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Synthesis and Luminescence of Distyrylstilbenes with Asymmetrically Substituted Functionalized Dendrons Chi Chung Kwok and Man Shing Wong* Department of Chemistry, The Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR China Received February 15, 2002. Revised Manuscript Received May 9, 2002
Highly luminescent distyrylstilbenes (DSBs) bearing either propoxy or oxadiazole-surfacefunctionalized poly(benzyl ether)-type dendritic wedges at one end have been synthesized by a stepwise Wadsworth-Emmon reaction. We show for the first time that an emissive core bearing asymmetrically substituted surface-functionalized dendrons exhibits more favorable luminescence properties than the symmetrically dendron-substituted core. All of these asymmetrically dendron-substituted DSBs display very similar optical characteristics in both solution and solid state, except for a few nanometers of blue shift in the absorption and emission maxima as compared to the corresponding symmetrically dendron-substituted DSBs. Their solution fluorescence lifetimes and photoluminescence quantum efficiencies were found to be smaller than those of the symmetrically dendron-substituted counterparts, which was attributed to the inefficient shielding of the surface-functionalized dendritic wedges of the asymmetrically dendron-substituted DSBs. However, the energy transfer efficiency of the oxadiazole-surface-functionalized G1-dendron-substituted DSB, 2, reaches 75%, which is the highest among all of the dendritic DSBs synthesized so far. Single-layer light-emitting diodes (LEDs) using dendritic DSB doped poly(N-vinylcarbazole) (PVK) film as an emissive layer with a structure of (ITO/DSB:PVK/Al) have been fabricated and investigated. Importantly, LEDs based on these asymmetrically dendron-substituted DSBs exhibit better device performance than do LEDs based on the corresponding symmetrically dendronsubstituted counterparts. Furthermore, there is a remarkable enhancement in device performance, particularly for oxadiazole-surface-functionalized asymmetrically dendronsubstituted DSB-based LEDs, when the emissive layer is blended with diphenylamine.
Introduction Organic electroluminescent materials, which can offer fast responses, high brightnesses, low drive voltages, large areas, low production costs, and ease of color tuning, have been extensively investigated for various flat-panel display applications in recent years.1-3 After the breakthroughs introduced by Tang et al. in 19874 and Burroughes et al. in 1990,5 significant progress has been made in enhancing light-emitting diode (LED) performance characteristics such as efficiency, stability, and brightness. Electroluminescence arises from the radiative decay of excitons of emissive molecules/ polymers formed by the recombination of holes and electrons injected from the opposite electrodes. An efficient LED thus requires the balance of charge transport and injection in the emissive material. However, many organic electroluminescent materials exhibit good hole-transport properties with lower injection * To whom correspondence should be addressed. (1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. Engl. 1998, 37, 402. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (3) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471. (4) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 52, 913. (5) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539.
barriers for holes than for electrons, which results in decreasing device efficiency, especially in single-layer LEDs. One widely adopted approach to improving the charge imbalance in polymeric materials is to incorporate electron-affinitive substituents such as oxadiazole,6,7 triazole,8 and triazine9 moieties, into the emissive π-conjugated main chains or to attach them as side chains. On the other hand, there are only a few examples of this strategy being used to design small molecules or π-conjugated oligomers with a balanced charge character. The merits of using monodisperse, well-defined molecules or oligomers as active components in LEDs are that they can easily be prepared in high purity and fabricated in thin films.3,10 They can also serve as a model for the related polymeric materials.11 Dendritic macromolecules, because of their unique dendritic architecture, have been widely investigated for various functional properties12-22 such as catalysis, (6) Peng, Z.; Bao, Z.; Galvin, M. E. Adv. Mater. 1998, 10, 680. (7) Bao, Z.; Peng, Z.; Galvin, M. E.; Chandross, E. A. Chem. Mater. 1998, 10, 1201. (8) Burn, P. L.; Grice, A. W.; Tajbakhsh, A.; Bradley, D. D. C.; Thomas, A. Adv. Mater. 1997, 9, 1171. (9) Po¨sch, P.; Fink, R.; Thelakkat, M.; Schmidt, H. W. Acta Polym. 1998, 49, 487. (10) Segura, J. L.; Martin, N. J. Mater. Chem. 2000, 10, 2403. (11) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, Germany, 1998.
10.1021/cm020214a CCC: $22.00 © 2002 American Chemical Society Published on Web 06/22/2002
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Figure 1. Molecular structures of distyrylstilbenes with asymmetrically substituted G1-surface-functionalized dendrons (1, 2) and the corresponding symmetrically dedron-substituted counterparts (1′, 2′).29
molecular recognition, self-assembly, energy transduction, and encapsulation. Recently, functional dendrimers have also been exploited as an active component in LED such as electron-transport layers,23 hole-transport layers,24 and emissive materials.25 The attractions of using dendritic luminescent molecules for LEDs are that the dendritic wedges/structures do not affect the fluorescent color of the emissive core and can shield the emissive cores from interacting with one another, which can alleviate the aggregation problem that is known to be detrimental to device efficiency.26-28 In addition, with an incorporation of proper charge-transport functionalities at the periphery of the dendritic wedges, the dendritic luminophore will become bifunctional, which might improve the imbalanced charge character of the luminophore. We have previously found that the functional groups on the surface of the dendritic wedges have a profound effect on the molecular properties of the luminescent dendrimers, including their energy transfer efficiency and their LED device performance. (12) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers; J. Wiley and Sons: New York, 2001. (13) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concept, Synthesis, Perspectives; VCH: Weinheim, Germany, 1996. (14) Fre´chet, J. M. J.; Hawker, C. J.; Aggarwal, S. L. Russo, S., Eds.; Pergamon Press: Oxford, U.K., 1996; p 140. (15) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1. (16) Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353. (17) Frey, H.; Lach, C.; Lorenz, K. Adv. Mater. 1998, 10, 279. (18) Fischer, M.; Vo¨gtle, F. Angew. Chem., Intl. Ed. 1999, 38, 885. (19) Bosman, A. W.; Jansen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (20) Majoral, J. P.; Caminade, A. M. Chem. Rev. 1999, 99, 845. (21) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701. (22) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74. (23) Kraft, A. J. Chem. Soc., Chem. Commun 1996, 77. (24) Kuwabara, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. Adv. Mater. 1994, 6, 677. (25) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv. Mater. 1996, 8, 237. (26) Halim, M.; Pillow, J. N. G.; Samuel, I. D. W.; Burn, P. L. Adv. Mater. 1999, 11, 371. (27) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson, M. E.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 12385. (28) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Burn, P. L.; Ba¨ssler, H. Adv. Mater. 2001, 13, 258.
The propoxy-surface-functionalized dendritic distyrylstilbene-based single-layer LED generally exhibits better device performance and stability than the oxadiazolesurface-functionalized dendrimer-based device.29 Continuing our effort to probe the structural factor(s) that can enhance the functional properties of luminescent dendrimers, we report herein the synthesis of a new series of bifunctional dendritic distyrylstilbenes (DSBs), 1-4 (see Figures 1 and 2), in which DSB core was asymmetrically incorporated with propoxy- or oxadiazole-surface-functionalized poly(benzyl ether)-type dendritic wedge(s) at one end and an investigation of the influence of the dendritic wedge(s) on various molecular properties, including the light-emitting properties. The effects of dopant on the device performance of these newly synthesized asymmetrically dendronsubstituted DSB-based single-layer LEDs were also pursued. Experimental Section All of the new dendrimers were fully characterized with standard spectroscopic techniques. 1H NMR spectra were recorded using a JEOL JHM-EX270 FT NMR spectrometer and are referenced to the residual CHCl3 peak at 7.24 ppm. 13 C NMR spectra were recorded using a Varian INOVA-400 FT NMR spectrometer and are referenced to the CDCl3 peak at 77 ppm. All physical measurements were performed in CHCl3. Electronic absorption (UV-vis) and fluorescence spectra were recorded using a Varian Cary 100 Scan spectrophotometer and a PTI luminescence spectrophotometer, respectively. The fluorescence quantum yields in chloroform using 9,10-diphenylanthrancene as the standard were determined by the dilution method as described by Parker et al.30 The fluorescence decay curves were recorded on a PTI fluorescence Master 2M1 luminescence spectrophotometer using a PTI G23300 nitrogen laser for excitation. The lifetimes were estimated from the measured fluorescence decay using an iterative fitting procedure. Thermal stabilities were determined by using a Perkin-Elmer TGA-6 thermal gravimetric analyzer with a heating rate of 10 °C/min. Dendritic DSB-doped single-layer LEDs were fabricated by spin-coating a PVK polymer solution containing 0.133 mmol/g (29) Kwok, C. C.; Wong, M. S. Macromolecules 2001, 34, 6821. (30) Parker, C. A.; Rees, W. T. Analyst 1960, 85, 587.
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Figure 2. Molecular structures of distyrylstilbenes with asymmetrically substituted G2- surface-functionalized dendrons (3, 4) and the corresponding symmetrically dedron-substituted counterparts (3′, 4′).29 of DSB onto ITO glass substrates. The total solid concentration of the polymer solution was 15.4-17.2 mg/mL in chloroform. The DSB/PVK solution was filtered through a 2-µm PTFE filter, and the ITO glass, which had a sheet resistance of 80 Ω/m2, was cleaned with UV ozone before use. The typical polymer film thickness was about 100 nm. After the film had been baked for 48 h at 60 °C under vacuum, the aluminum cathode was vapor deposited onto the polymer film. The
thickness of the cathode was typically 100 nm. Evaluation of the LEDs was performed under ambient conditions. The external quantum efficiency was calculated from the measured EL intensity divided by the current density passing through the device. General Procedure for the Wadsworth-Emmons Reaction. To a solution of 1:1 equiv of (4-styryl)benzylphosphonate ester and the corresponding G1 or G2-dendron-substi-
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Scheme 1. General Scheme for the Synthesis of First-Generation and Second-Generation Asymmetrically Dendritic-Wedge-Substituted Distyrylstilbenes, 1-4
tuted benzaldehyde in anhydrous THF was slowly added 1.2 equiv of potassium t-butoxide. After being stirred overnight, the reaction mixture was quenched with water. The crude product was extracted twice with CH2Cl2, dried over anhydrous MgSO4, and evaporated to dryness. The crude product was then purified by silica gel chromatography. For 1, the above procedure was followed using 99 mg (0.31 mmol) of phosphonate, 165 mg (0.3 mmol) of aldehyde, and 42 mg (0.37 mmol) of potassium t-butoxide. The pure product was separated by silica gel column chromatography using CH2Cl2 as the eluent to afford 150 mg (68.8%) of a yellow solid. 1H NMR (270 MHz, CDCl3, δ): 7.52-7.48 (m, 5H), 7.38-7.27 (m, 4H), 7.10 (s, 2H), 7.03 (s, 2H), 6.75 (d, J ) 1.89 Hz, 2H), 6.57 (d, J ) 2.16 Hz, 4H), 6.53 (t, J ) 2.02 Hz, 1H), 6.41 (t, J ) 2.16 Hz, 2H), 4.98 (s, 4H), 3.90 (t, J ) 6.62 Hz, 8H), 1.851.72 (m, 8H), 1.02 (t, J ) 7.43 Hz, 12H). 13C NMR (67.8 MHz, CDCl3, δ): 160.4, 160.0, 139.2, 138.9, 137.2, 136.7, 136.4, 128.7, 128.6, 128.5, 128.4, 128.1, 127.6, 126.8, 126.7, 126.4, 105.7, 101.6, 100.8, 70.2, 69.6, 22.7, 10.7. MS (FAB): m/z 727 (M+). HRMS (MALDI-TOF): calcd for C48H55O6NaK, 789.3533; found, 789.2993 (M+ + H + Na + K). mp 51-53 °C; decomposition temperature 227 °C. For 2, the above procedure was followed using 120 mg (0.364 mmol) of phosphonate, 250 mg (0.348 mmol) of aldehyde, and 50 mg (0.44 mmol) of potassium t-butoxide. The pure product was separated by silica gel column chromatography using CH2Cl2 as the eluent to afford 290 mg (93%) of a greenish-yellow solid. 1H NMR (270 MHz, CDCl3, δ): 8.16 (d, J ) 8.37 Hz, 4H), 8.05 (d, J ) 8.64 Hz, 4H), 7.61 (d, J ) 8.37 Hz, 4H), 7.54 (d, J ) 8.64 Hz, 4H), 7.49 (s, 2H), 7.38-7.32 (m, 7H), 7.10 (bs, 2H), 7.05 (d, J ) 2.43 Hz, 2H), 6.80 (d, J ) 1.89 Hz, 2H), 6.55 (t, J ) 2.03 Hz, 1H), 5.17 (s, 4H), 1.36 (s, 18H). 13C NMR (100 MHz, CDCl3, δ): 164.7, 164.1, 159.8, 155.4, 140.6, 139.6, 136.9, 136.2, 129.1, 128.7, 128.1, 127.7, 127.6, 127.1, 126.9, 126.8,
126.7, 126.5, 126.0, 123.5, 121.0, 105.9, 101.6, 69.4, 35.1, 31.1. MS (FAB): m/z ) 895 (M+). HRMS (MALDI-TOF): calcd for C60H54N4O4, 894.4145; found, 894.4237 (M+). mp 121-124 °C; decomposition temperature 294 °C. For 3, the above procedure was followed using 115 mg (0.348 mmol) of phosphonate, 422 mg (0.35 mmol) of aldehyde, and 45 mg (0.40 mmol) of potassium t-butoxide. The pure product was separated by silica gel column chromatography using CH2Cl2 as the eluent to afford 337 mg (69.6%) of a yellow solid. 1H NMR (270 MHz, CDCl3, δ): 7.52-7.48 (m, 5H), 7.38-7.27 (m, 4H), 7.10 (s, 2H), 7.04 (s, 2H), 6.75 (d, J ) 2.16 Hz, 2H), 6.68 (d, J ) 2.16 Hz, 4H), 6.55-6.54 (m, 11H), 6.39 (t, J ) 2.16 Hz, 4H), 4.99 (s, 4H), 4.95 (s, 8H), 3.88 (t, J ) 6.48 Hz, 16H), 1.841.71 (m, 16H), 1.00 (t, J ) 7.23 Hz, 24H). 13C NMR (67.8 MHz, CDCl3, δ): 160.3, 160.0, 159.9, 139.3, 139.1, 138.8, 137.2, 136.7, 136.3, 128.8, 128.7, 128.6, 128.5, 128.3, 128.1, 126.8, 126.7, 126.4, 126.3, 106.3, 105.7, 101.5, 100.8, 70.2, 70.1, 69.5, 22.6, 10.6. MS (FAB): m/z 1384 (M+). HRMS (MALDI-TOF): calcd for C88H103O14NaK, 1445.6882; found, 1445.6006 (M+ + H + Na + K). mp 45-48 °C; decomposition temperature 259 °C. For 4, the above procedure was followed using 33 mg (0.1 mmol) of phosphonate, 156 mg (0.1 mmol) of aldehyde, and 15 mg (0.13 mmol) of potassium t-butoxide. The pure product was separated by silica gel column chromatography using CH2Cl2 as the eluent to afford 40 mg (23.3%) of a yellow solid. 1H NMR (270 MHz, CDCl3, δ): 8.11 (d, J ) 7.83 Hz, 8H), 8.02 (d, J ) 8.10 Hz, 8H), 7.56-7.31 (m, 25H), 7.07 (bs, 2H), 6.95 (bs, 2H), 6.69 (bs, 6H), 6.57 (bs, 2H), 6.46 (bs, 1H), 5.13 (s, 8H), 5.03 (s, 4H), 1.34 (s, 36H). 13C NMR (67.8 MHz, CDCl3, δ): 164.5, 163.9, 159.8, 159.7, 155.2, 140.4, 140.0, 139.9, 137.1, 136.7, 136.1, 128.6, 128.1, 127.9, 127.6, 127.5, 127.0, 126.8, 126.7, 126.6, 126.4, 126.0, 123.5, 120.9, 106.4, 106.3, 104.1, 103.9, 69.4, 35.1, 31.1. MS (FAB): m/z 1722 (M+). HRMS (MALDI-TOF): calcd for C112H102N8O10, 1718.7719; found,
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Table 1. Summary of Physical Measurements of Asymmetrically Dendritic-Wedge-Substituted Distyrylstilbenes 1-4
1 2 3 4
λmaxa/nm (max/10-4 M-1 cm-1)
λdena/nm (max/10-4 M-1 cm-1)
emission band maximaa/nm
FL quantum yieldb
energy transfer efficiencyb/%
FL lifetime in solution/nsa
FL lifetime in thin film/nsd
decomposition tempe/°C
359 (5.40) 359 (5.44) 359 (4.39) 360 (4.56)
284 (1.04) 295 (5.40) 284 (1.88) 294 (11.6)
395 418 394 417 395 417 396 418
0.88 0.93 0.89 0.91
52 75 31 61
0.92 0.79 0.94 0.90
2.01 2.25 2.06 2.44
227 294 259 350
a Measured in CHCl . b Determined by the dilution method. c Determined by comparing the absorption maxima of dendritic wedges in 3 the absorption and fluorescence excitation spectra. d Measured in DSB-doped PVK thin films. e Decomposition temperature determined by TGA.
Figure 3. Normalized photoluminescence and electroluminescence spectra of one-side dendritic-wedge-substituted DSBs 1-4 doped in PVK thin films. For clarity, each pair of spectra is offset. 1719.7792 (M+ + H). mp 120-123 °C; decomposition temperature 350 °C.
Results and Discussion The general synthetic pathway for the preparation of distyrylstilbenes bearing asymmetrically substituted surface-functionalized dendrons, 1-4, is outlined in Scheme 1. 4-(Styryl)benzylphosphonate ester was prepared by the dropwise addition of benzaldehyde to excess bis(diethyl)-p-xylylene(bis(phosphonate)) and NaH in DME.31 First-generation (G1) and second-generation (G2) surface-functionalized poly(benzyl ether)-type dendritic benzaldehydes were prepared according to previously published procedure.29 The stereoselective Wadsworth-Emmons reaction of 4-(styryl)benzylphosphonate ester and the corresponding surface-functionalized (G1 or G2) dendron-substituted benzaldehyde afforded the desired G1 and G2 dendritic distyrylstilbenes 1-4. All of the new dendrimers were fully characterized with standard spectroscopic techniques, including 1H NMR spectroscopy, 13C NMR spectroscopy, low-resolution mass spectroscopy (FAB), and high-resolution mass (31) Wong, M. S.; Li, Z. H.; Shek, M. F.; Chow, K. H.; Tao, Y.; D’Iorio, M. J. Mater. Chem. 2000, 10, 1805.
spectroscopy (MALDI-TOF). All of the G1 and G2 asymmetrically dendron-substituted DSBs are highly soluble in common organic solvents. All of these asymmetrically dendron-substituted DSBs 1-4 exhibit very similar absorption behaviors and vibronic structures with absorption maxima of the DSB core at 359 nm (λmax) and of dendritic wedge, λden, at 284 nm (for 1 and 3) or 295 nm (for 2 and 4). Although the surface-functionalized dendritic wedges are attached at the unconjugated position of the DSB core, there is consistently a 3-nm blue shift in λmax compared to that of the corresponding symmetrically dendron-substituted counterparts. A monotonic increase in the absorbance of the dendritic wedges is observed as the generation/ the number of the wedges increases (Table 1). In view of the emission spectra, all of the G1 and G2 dendrimers exhibit apparently identical blue light emissions with bands at 395 and 417 nm when excited at 359 nm. These emission bands are also slightly blue-shifted (by 3 nm) relative to those of the symmetrically dendron-substituted DSBs. Direct irradiation of the surface-functionalized dendritic wedges of all of these dendrimers leads to very strong blue light emission from the DSB core. This clearly suggests that the energy is funneled from the surface-functionalized dendrons to the DSB core. To
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Figure 4. Current-voltage and light-voltage characteristics of single-layer LEDs with a structure of (ITO/DSB:PVK/Al) doped with asymmetrically propoxy-functionalized dendritic-wedge-substituted DSBs 1 and 3 and their corresponding symmetrically dendritic-wedge-substituted DSBs 1′ and 3′. The inset shows the curves of external efficiency as a function of the current density.
estimate the energy transfer efficiency of these dendrimers, the difference in intensity at the absorption maxima of the dendritic wedges in the absorption spectrum and in the fluorescence excitation spectrum recorded at the emission of the core of the corresponding DSB was compared.32 The results are presented in Table 1. It was consistently found that the energy transfer efficiency decreased as the generation of the surfacefunctionalized poly(benzyl ether)-type dendritic wedge increased. The oxadiazole-surface-functionalized asymmetrically dendron-substituted DSBs 2 and 4 (75 and 61%, respectively) exhibited substantially higher energy transfer efficiencies than did their corresponding symmetrically dendron-substituted counterparts (59 and 53%, respectively).29 It is important to note that the energy transfer efficiency of 2 reaches 75%, which is the highest among all dendritic DSBs. This indicates that the oxadiazole-surface-functionalized G1 dendritic wedge is a very efficient light-harvesting moiety for funneling energy to the core and the more oxadiazole-surfacefunctionalized dendritic wedge incorporated, the lower the energy transfer efficiency. On the other hand, the energy transfer efficiencies of the propoxy-surfacefunctionalized asymmetrically dendron-substituted DSBs 1 and 3 (52 and 31%, respectively) are comparable to those of their symmetrically dendron-substituted counterparts (48 and 32%, respectively).29 In addition, the number of surface-functionalized dendritic wedges attached has a pronounced influence on the luminescence properties of the emissive core. All of these asymmetrically dendron-substituted DSBs show very high fluorescence quantum yields (88-93%) because of the rigidity and coplanarity of their DSB cores; however, their yields appear to be slightly lower than those of (32) Stryer, L.; Haugland, R. P. Proc. Natl. Acad. Sci. U.S.A. 1967, 58, 719.
the symmetrically dendron-substituted counterparts (93-99%). Furthermore, the asymmetrically dendronsubstituted DSBs exhibit shorter fluorescence lifetimes (0.79-0.94 ns) than the corresponding symmetrically dendron-substituted ones (1.39-1.48 ns). These results consistently indicate that the dendritic wedges play an important role in shielding/isolating the DSB core. The thermal gravimetric analysis shows that all dendritic DSBs exhibit good thermal stabilities, with decomposition temperatures in the range of 227-350 °C under nitrogen atmosphere. The solid-state photoluminescence (PL) spectra were measured from dendritic-DSB-doped poly(N-vinylcarbazole) (PVK) film. Single-layer LEDs using dendritic DSB-doped PVK film as the emissive layer with a structure of (ITO/DSB:PVK/Al) were fabricated and investigated. The PL and electroluminescence (EL) spectra of dendrimers 1-4 are shown in Figure 3. The spectral characteristics of the thin-film PL and EL spectra are very similar to those measured in solution. This suggests that both the PL and EL emissions originate from the asymmetrically dendron-substituted DSBs and not from the PVK host, which has a peak emission at 372 nm. There is about a 10-nm red shift in the emission bands in both the thin-film PL and EL spectra exhibited as compared to those measured in solution. The solid-state fluorescence lifetimes of these asymmetrically dendron-substituted DSBs (2.01-2.44 ns) are comparable to those of their symmetrically dendron-substituted counterparts (2.08-2.74 ns). To probe and understand the correlations between the structure and light-emitting properties of 1-4, the corresponding symmetrically dendron-substituted DSBbased LEDs were also fabricated and investigated under the same conditions. The turn-on voltages for the light and the current are similar for both the 1- and 3-based
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Figure 5. Current-voltage and light-voltage characteristics of single-layer LEDs with a structure of (ITO/DSB:PVK/Al) doped with asymmetrically oxadiazole-functionalized dendritic-wedge-substituted DSBs 2 and 4 and their corresponding symmetrically dendritic-wedge-substituted DSBs 2′ and 4′. The inset shows the curves of external efficiency as a function of the current density.
Figure 6. Normalized photoluminescence and electroluminescence spectra of asymmetrically propoxy-functionalized dendriticwedge-substituted dendrimer 1 blended with different molar ratios of diphenylamine (DPA) doped in PVK thin films as an emissive layer. For clarity, each pair of spectra is offset.
LED devices, indicating reasonably balanced charge carriers (Figure 4). However, the turn-on voltage for a 1- or 3-based device is slightly lower than that of a device based on the corresponding symmetrically dendron-substituted DSB (1′ or 3′, respectively). The improvement in device performance of the asymmetrically dendron-substituted DSB-based LEDs such as the turnon voltage and brightness in the oxadiazole-functional-
ized series is more dramatic, as shown in Figure 5. The imbalance in charge carriers is also alleviated in the 2and 4-based devices. This is in sharp contrast to the devices based on the symmetrically dendron-substituted DSBs, in which the onset of the light occurs earlier than the onset of the current. This is presumably due to the effective shielding of the DSB core by the dendritic wedges, which increases the barrier (or potential) for
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Figure 7. Normalized photoluminescence and electroluminescence spectra of asymmetrically oxadiazole-functionalized dendriticwedge-substituted dendrimer 2 blended with different molar ratios of diphenylamine (DPA) doped in PVK thin film as an emissive layer. For clarity, each pair of spectra is offset.
Figure 8. Current-voltage and light-voltage characteristics of single-layer LEDs based on a blend of asymmetrically oxadiazolefunctionalized dendritic-wedge-substituted dendrimer 2 with different molar ratios of diphenylamine (DPA) with the device structure (ITO/DSB:PVK/Al). The inset shows the curves of external efficiency as a function of the current density.
charge recombination at the emissive core. The shielding is more effective in the symmetrically dendronsubstituted DSBs than in the asymmetrically dendronsubstituted ones. In addition to the shielding effect of the dendritic wedges, the large number of oxadiazole moieties at the periphery of the dendritic wedges, which can act as electron traps, is responsible for the poor device performance of the oxadiazole-surface-function-
alized dendron-substituted DSB-based LEDs. The device performance of G1 dendritic DSB-based LEDs is again better than that of the corresponding G2 dendrimer-based devices. To further enhance the device performance of the oxadiazole-based LEDs, a hole-transporting/blocking material, diphenylamine (DPA), was blended into the emissive layer. There is no apparent difference between
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Figure 9. Current-voltage and light-voltage characteristics of single-layer LEDs based on a blend of asymmetrically propoxyfunctionalized dendritic-wedge-substituted dendrimer 1 with different molar ratios of diphenylamine (DPA) with the device structure (ITO/DSB:PVK/Al). The inset shows the curves of external efficiency as a function of the current density.
the EL and PL spectra of the doped and undoped devices (Figures 6 and 7). Figure 8 shows the I-V-L characteristics of the LEDs based on the blend of 2 with different molar ratios of DPA as an emissive layer. It is important to note that there is significant improvement in the maximum luminance (up to a 4-times increase in brightness), external quantum efficiency (more than 1 order of magnitude in enhancement), and charge balance of the LEDs with increasing content of DPA. This further supports our presumption that an excess number of oxadiazole moieties can cause detrimental effect in the device performance. For the propoxy-based (1-based) devices, blending with DPA can also enhance the device performance substantially, provided that the doping level does not reach saturation (Figure 9). As our focus is mainly on an investigation of the structural factors that can enhance various molecular properties in the present studies, no attempt was made to further optimize the device performance, although device performance can be enhanced by various means such as using low-resistance ITO glass, low-work-function cathodes, or multilayer device structures. Conclusions In summary, we have successfully synthesized a new series of blue-light-emitting distyrylstilbenes (DSBs) bearing either propoxy or oxadiazole moieties on the outer surface of the first- and second-generation poly(benzyl ether)-type dendritic wedges at one end. Both the absorption and the emission maxima of the DSB cores of these asymmetrically dendron-substituted dendrimers are slightly blue-shifted as compared with those of the asymmetrically dendron-substituted ones. The energy transfer efficiencies of the oxadiazole-surface-
functionalized dendron-substituted DSBs are substantially higher than those of their symmetrically dendronsubstituted counterparts. On the other hand, the solution fluorescence lifetimes and photoluminescence quantum efficiencies were smaller, which is due to the inefficient shielding of the dendritic wedges substituted asymmetrically at one end only. However, both asymmetrically and symmetrically dendron-substituted DSBs show comparable fluorescence lifetimes in the solid state. The asymmetrically dendron-substituted DSBbased LEDs consistently exhibited superior device performance compared to LEDs based on the symmetrically dendron-substituted counterparts, particularly for the oxadiazole-surface-functionalized dendronsubstituted DSBs. With blending of diphenylamine in the emissive layer, the external quantum efficiencies and charge balance properties of the single-layer LEDs are greatly enhanced. The poor device performance of the oxadiazole-surface-functionalized dendron-substituted DSB-based LEDs is presumably due to a large excess of the electron-affinitive oxadiazole moieties which could act as electron traps, leading to a reduction of electron transport. Acknowledgment. This work was partially supported by an Earmarked Research Grant (HKBU 2051/ 01P) from the Research Grants Council, Hong Kong. We gratefully acknowledge Dr. Y. Tao at NRC, Canada, for the MALDI-TOF mass spectroscopy measurements and Dr. S. K. So at the Department of Physics, HKBU, for access to his research facility to perform the EL experiments. CM020214A
