High-Efficiency Polymer-Based Electrophosphorescent Devices

Received: May 28, 2001 Final version: January 28, 2002

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[1] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001, 11, 15. [2] J. Rostalski, D. Meissner, Sol. Energy Mater. Sol. Cells 2000 61, 87. [3] a) P. Peumans, V. Bulovic, S. R. Forrest, Appl. Phys. Lett. 2000, 76, 2650. b) P. Peumans, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 126. [4] U. Bach, D. Lupo, R. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Grätzel, Nature 1998, 395, 583. [5] M. Granström, K. Petritsch, A. C. Arias, A. Lux, M. R. Andersson, R. H. Friend, Nature 1998, 395, 257. [6] C. W. Tang, Appl. Phys. Lett. 1986, 48, 183. [7] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [8] B. O'Regan, M. Grätzel, Nature 1991, 353, 737. [9] J. Hagen, D. Haarer, Synth. Met. 1997, 89, 215. [10] W. Kubo, K. Murakoshi, T. Kitamura, Y. Wada, K. Hanabusa, H. Shirai, S. Yanagida, Chem. Lett. 1998, 12, 1241. [11] L. S. Roman, W. Mammo, L. A. A. Pettersson, M. R. Andersson, O. Inganäs, Adv. Mater. 1998, 10, 774. [12] L. Ouali, V. V. Krasnikov, U. Stalmach, G. Hadziioannou, Adv. Mater. 1999, 11, 1515. [13] D. Wöhrle, B. Tennigkeit, J. Elbe, L. Kreienhoop, G. Schnurpfeil, Mol. Cryst. Liq. Cryst. 1993, 228, 221. [14] T. Tsutsui, T. Nakashima, Y. Fujita, S. Saito, Synth. Met. 1995, 71, 2281. [15] U. Bach, M. Graetzel, D. Lupo, J. Salbeck, F. Weisoertel, German Patent DE 19 711 713, 1998. [16] H. K. Pulker, G. Paesold, E. Ritter, Appl. Opt. 1976, 15, 2986. [17] H. K. Pulker, U.S. Patent 3 927 228, 1975. [18] C. Schmitz, M. Thelakkat, H.-W. Schmidt, Chem. Mater. 2000, 12, 3012. [19] C. Schmitz, P. Pösch, M. Thelakkat, H.-W. Schmidt, Phys. Chem. Chem. Phys. 1999, 1, 1777. [20] C. Schmitz, M. Thelakkat, H.-W. Schmidt, Adv. Mater. 1999, 11, 821. [21] N. B. McKeown, Phthalocyanine Materials, Cambridge University Press, Cambridge 1998. [22] High resolution SEM was carried out using a LEO 1530 instrument with a Gemini thermal field emission column. EDX analysis was carried out in situ using an Inca EDX system from Oxford Instruments.

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High-Efficiency Polymer-Based Electrophosphorescent Devices** By Xiong Gong, Matthew R. Robinson, Jacek C. Ostrowski, Daniel Moses, Guillermo C. Bazan,* and Alan J. Heeger* Organic light-emitting diodes (OLEDs) are under active investigation because of their potential for application in flat panel displays.[1,2] Research on OLEDs has focused on the improvement of the emission efficiency by developing highefficiency fluorescent materials and on the use of novel device architectures. The existence of an upper limit for the electroluminescence (EL) internal quantum efficiency (QE) has been widely discussed. Since charge carrier recombination in p-conjugated systems can produce both emissive singlet and non-emissive triplet excited states, a simple statistical argument leads to an upper limit for the internal QE of 25 %.[3] The 25 % upper limit assumes similar cross sections for the formation of singlets and triplets.[4] Experiments have recently shown, however, that in luminescent semiconducting polymers, the singlet cross section is considerably larger than the triplet cross section (by a factor of 3±4).[5] As a result, electroluminescent quantum efficiencies as high as 50 % have been reported.[3] Nevertheless, by utilizing a phosphorescent dye that captures both singlet and triplet excited states, the OLED internal efficiency can, in principle, be increased to 100 %. Utilization of the triplet excitations in OLEDs was proposed several years ago.[6,7] These devices incorporate a heavy metal atom with strong spin±orbit coupling that enhances intersystem crossing and mixes the singlet and triplet states. In this way, the lowest triplet state is efficiently populated and can produce light emission via phosphorescence. Recently, electrophosphorescent OLEDs based on small molecules as the transport materials have achieved considerable success.[8,9,10,11] For example, an external quantum efficiency (QEext) of 13.7 % and power efficiency of 38.3 lm/W at 0.215 mA cm±2 were reported using [Ir(ppy)3] as the phosphorescent dopant for green-light

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[*] Prof. A. J. Heeger Department of Physics and Materials Department University of California at Santa Barbara Santa Barbara, CA 93106 (USA) E-mail: [email protected] Prof. G. C. Bazan Department of Chemistry and Materials University of California at Santa Barbara Santa Barbara, CA 93106 (USA) E-mail: [email protected] Dr. X. Gong, M. R. Robinson, J. C. Ostrowski, Dr. D. Moses Institute for Polymers and Organic Solids Santa Barbara, CA 93106-5096 (USA)

[**] This work was supported by the Mitsubishi Chemical Center for Advanced Materials at UCSB and, in the initial phase, by the Air Force Office of Scientific Research, F49620-99-1-0031, Charles Lee, Program Officer. The authors are grateful to Dr. Gang Yu (UNIAX Corporation) for valuable suggestions and discussions, and for help with calibration. We also thank Dr. M. B. O'Regan at UNIAX Corporation for useful discussions. Drs. C. Y. Yang, Jian Wang, Deli Wang, and V. Srdanov are gratefully acknowledged for assistance in various aspects of the experimental work.

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In our opinion, the concept of fully vapor-deposited thinlayer TiO2 solar cells presented here opens a very promising approach in the expanding field of solar cell research, to achieve inexpensive and stable devices for large field of application. This cell fabrication method enables a direct integration in the existing technology of vapor-deposited electrooptic thin-layer devices. We believe that the performance of TiO2 cells under solar conditions can be further improved by the use of narrow bandgap dyes, with the aims of expanding the spectral absorption domain and to match the photon flux spectrum of the Sun. Furthermore, by suitable device engineering the interface area between TiO2 and the dye responsible for charge separation can be increased, resulting in more efficient devices. Moreover, the limitation caused by the very small diffusion length of charge carriers, in the range of 10 nm, can be overcome by optimizing the individual layer thicknesses in the device into the same range as the diffusion length of charge carriers. The concepts of increased light harvesting as recently demonstrated in vapor-deposited thinlayer cells by Forrest et al.[3] and the use of dopants[2] to decrease the series resistance in the device can also lead to improved device performance.

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emission.[9] Forrest et al. reported QEext = 12.3 % in devices fabricated by co-evaporating the iridium complex and 4,4¢N,N¢-dicarbazolebiphenyl (CBP) in a multi-layer structure.[12] Recently, a QEext »1.8 % was achieved by incorporating the phosphorescent dopant, fac-tris[2-(4¢,5,-difluorophophenyl)pyridine-C¢2,N] iridium(III) [F(Irppy)3], into poly(N-vinylcarbazole) (PVK) based LEDs.[13] These high efficiencies demonstrate the advantages of using phosphorescent dopants as emission centers in organic LEDs. In this communication, we report on high-efficiency yellow±green electrophosphorescent OLEDs, fabricated by doping tris[9,9-dihexyl-2-(pyridinyl-2¢)fluorene] iridium(III) (Ir(DPF)3) into a host polymer matrix of PVK blended with the electron transport molecule, 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD). Depending on the [Ir(DPF)3] concentration, the devices showed external quantum efficiencies as high as 10 % photon/electron and luminous efficiencies as high as 36 cd A±1. The operating voltage of the electrophosphorescence increased with increased [Ir(DPF)3] concentration. Brightness in excess of 8000 cd m±2 was achieved at 75 mA cm±2 (55 V). Emission from the dopant molecules in such devices involves localization of the injected electron and hole on the metal-organic center. This can occur by a variety of mechanisms, including Förster and/or Dexter energy transfer from the host transport material to the dopant, and direct trapping of both electrons and holes on the metal-organic center. In the Förster mechanism,[14,15] the dipole±dipole interaction results in efficient transfer of the singlet excited-state energy from host to guest. The efficiency of the Förster mechanism is dependent on the spectral overlap between the host emission spectrum and the guest absorption spectrum and varies as the inverse sixth power on the host±guest separation. Typically, the maximum distance over which Förster energy transfer can occur is 30±50 Š. Dexter transfer requires direct quantum mechanical tunneling of electrons between the host and guest. It is therefore a shorter range process that requires separations of only a few Angstroms. In addition to singlet±singlet energy transfer, the Dexter mechanism also allows triplet± triplet energy transfer. In the direct electron and hole trapping mechanism,[16,17] an excited dopant molecule is formed by the sequential trapping of a separate hole and electron onto the dopant metal-organic complex. Electron and hole trapping is most favorable if the highest occupied molecular orbital (HOMO) level of the guest is above (closer to vacuum level) that of the host, and if the lowest unoccupied molecular orbital (LUMO) level of the guest is below (farther from vacuum level) that of the host. Charge trapping and localization onto the guest requires overlap of the molecular orbitals of the host and guest molecules. PVK was selected as host because its emission spectrum overlaps with the absorption spectrum of [Ir(DPF)3]; PVK emits blue light.[18,19] PVK is known as a good hole-transporting material. It is not, however, an electron-transporting material.[20]] Thus, in order to enable the host to transport both electrons and holes, PBD was mixed with PVK.[18] The molec-

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Fig. 1. a) The molecular structures of [Ir(DPF)3], PVK, and PBD. b) The normalized absorption spectra of PVK-PBD (40 wt.-%) film (n) and neat [Ir(DPF)3] film (l), and photoluminescence spectra of PVK-PBD (40 wt.-%) film (u) and neat [Ir(DPF)3] film (~).

ular structures of [Ir(DPF)3], PVK, and PBD are shown in Figure 1a. For device fabrication, we employed only the single-activelayer configuration with poly(3,4-ethylene dioxythiophene), PEDOT, on indium tin oxide (ITO) as the hole-injecting bilayer electrode. The device structure is as follows: (ITO)/PEDOT/PVK-PBD (40 wt.-%):[Ir(DPF)3]/Ca/Ag. PEDOT was spin-cast onto a pre-cleaned ITO surface and baked at 120 C for 2 h. The emitting layer, [Ir(DPF)3] doped in PVK-PBD (40 wt.-%) was then spin cast onto the surface of PEDOT. Thicknesses of the doped PVK-PBD layers were in the range 100±200 nm. Transmission electron microscopy (TEM) of the PVK-PBD (40 wt.-%) films doped with [Ir(DPF)3] showed no evidence of phase separation. The synthesis and characteristics of the phosphorescent [Ir(DPF)3] are described elsewhere.[21] The concentrations of [Ir(DPF)3] used were 0.01 %, 0.05 %, 0.1 %, 0.3 %, 0.5 %, 1 %, 3 %, 5 %, and 8 % by weight. Note that since the molecular weight of the Ir-complex is 1660, the concentration of the dopant per repeat unit of the PVK is approximately an order of magnitude less. The Ca/Ag cathode was deposited through a shadow mask by thermal evaporation at 2 ” 10±7 torr. The current±voltage and brightness±voltage characteristics were measured using a Keithley 236 source measurement unit

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and a calibrated silicon photodiode[22,23] (computer interfaced with LabViewTM supplied by National Instruments). The QEext was measured with an integrating sphere and optometer from UDT Corp. The sample was mounted at the center of the sphere, and a silicon photodiode of known spectral response was mounted at the exit port.[3,24] The EL spectra were measured with a single-grating monochromator equipped with a photometric charge coupled device (CCD) camera as detector. The luminous efficiency (LE) is defined as follows: LE = B/j [cd A±1], where B is the brightness and j is the current density; power efficiency = pB/jV [lm/W], where V is applied voltage. All device fabrication and testing were carried out in a controlled atmosphere dry-box in N2 atmosphere. At each doping concentration, at least four different (identical) devices were fabricated. For each of the devices, the current±voltage curve, the brightness±voltage curve and the QEext were measured. All data reported here are reproducible. Figure 1b shows the absorption and photoluminescence (PL) spectra of a PVK-PBD (40 wt.-%) film, and a neat film of [Ir(DPF)3]. As required for efficient Förster transfer from the singlet excited state in the host to the metal-ligand charge transfer (MLCT) band of the Ir-complex, there is a good overlap between the fluorescence spectrum of PVK-PBD (40 wt.-%) and the 400 nm MLCT absorption band of [Ir(DPF)3]. Consequently, fast intersystem crossing to the triplet state of the iridium complex and subsequent emission from this state are built-in features. In order to test the efficiency of the Förster energy transfer, films consisting of blends of 0.1 %, 0.3 %, 0.5 %, 1 %, 3 %, 5 %, 8 % by weight of [Ir(DPF)3] in PVK-PBD (40 wt.-%) were made and excited optically. Figure 2a shows the PL spectra of PVK-PBD (40 wt.-%) film (excited at 330 nm) and the PL spectra of the blends of [Ir(DPF)3] doped into PVK-PBD (40 wt.-%) (excited at 325 nm). We also measured the absolute PL efficiencies:[25] the 0.1 wt.-%, 3 wt.-%, and 8 wt.-% blends of [Ir(DPF)3] in PVK-PBD (40 wt.-%) have PL quantum efficiencies of 24 %, 30 %, and 25 %, respectively. To completely quench the host PL, 8 wt.-% [Ir(DPF)3] is required. At lower concentrations, the energy transfer from the host to the Ir-complex is incomplete because the average distance from a photoexcited polymer chain to the nearest Ir-complex is too large.[22] At higher concentrations, all of the energy is transferred to the iridium complex, but the fluorescence efficiency is evidently reduced by concentration quenching. To further examine the effects of concentration quenching, films of pure [Ir(DPF)3] were also studied. The PL quantum efficiency of the pure film of [Ir(DPF)3] is only approximately 1.5 %.[21] These results indicate that there is complete Förster energy transfer to the guest at very low dopant concentrations (approximately 1 % per repeat unit) as expected for the relatively long-range (dipole±dipole) Förster coupling.[8,11] Figure 2b shows the emission spectra from a device made with 0.01 wt.-% [Ir(DPF)3] dopant at different applied voltages. The EL spectra show the characteristic spectrum of [Ir(DPF)3], with a peak at 550 nm, and secondary band at 600 nm. Although 8 wt.-% [Ir(DPF)3] was needed to quench

Fig. 2. a) PL spectra of PVK-PBD (40 wt.-%) and PVK-PBD (40 wt.-%) with different [Ir(DPF)3] concentrations. All films had approximately the same thickness (for direct comparison of the emission intensities). b) EL spectra of LED devices made from 0.01 wt.-% [Ir(DPF)3] doped into PVK-PBD (40 wt.-%) at different bias voltages.

the PVK-PBD (40 wt.-%) emission when the blend was photoexcited, there is no PVK-PBD (40 wt.-%) emission from any of the LEDs that contain the Ir-complex, even at the very lowest doping concentration (approx. 0.001 % per repeat unit of the PVK polymer). Tessler et al. also noted efficient electrophosphorescence at concentrations as low as 0.01 wt.-% in the PtOEP system.[26] The absence of PVK emission peak at both low and high bias is consistent with charge trapping on the Ir-complex (rather than Förster transfer) as the dominant mechanism in the LEDs. Table 1 summarizes the operating conditions and the characteristics of the LEDs fabricated from [Ir(DPF)3] doped into the PVK-PBD (40 wt.-%) blend. The substantial difference between the PL and EL spectra for [Ir(DPF)3] doped into the PVK-PBD (40 wt.-%) blend implies that different mechanisms are involved.[22] Under photoexcitation, singlet excited states are created on the carbazole unit and with subsequent transfer to the metal-organic complex (by Förster transfer). In contrast, in the LEDs, the

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Table 1. Summary of the operating conditions and the characteristics of the LEDs fabricated from [Ir(DPF)3] doped into the PVK-PBD (40 wt.-%).

[a] see Organic Eelectroluminescent Materials and Devices (Ed: S. Muyata) [20]. [b] Device structure is same as others, but without PBD.

electrons and holes are trapped on the metal-organic complex after injection of an electron at one side of the film and a hole from the opposite side. Although both Förster energy transfer and charge trapping might occur concurrently in the LEDs to some extent, the increase in the operating voltages with increasing [Ir(DPF)3] concentration provides additional evidence of charge trapping in this system (see Table 1). Figure 3 shows the j±voltage and B±voltage characteristics of the device made from 1 wt.-% doping concentration in PVK-PBD (40 wt.-%). The device turns-on at approx. 10±

Fig. 3. Brightness (j) and current density (d) characteristics of 1 wt.-% doped ITO/PEDOT/[Ir(DPF)3]:PVK-PBD/Ca/Ag device as function of applied voltages.

11 V, with maximum brightness in excess of 8300 cd m±2 (at 55 V). Figure 4 shows the LE and QEext as a function of the current density for the devices made with 1 wt.-% [Ir(DPF)3] doped into PVK-PBD 40 wt.-%. The LE and QEext first increased and then decreased with increasing j, similar to the QEext reported for devices containing [Ir(ppy)3].[27] The inset shows the QEext vs. the [Ir(DPF)3] doping concentration at constant current density, j = 4 mA cm±2. The highest QEext, approx. 10 % photon/electron, was obtained from the device with 0.3 wt.-% [Ir(DPF)3] in PVK-PBD (40 wt.-%). As the doping concentration was increased, the QEext decreased, possibly due to aggregation of [Ir(DPF)3].[28] At doping concentrations of 0.3±1 wt.-%, the devices have highest luminous efficiency; e.g., for 0.3 wt.-%, LE = 36 cd A±1 (power efficiency of »2.5 lm/W) at 45 V. The highest brightness, B = 8320 cd m±2, was observed at a doping concentra-

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Fig. 4. The luminous efficiency, LE = B/j, (d) and quantum efficiency (j) versus current density of [Ir(DPF)3]/PVK-PBD (40 wt.-%) LEDs with 1 wt.-% doping concentration of [Ir(DPF)3]. The QEext was obtained from the LE using the standard relation [31]. Inset: The external quantum efficiency, measured with the integrating sphere, versus doping concentration (wt.-%) of [Ir(DPF)3]/ PVK-PBD (40 wt.-%) LEDs at the constant current density of 4 mA cm±2.

tion of 1 wt.-% at 75 mA cm±2 (55 V). At 1 wt.-%, LE = 35 cd A±1 (» 3 lm/W) at 37 V. Even at a doping concentration of 5 wt.-%, B » 1200 cd m±2 and LE = 22 cd A±1 (1.3 lm/ W) at 55 V. Assuming the emission pattern is Lambertian, 1 cd = 3.14 lumens. Based on the emission spectra, these luminous efficiencies yield external quantum efficiencies of 9.1 %, 8.8 %, and 5.6 % for 0.3 wt.-%, 1 wt.-%, and 5 wt.-% doping concentration, respectively (for 1 wt.-%, the data are plotted versus j in Fig. 4).[29] These QEext values (inferred from the LE measurements) are consistent with the external quantum efficiencies measured directly with the integrating sphere (see the insert in Fig. 4). Although the operating voltages of these devices are quite high, the measurements were reproducible. For example, devices made from blends with 0.5 wt.-% [Ir(DPF)3] concentration exhibit a lifetime of several hours when operated at constant current (0.3 mA). It is remarkable that these thin film devices can withstand relatively high voltages without breakdown. Compared with the devices made from [Ir(ppy)3] in small molecules,[9,11] these polymer-based devices exhibit high external quantum efficiency and high luminous efficiency at remarkably low doping concentrations. We assume that the electron-hole trapping on the Ir-complex was improved by adding the fluorene unit to the ligand.[30,31] In summary, we have demonstrated high-efficiency polymer-based electrophosphorescent LEDs using [Ir(DPF)3] doped into PVK-PBD (40 wt.-%). The highest external quantum efficiency was 10 % photon/electron at a concentration of 0.3 wt.-% [Ir(DPF)3] in PVK-PBD (40 wt.-%). The highest luminous efficiency of 36 cd A±1 was observed (power efficiency of 2.5 lm/W) at 45 V. Our results demonstrate that high-efficiency electrophosphorescent light-emitting devices can be realized with polymers as host and heavy metal complexes as guest, and that these devices can be fabricated by

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Received: September 4, 2001 Final version: January 18, 2002

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[1] R. H. Friend. R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. BrØdas, M. Lögdlund, W. R. Salaneck, Nature 1999, 397, 121. [2] A. J. Heeger, Angew. Chem. Int. Ed. 2001, 40, 2591. [3] Yong Cao, I. D. Parker, Gang Yu, Chi Zhang, Alan J. Heeger, Nature 1999, 397, 414. [4] Z. Shuai, D. Beljonne, R. J. Silbey, J. L. BrØdas, Phys. Rev. Lett. 2000, 84, 131. [5] M. Wohlgenannt, K. Tandon, S. Mazumdar, S. Ramasesha, Z. V. Vardeny, Nature 2001, 409, 494. [6] J. Kido, H. Haromichi, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 1994, 65, 2124. [7] X. Zhang, R. Sun, Q. Zheng, T. Kobayashi, W. Li, Appl. Phys. Lett. 1997, 71, 2596. [8] M. A. Baldo, M. E. Thompson, S. R. Forrest, Nature 2000, 403, 750. [9] T. Tsutsui, M.- J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys. 1999, 38, L1502-L1504. [10] C. Adachi, M. A. Baldo, S. R. Forrest, Appl. Phys. Lett. 2000, 77, 904. [11] C.-L. Lee, K. B. Lee, J.-J. Kim, Appl. Phys. Lett. 2000, 77, 2280. [12] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, M. E. Thompson, J. Am. Chem. Soc. 2001, 123, 4304. [13] S. Lamansky, R. C. Kwong, M. Nugent, P. I. Djurovich, M. E. Thompson, Org. Electron. 2001, 2, 53. [14] T. Förster, Discuss. Faraday Soc. 1959, 7, 27. [15] D. L. Dexter, J. Chem. Phys. 1953, 21, 836. [16] K. Utsugi, S. Takano, J. Electrochem. Soc. 1992, 139, 3610. [17] H. Suzuki, A. Hoshino, J. Appl. Phys. 1996, 79, 8816. [18] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 1993, 63, 2627. [19] C. Zhang, H. von Seggern, K. Pakbaz, B. Kraabel, H.-W. Schmidt, A. J. Heeger, Synth. Met. 1994, 62, 35. [20] Organic Electroluminescent Materials and Devices (Ed: S. Muyata), Gordon and Breach Publishers, London 1997, p. 218. [21] J. C. Ostrowski, M. R. Robinson, X. Gong, A. J. Heeger, G. C. Bazan, unpublished. [22] M. D. McGehee, T. Bergstedt, C. Zhang, A. P. Saab, M. B. O'Regan, G. C. Bazan, V. I. Srdanov, A. J. Heeger, Adv. Mater. 1999, 11, 1349. [23] J. Gao, Y. Li, G. Yu, A. J. Heeger, J. Appl. Phys. 1999, 86, 4594. [24] N. C. Greenham, R. H. Friend, D. D. C. Bradley, Adv. Mater. 1994, 6, 491. [25] N. C. Greenham, I. D. W. Samuel. G. R. Hayes, R. T. Phillips, Y. A. R. R. Kessener, S. C. Moratti, A. B. Holmes, R. H. Friend, Chem. Phys. Lett. 1995, 241, 89. [26] N. Tessler, P. K. H. Ho, V. Cleave, D. J. Pinner, R. H. Friend, G. Yahioglu, P. Le. Barny, J. Gray, M. de Souza, G. Rumbles, Thin Solid Films 2000, 363, 64. [27] M. Lkai, Shizuo T. Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 2001, 79, 156. [28] M. A. Buldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151. [29] The external quantum efficiency is proportional to the luminous efficiency QEext =

5:010 3 LE hv…ev†u…k†

(1)

where QEext is external quantum efficiency, LE is luminous efficiency (cd A±1), hm is the photon energy [eV] of emission peak, U (k) is the spectral eye sensitivity. [30] Z. Bao, A. J. Lovinger, J. Brown. J. Am. Chem. Soc. 1998, 120, 207. [31] Y. Wang, N. Herron, V. V. Grushin, D. LeCloux, V. Petrov, Appl. Phys. Lett. 2001, 79, 449.

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Gold Glyconanoparticles as Building Blocks for Nanomaterials Design** By Teresa C. Rojas, Jes‚s M. de la Fuente, Africa G. Barrientos, Soledad PenadØs, Laurence Ponsonnet, and Asunción Fernµndez* The design, synthesis, and characterization of surface-modified nanoparticles are of fundamental importance in controlling the mesoscopic properties of new materials and in developing new tools for nanofabrication.[1±4] Self-assembled monolayers of thiolates on colloidal gold clusters are therefore ideal substrates for investigating nanoparticle microfabrication techniques and the effect of surface-bound reagents on structure±properties relationships.[5±7] In a previous paper,[8] we have described the synthesis of water-soluble gold nanoparticles functionalized with a monolayer of 11-thioacetateundecanol derivatized neoglycoconjugates of two biologically significant oligosaccharides: the lactose disaccharide (Lac; Galb(1®4)Glcb1-OR) and the trisaccharide Lex antigen (Lex; Galb(1®4)[Fuca(1®3)]GlcNAcb1-OR). In that paper,[8] we showed how these tailored globular carbohydrate models can be used to mimic glycosphingolipid clusters in plasma membrane, to investigate in solution a novel mechanism of cell adhesion through carbohydrate±carbohydrate interactions.[9±11] In this present paper, we report how these interactions can be used to guide the assembly of the gold clusters. New interactions through functionalization of the gold particles with fluorescein molecules were also studied and different nanostructures will be presented. In addition, size-selection processes by using protein filter devices will be reported. The glyconanoparticles, Lac-Au and Lex-Au, were obtained by ªin situº functionalization of gold nanoparticles following the procedure of Brust et al.[12] for the synthesis of monolayer-protected gold nanoclusters, as described by Fernµndez and co-workers.[8] The Lac-Au and Lex-Au glyconanoparticles are stable, water soluble, and can be manipulated as a watersoluble biological macromolecule. They were purified by dialysis or centrifugal filtering devices and characterized by 1 H-NMR, UV-vis, and FTIR spectroscopies, elemental analysis, and transmission electron microscopy (TEM).[8]

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[*] Dr. A. Fernµndez, Dr. T. C. Rojas Instituto de Ciencia de Materiales, CSIC-UNSE Isla de la Cartuja, AmØrico Vespucio s/n, E-41092 Sevilla (Spain) E-mail: [email protected] J. M. de la Fuente, A. G. Barrientos, Dr. S. PenadØs Grupo de Carbohidratos Instituto de Investigaciones Químicas, CSIC-UNSE Isla de la Cartuja, AmØrico Vespucio s/n, E-41092 Sevilla (Spain) Dr. L. Ponsonnet École Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Syst›mes, UMR 5513 ECL/CNRS B.P. 163, F-69131 Ecully Cedex (France)

[**] This work was supported by the DGICYT (PB96-0820 and PB96-0863C02), J.M.d.l.F. thanks the MEC for a predoctoral fellowship. A.G.B. thanks CSIC for financial support.

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processing the luminescent layer from solution. The results demonstrate an opportunity to realize high-performance LEDs with low operating voltages using Ir-complex in combination with conjugated semiconducting polymers as host materials.