Highly Efficient Blue Organic Light-Emitting Devices Based on

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J. Phys. Chem. C 2008, 112, 12024–12029

Highly Efficient Blue Organic Light-Emitting Devices Based on Improved Guest/Host Combination Feng He,† Leilei Tian,† Weijie Xie,† Mao Li,† Qi Gao,† Muddasir Hanif,† Yingfang Zhang,‡ Gang Cheng,‡ Bin Yang,† Yuguang Ma,*,† Shiyong Liu,‡ and Jiacong Shen† State Key Laboratory for Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China. Fax: +86-431-85168480, National Laboratory of Integrated Optoelectronics, Jilin UniVersity, Changchun130012, P. R. China ReceiVed: April 3, 2008; ReVised Manuscript ReceiVed: May 16, 2008

We present here a cruciform oligo(phenylenevinlyene) 2,5,2′,5′-tetrastyrylbiphenyl (TSB) exhibits excellent properties as host material for blue emitting guest molecule 1,4-di(4′-N,N-diphenylaminostyryl)benzene (DPA-DSB). TSB, which is constructed by linking two rigid distyrylbenzene (DSB) through the phenyl-phenyl bond, shows good optical and electronic properties e.g. high photoluminescent (PL) efficiency and wide band gap similar to DSB. Meanwhile, the central biphenyl core in TSB, which allows the relatively free rotation of two DSB segments along the biphenyl bond, makes it to be conformational multiformity and relatively flexible. As the result, TSB shows the good film forming property and larger loading ability to the guest molecules. The PL efficiencies of guest-host films of DPA-DSB and TSB arrive at the high level around 70% (68% for the 2 wt % DPA-DSB doped film and 72% for the 8 wt % DPA-DSB doped film), which is approaching the PL efficiency of the guest DPA-DSB in dilute solution (78%), indicating that the host TSB can sufficiently disperse the guest DPA-DSB with little aggregation. The organic light-emitting devices using DPA-DSB (2 wt %) doped TSB as blue emitting layer show the maximum efficiency of 12.2 cd/A (6.2%) and 6.39 lm/W, and the maximum brightness of 17350 cd/m2. Upon further analysis, it has been revealed that the Fo¨rster energy transfer and charge trapping are demonstrated to cooperatively work in this doping system. 1. Introduction Organic light emitting diodes (OLEDs) continue to be actively investigated for their potential applications in flat panel displays.1–4 The enhancement of the device performance that includes efficiency, color and operational stability, especially for blueemitting devices, is still an important issue for practical applications. The guest/host systems have been demonstrated as a virtual approach to achieve high performance devices, and much work had been done in developing the guest and host materials and their combinations.5–12 Generally, the good host material is required to possess a wide energy gap, carrier transporting abilities as well as film forming abilities. The most used host materials involve di(styryl)arylene derivatives,10 terfluorenes,13 tetra(phenyl)silyl,14 and anthracene derivatives.15 The guest materials are required to possess high luminescent efficiency as well as the ability of accepting energy from host materials via the Fo¨rster energy transfer16 and/or the direct recombination via the charge trapping.17,18 Such guest molecules typically include some classical dyes: green-emitting coumarin,19 red-emitting DCM,20 rubrene21 and recently reported blueemitting 1,4-di(4′-N,N-diphenylaminostyryl)benzene (DPADSB).22,23 DPA-DSB is a prominent blue-emitting molecule with photoluminescence (PL) quantum yield of ∼80% in dilute solution,24 but its emission is suppressed and red-shifts in the solid state due to strong intermolecular π-π stacking.25 As a result, the efficiencies of DPA-DSB-doped devices are strongly * Corresponding author. E-mail: [email protected]. † State Key Laboratory for Supramolecular Structure and Materials, Jilin University. ‡ National Laboratory of Integrated Optoelectronics, Jilin University.

dependent on the host materials and the doping concentration. Park and co-workers reported an EL efficiency of 4.1 cd/A utilizing DPA-DSB (1%)/ DPVBi (4,4′-bis(2,2′-diphenylvinyl)1,1′-biphenyl) combination.22 Then the EL efficiency increased to 9.7 cd/A when MADN (2-methyl-9,10-di(2-napthyl)anthracene) was used as the host material (DPA-DSB (3%)/ MADN) developed by Chen et al.23 Obviously, a host with powerful compatible ability can availably enhance the performance of the devices, and the host, which is designed totally to aim at the problem of guest molecules, may redound to increase the device efficiencies ulteriorly. More recently, we have demonstrated that oligo(phenylenevinylene) 2,5,2′,5′-tetrastyrylbiphenyl (TSB) could form highly quality vacuum evaporated films as well as highly efficient blue emission.26,27 TSB is a cruciform oligomer28 in which two distyrylbenzenes (DSB) chains are linked through a central biphenyl core, which is a special tether for its relatively free rotation and results in the conformational multiformity and relatively flexible in TSB. Therefore, being benefit from its mild flexibility, TSB molecule can provide larger loading ability for the guest DPA-DSB compared with some entirely rigid host materials. Herein, we report the high efficient blue organic light emitting devices based on the improved guest/host combination. (See Chart 1 for structures.) 2. Experimental Section The NMR spectra were recorded on AVANCZ 500 spectrometer at 298 K utilizing deuterated chloroform, dimethyl sulfoxide (DMSO) as solvent and tetramethylsilane (TMS) as standard. The mass spectra were recorded on a Kratos MALDI-TOF mass system.

10.1021/jp8029049 CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

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CHART 1: Molecular Structure of DPA-DSB and TSB

UV-vis absorption and fluorescence spectra were recorded on a UV-3100 and RF-5301PC spectrophotometer, respectively. The PL efficiencies of the films were measured using a calibrated integrating sphere. The films for PL experiments were formed on precleaned quartz plate at air atmosphere. Electrochemical measurements were performed with a BAS 100W Bioanalytical Systems, using a glassy carbon disk (Φ ) 3 mm) as working electrode, platinum wire as auxiliary electrode, with porous glass wick Ag/Ag+ as reference electrode. Cyclic voltammogram (CV) studies were carried out at a scan rate of 100 mV/s and in DMF solutions containing 0.1 M NBu4BF4 as supporting electrolyte. The electroluminescent (EL) spectra and Commission Internationale de l’Eclairage (CIE) coordination of the devices were measured by a PR650 spectroscan spectrometer. The luminancecurrent density-voltage characteristics were recorded simultaneously with the measurement of the EL spectra by combining the spectrometer with a Keithley model 2400 programmable voltage-current source. All measurements were carried out at room temperature under ambient conditions. Synthesis: The synthesis of these two PPV oligomers had been reported in the previous literature.22,26 Here we just present

some characteristic information about the materials used in the experiments. 1,4-Di(4′-N,N-diphenylaminostyryl)benzene (DPA-DSB). 1H NMR (500 MHz, DMSO-d ): δ [ppm] 7.562 (s, 4H, ArH), 6 7.521-7.510 (d, 4H, ArH), 7.342-7.310 (t, 2H, ArH), 7.235-7.202(d, 2H, -CHd), 7.131-7.099 (d, 2H, -CHd), 7.087-7.037 (m, 12H, ArH), 6.970-6.953 (d, 2H, ArH). 13C NMR (125 MHz, CDCl3): δ [ppm] 147.6, 147.4, 136.7, 131.6, 129.3, 127.9, 127.4, 126.7, 126.6, 124.5, 123.6, 123.1. MALDI-TOF MS: m/z ) 617.2 ([M + H]+)); calcd for C46H36N2, 616.29. 2,5,2′,5′-Tetrastyrylbiphenyl (TSB). 1H NMR (500 MHz, DMSO-d6): δ [ppm] 8.000-7.984 (d, 2H, ArH), 7.770-7.753 (d, 2H, ArH), 7.625-7.610 (d, 4H, ArH), 7.560 (s, 2H, ArH), 7.419-7.386 (d, 2H, -CHd), 7.375-7.362 (d, 4H, ArH), 7.362-7.329 (d, 2H, -CHd), 7.286-7.230 (m, 12H, ArH and -CHd), 7.180-7.167 (m, 2H, ArH), 6.781-6.748 (d, 2H, -CHd). 13C NMR (125 MHz, DMSO-d6): δ [ppm] 139.7, 137.0, 136.3, 134.5, 129.2, 128.9, 128.6, 128.4, 127.6, 126.4, 126.2, 126.0, 125.7, 125.2. MALDI-TOF MS: m/z ) 562.3 ([M]+)); calcd for C44H34, 562.27.

Figure 1. AFM images of the vacuum evaporated films: (a) TSB; (b) TSB:DPA-DSB (2 wt %); (c) TSB:DPA-DSB (8 wt %); (d) DPADSB.

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Figure 2. Calculated energy profile of the host TSB for the rotation around the biphenyl bond.

Figure 3. PL spectra of the TSB in film (black line) and the DPA-DSB-doped systems: 2 wt % (red line), 4 wt % (green line), and 8 wt % (blue line).

3. Results and Discussion 3.1. Morphology Characterization. The surface topographies of the films were investigated by atomic force microscopy (AFM). As shown in Figure 1, the vacuum evaporated films of the host TSB and TSB/DPA-DSB (concentration: 2-8 wt %) are very smooth with the height variation in the range of (1 nm (Figure 1a-c), which is much smaller than that of the cluster-shaped DPA-DSB film ((40 nm, Figure 1d). It indicates that the DPA-DSB/TSB combined films are homogeneous with little phase separation, which will certainly benefit for the next step of the fabrication of blue light emitting devices.29 3.2. The structure Particularity of the Host TSB. As mentioned above, TSB is a cruciform oligomer by linkage two distyrylbenzenes (DSB) through a central biphenyl core (inset of Figure 2), in which there is a dihedral angle of about 65.7 degrees between two linear DSB chromophore groups. This cruciform structure can greatly suppress the intermolecular interaction in the solid-state and prevent the forming of the big cluster aggregation in films during the vacuum deposition. As a result, the film-forming ability of the host TSB has been greatly improved and it shows strong blue light emitting in the solidstate. On the other hand, the biphenyl core is a flexible building

Figure 4. PL efficiencies of the neat DPA-DSB and TSB/DPA-DSB (2-8 wt %) films as well as DPA-DSB in dilute solution (10-5 mol L-1).

block, which will make it more advantages as host material for device fabrication. The flexible feature of TSB molecule is demonstrated by theoretical investigation through a total energy calculation of the of TSB molecule as rotating biphenyl bond. The energy calculation is based on the density functional theory (DFT) at B3LYP/6-31G level as implemented in Gaussian 98 for diversified configurations with fixed dihedral angles.30 The results reveal that in a wide-angle range of 60-120°, the energies of TSB molecule are very closed (Figure 2). There are two different minima for dihedral angle at 65.7 and 120°, respectively, and a series of conformations show close energy just around the two minima within a wide range approximately from 60 to 120°. Among these conformations, the energy barrier of rotation along the biphenyl bond is estimated to be about 12 meV, which is lower than the thermal energy kT at room temperature (25 meV). It means that there is a relatively free rotation of two rigid styrene units in TSB molecule along the biphenyl bond in the angle range of 60-120°. The energy analysis also suggests a big limitation as angles overrun the 60-120° due to strong intramolecular steric-hindrance. Thus

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Figure 5. Voltage-current density-luminance curve of the TSB and TSB/DPA-DSB (2 wt %) devices with structure of ITO/PEDOT:PSS/TCTA (40 nm)/TSB: × wt % DPA-DSB (20 nm)/BAlq (30 nm)/LiF (0.5 nm)/Al, where x ) 0, 2, and 8.

Figure 6. EL efficiency-voltage characteristics of the undoped and DPA-DSB doped devices with structure of ITO/PEDOT:PSS/TCTA (40 nm)/TSB: x wt % DPA-DSB (20 nm)/BAlq (30 nm)/LiF (0.5 nm)/ Al, where x ) 0, 2, 4, and 8.

Figure 7. EL and PL spectra of 2 wt % DPA-DSB-doped film. The insert is the magnified image from 390 to 450 nm. The device’s structure is ITO/PEDOT:PSS/TCTA (40 nm)/TSB: 2 wt % DPA-DSB (20 nm)/ BAlq (30 nm)/LiF (0.5 nm)/Al.

during film-forming process, the molecular stacking induced repulsion can trigger the mild rotation of the arms in TSB along the biphenyl bond to adjust its conformation for compact stacking. And as a host material, such partial flexibility provides an ability to contain guest molecules for larger loading amount through this kind of molecular conformation adjusting, which is superior with its partial flexibility over other fully rigid host materials such as 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi).31 3.3. Photoluminescent Properties. The guest DPA-DSB shows a strong blue emission at λmax ∼ 466 nm in dilute toluene solution, and the emission spectrum shifts to green region (502 nm) at solid state due to intermolecular aggregation (Supporting Information). Figure 3 shows the PL spectra of TSB/DPA-DSB combined films with doping concentration of 2∼ 8 wt %. Except there is a weak emission band (∼ 420 nm) from the host TSB in the low doping concentration (2 wt %), the doped films show strong emission at λmax ∼ 470 nm, which is close to the emission of DPA-DSB in dilute solution, indicating this crystalline guest has little aggregation in the host TSB substrate and the energy transfer from TSB host to DPA-DSB guest is very efficient.

The PL efficiencies of the DPA-DSB doped films have been measured using a calibrated integrating sphere.32 As shown in Figure 4, when DPA-DSB is doped into the host TSB in a doping concentration range from 2 to 8 wt %, the resulted films showobservablyenhancedfluorescencecomparedwithDPA-DSB neat film (PL efficiency: 13%). For example, the PL efficiency is 68% for the 2 wt % doped film and 72% for the 8 wt % doped film, respectively. These values almost approach the PL efficiency of the DPA-DSB in dilute toluene solution (78%), which further indicates that the host TSB can provide “comfortable accommodation” for the guest DPA-DSB, that is to say, the guest DPA-DSB can form evenly molecularly dispersed “solid solution’” in its the host material TSB and maintain the strong blue emission as that in dilute solution. Combined with the results of AFM investigation, it has been demonstrated that there is good chemical compatibility of DPA-DSB/TSB guest/ host systems. 3.4. Electroluminescent Properties. The EL properties of the guest/host system with different doping concentrations were tested by utilizing them as emissive layer with a device structure of ITO/PEDOT:PSS/TCTA (40 nm)/TSB: DPA-DSB (x wt %,

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Figure 8. (a) Cyclic voltammograms of the guest and host materials recorded in N,N-dimethylformamide (DMF) solutions (scan rate 100 mV/s). (b) The energy level diagram of the doped layer.

TABLE 1: EL Performances of TSB and TSB/DPA-DSB (x wt %) devices.

c

DPA-DSB (wt %)

max. brightness (cd/m2)

ηa (cd/A)

ηpb (lm/W)

0 2 4 8

3144 17350 15020 12720

1.61 12.2 9.9 8.9

0.78 6.39 5.18 4.66

a Maximal luminance efficiency; Sky blue.

b

CIE at 10 V x

y

0.16 0.15 0.16 0.16

0.19 0.26c 0.27c 0.28c

Maximal power efficiency;

20 nm)/BAlq (30 nm)/LiF (0.5 nm)/Al, where × ) 0, 1, 2, 4, and 8. The PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)/ Polystyrenesulphonate) combining with TCTA (4,4′′-tri(Ncarbazolyl)triphenylamine) were used as hole transporting layer, and BAlq (bis(2-methyl-8-quinolinolato)(p-phenylphenolate)aluminum(III)) was used as electron transporting layer. The EL spectra of DPA-DSB-doped devices show a peak at 468 nm with CIE coordinate at (0.16, 0.26∼0.28). Figure 5 is the voltage- current density- luminance characteristics of the TSB and TSB/DPA-DSB (2 wt %) devices. The devices exhibit gradually enhanced turn-on voltage along with the increasing of the doping concentration and the luminance of these doped devices are all over 10000 cd/m2. The details of the EL performances of these devices are summarized in Table 1. From the summarized data, it has been revealed that the optimized doping concentration is 2 wt %. At this doping level, the maximum luminous efficiency of 12.2 cd/A (corresponding to an external quantum efficiency of 6.2%) is obtained, and the maximum power efficiency is 6.4 lm/W as well as the maximum brightness is 17350 cd/m2. In addition, it is worth noting that, for the high doping concentration (4 wt % and 8 wt %), the devices can still maintain the high efficiency level of about 10 cd/A (Figure 6, ∼5 lm/W), which is approaching the highest EL efficiency in the blue guest/host fluorescent systems and observably extends the application for fabricating high performance devices in a broad doping concentration. As shown in Figure 5, the current density decreases at the same voltage as the doped concentration increased, reflecting another important feature of this guest/host system, namely the charge trapping mechanism. The differences between EL spectrum and the corresponding PL spectrum can also provide

the evidence of trapping mechanism existing in DPA-DSBdoped devices. As shown in Figure 7, at low doping concentration of 2 wt %, there is no emission from the host can be detected in the EL spectrum while obvious emission can be observed in the PL spectrum as shown in the inset. Thus it has been seen that except the efficient Fo¨rster energy transfer from the host TSB to the guest DPA-TSB, the trapping mechanism also play a role in this system. The energy levels of host and guest, which are obtained by cyclic voltammetry (CV) measurements, indicate that the trapping mechanism is very possible in DPA-DSB/TSB based devices. Figure 8 shows the CV of DPA-DSB guest and TSB host, and these two materials both show reversible reduction processes. The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels of the guest and host are also obtained through CV studies: the HOMO of TSB and DPA-DSB are at -5.47 and -5.06 eV, and the LUMO of TSB and DPA-DSB are at -2.53 and -2.55 eV, respectively. In general, electron and hole trapping will be most favorable when the LUMO level of host is higher and the HOMO level is lower than that of guest. Here, it is demonstrated that the energy gap of DPA-DSB (∼2.51 eV) is much smaller than that of TSB (∼ 2.93 eV) and just accord with what is required by charge trapping. From the energy diagram of the doped layer (Figure 8b), it can be found that there is a difference of 0.41 eV between the HOMO of the two materials but much smaller difference between LUMO (0.02 eV), indicating DPA-DSB may trap the holes from TSB. Previous investigations have demonstrated that the injection of hole predominates for PPV (poly(phenylenevinylene)) based polymers and oligomers,33,34 thus trapping holes (majority of charge carriers in emissive layer) may be favorable for the balance of the electrons and holes in the emissive layer. The improvement of the electrons and holes recombination will create a favorable positive effect for device performance. 4. Conclusion In summary, we have achieved highly efficient blue-emitting devices by doping DPA-DSB into the cruciform host material TSB. Combining rigid backbone structure and mild flexible property, the host TSB exhibits large loading ability to the guest DPA-DSB, and the doped films demonstrate good uniformity and chemical compatibility for a broad range of doping

Blue Organic Light-Emitting Devices concentration. From further research of the devices, it has been revealed that the Fo¨rster energy transfer and charge trapping processes both worked in this doping system. The host comfortably accommodates the guest and they are compatible in this analogical doping system, which makes them to be the promising candidates for blue guest/host EL materials. Acknowledgment. We are grateful for financial support from National Science Foundation of China (Grant Nos. 20573040, 20474024, 90501001, 50303007), the Ministry of Science and Technology of China (Grant Nos. 2002CB6134003 and 2003CB314703), China Postdoctoral Science Foundation (Grant No. 2005037703), 111 project (Grant No. B06009), and PCSIRT. Supporting Information Available: Text and table discussing single crystal X-ray diffraction data of DPA-DSB, synthesis route of oligomers, figures showing 1H NMR spectra, MALDITOF mass spectra of the TSB and DPA-DSB and the UV-vis and PL spectra of the guest DPA-DSB., and a scheme showing the synthesis route. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-EmittingDiodes Based on Conjugated Polymers. Nature 1990, 347 (6293), 539– 541. (2) Gong, X.; Wang, S.; Moses, D.; Bazan, G. C.; Heeger, A. J. Multilayer polymer light-emitting diodes: White-light emission with high efficiency. AdV. Mater. 2005, 17, 2053–2058. (3) Muller, D. C.; Braig, T.; Nothofer, H. G.; Arnoldi, M.; Gross, M.; Scherf, U.; Nuyken, O.; Meerholz, K. Efficient blue organic light-emitting diodes with graded hole-transport layers. ChemPhysChem 2000, 1 (4), 207– 211. (4) Chiang, C. L.; Tseng, S. M.; Chen, C. T.; Hsu, C. P.; Shu, C. F. Influence of molecular dipoles on the photoluminescence and electroluminescence of dipolar spirobifluorenes. AdV. Funct. Mater. 2008, 18 (2), 248– 257. (5) Tang, C. W.; Vanslyke, S. A.; Chen, C. H. Electroluminescence of Doped Organic Thin-Films. J. Appl. Phys. 1989, 65, 3610–3616. (6) Shi, J. M.; Tang, C. W. Doped organic electroluminescent devices with improved stability. Appl. Phys. Lett. 1997, 70, 1665–1667. (7) Shi, J. M.; Tang, C. W. Anthracene derivatives for stable blueemitting organic electroluminescence devices. Appl. Phys. Lett. 2002, 80, 3201–3203. (8) Kan, Y.; Wang, L. D.; Duan, L.; Hu, Y. C.; Wu, G. S.; Qiu, Y. Highly-efficient blue electroluminescence based on two emitter isomers. Appl. Phys. Lett. 2004, 84, 1513–1515. (9) Chen, C. H.; Tang, C. W.; Shi, J.; Klubek, K. P. Recent developments in the synthesis of red dopants for Alq(3) hosted electroluminescence. Thin Solid Films 2000, 363 (1-2), 327–331. (10) Tokailin, H.; Higashi, H.; Hosokawa, C. No. 513,063,0, 1992. (11) Hosokawa, C.; Higashi, H.; Nakamura, H.; Kusumoto, T. Highly efficient blue electroluminescence from a distyrylarylene emitting layer with a new dopant. Appl. Phys. Lett. 1995, 67, 3853–3855. (12) Ho, J. J.; Chen, C. Y.; Hsiao, R. Y.; Ho, O. L. The work function improvement on indium-tin-oxide epitaxial layers by doping treatment for organic light-emitting device applications. J. Phys. Chem. C 2007, 111, 8372–8376. (13) Chao, T. C.; Lin, Y. T.; Yang, C. Y.; Hung, T. S.; Chou, H. C.; Wu, C. C.; Wong, K. T. Highly efficient UV organic light-emitting devices based on bi(9,9-diarylfluorene)s. AdV. Mater. 2005, 17, 992–996.

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