 V and 0.54 cd/A at 11V, respectively. For the sake of

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Li et al. Acknowledgment. The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grants 20372060, 20340420326, 20490210, 206301040, and 20602035) and the MOST of China (“973” Program, Grant 2006CB601103). Supporting Information Available: X-ray crystallographic data (CIF) and the TGA-DTA curve for the Dy(PM)3(TP)2 complex. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Current density-voltage-luminance characteristics and current density-current efficiency characteristics (inset) of device G: ITO/CuPc (15 nm)/Dy(PM)3(TP)2 (70 nm)/BCP (20 nm)/AlQ (30 nm)/ LiF (1 nm)/Al (100 nm).

optimizing the EL performance of the device, the thickness of the Dy(PM)3(TP)2 complex layer was changed from 50 to 80 nm to extend the recombination zone. We make devices with a configuration of ITO/CuPc -Dy(PM)3(TP)2-BCP-AlQ-LiF-Al (15:X:20:30:1:100 nm) (X ) 50-80 nm). As a result, we find that when the thickness of the Dy(PM)3(TP)2 layer is X ) 70 nm, the device G shows optimal EL performances (luminance and efficiency). The luminance of this device was improved to 524 cd/m2 at 19 V (0.16 A/cm2), and the external current efficiency was 0.73 cd/A. The current density-voltageluminance (I-V-L) curve of this device is shown in Figure 8. As compared to device F, the reasons for the enhancement of performances of device G are that (1) thickening the Dy(PM)3(TP)2 emitting layer extends the recombination zone, which is beneficial to decreasing the quench probability and (2) reducing the total current density of the device makes the device stable and perform better. 4. Conclusion By investigating the PL and EL properties of this complex, we have found that the PM donor can match the level (4F9/2) of the Dy3+ ion, then improving the quantum yield of the Dy3+ ion (up to 3.5%). But, the quantum efficiency is lower than that of the Tb complex with the same ligand. This can be attributed to that the ∆E (T1 - 4F9/2) for this complex is not large enough to effectively prevent back-energy transfer from 4F 9/2 to T1 at room temperature. The EL performance of devices with Dy(PM)3(TP)2 as the emitting layer has also been detected. The results show that this complex has good carrier (electron and hole) transport properties.

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