ductivity at around n = 4. In light of our findings, we propose that materials below this conductivity maximum are best con- sidered as isolated fulleride nanowires and for this reason have conductivity patterns that more closely resemble the bulk state. The materials above n = 4.1, because of increased physi- cal or electronic continuity through the structure, show coop- erative behavior between the walls and the fulleride and thus higher conductivity. This explanation would certainly account for the observation that all materials in this study at greater than n = 4.1 conduct very well. If this pattern was associated only with band filling in the fulleride and not some other phe- nomenon, a single cusp (as with the n = 2.6 material or pure K 3 C 60 ) would be expected. An increase in physical continuity could be explained by a reordering of the fulleride in the pore to more continuous states, possibly aided by the increased con- centration of potassium ions in the channels. An increase in electronic continuity could be related to an Anderson transi- tion, a phenomenon common in amorphous solids, in which an increase in electron loading leads to filling of continuous bands directly above a series of highly localized states. There is also the possibility that the increased electron density at the fuller- ide in the n = 4.1 case allows for better overlap with the Nb 4d orbitals and formation of a new continuous hybrid band. At this stage our data does not allow us to distinguish between these possibilities, however, it is clear that this phenomenon can not be readily explained by trends in either fulleride chem- istry or the behavior of reduced mesoporous oxides. In conclusion, we have studied the dependence of conductiv- ity on oxidation state in a series of potassium fulleride compos- ites. The conductivity maximum at n = 2.6 provides the first compelling evidence that the phases other than n = 3 may be involved in conductivity in bulk K 3 C 60 , while that at n = 4.1 was unexpected and suggests a different mechanism for con- ductivity may be at work in these materials than in those with lower degrees of potassium. Because of the facility of oxida- tion-state tuning in these materials, they represent ideal model systems for the study of electron loading in the conduction band of one-dimensional molecular nanowires. Studies and computer modeling are ongoing to further elucidate the struc- tural and electronic nature of this new family of composites. Received: October 19, 2000 Final version: October 24, 2000 ± [1] P. Ball, Made to Measure, Princeton University Press, Princeton, NJ 1997. [2] H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162. [3] A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glar- um, T. T. M. Palstra, A. P. Ramirez, A. R.Kortan, Nature 1991, 350, 600. [4] P. J. Benning, J. L. Martins, J. H. Weaver, L. P. F. Chibante, R. E. Smalley, Science 1991, 252, 1417. [5] S. Chakraverty, M. P. Gelfand, S. Kivelson, Science 1991, 254, 970. [6] M. Kosaka, K. Tanigaki, K. Prassides, S. Margedonna, A. Lappas, C. M. Brown, A. N. Fitch, Phys. Rev. 1999, 59, 6628. [7] T. Yildrium, L. Barbedette, J. E. Fischer, C. L. Lin, J. Robert, P. Petit, T. T. M. Palstra, Phys. Rev. Lett. 1996, 77, 167. [8] T. T. M. Palstra, A. F. Hebard, R. C. Haddon, B. Littlewood, Phys. Rev. B 1994, 50, 3462. [9] R. W. Lof, M. A. van Veenendaal, B. Koopmans, H. T. Jonkman, G. A. Sawatzky, Phys. Rev. Lett. 1992, 68, 3924. [10] D. M. Antonelli,J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1996, 35, 426. [11] B. Ye, M. Trudeau, D. M. Antonelli, Adv. Mater. 2001, 13, 29. [12] a) M. Vettraino, M. Trudeau, D. M. Antonelli, Adv. Mater. 2000, 12, 337. b) S. Murray, M. Trudeau, D. M. Antonelli, Inorg. Chem. 2000, 34, 5901. c) S. Murray, M. Trudeau, D. M. Antonelli, Adv. Mater. 2000, 12, 1339. d)M. Vettraino, M. Trudeau, D. M. Antonelli, Inorg. Chem., in press. [13] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartulli, J. S. Beck, Nature 1992, 359, 710. [14] Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P.Sieger, A. Firouzi, B. F. Chmelka, F. Schuth, G. D. Stucky, Chem. Mater. 1994, 6, 1176. [15] C.-Y. Chen, S. L. Burkette, H.-X. Li, M. E. Davis, Microporous Mater. 1993, 2, 27. [16] P. T. Tanev, M. Chibwe,T. J.Pinnavaia, Nature 1994, 368, 321. [17] P. Behrens, Angew. Chem. Int. Ed. Engl. 1996, 35, 515. [18] D. M. Antonelli,J. Y. Ying, Curr. Opin. Colloid Interface Sci. 1996, 1, 523. [19] U. Ciesla, S. Schacht, G. D. Stucky, K. Unger, F. Schüth, Angew. Chem. Int. Ed. Engl. 1996, 35, 541. [20] D. M. Antonelli,J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1995, 34, 2014. [21] D. M. Antonelli, Adv. Mater. 1999, 11, 487. [22] Z. R. Tian, J. Y. Wang, N. G. Duan, V. V. Krishnan, S. L. Suib, Science 1997, 276, 926. [23] D. M. Antonelli, M. Trudeau, Angew. Chem. Int. Ed. 1999, 38, 1471. [24] J. Chen, Q. Li, H. Ding, W. Peng, R. Xu, Langmuir 1997, 13, 2050. [25] F. Bensebaa, B. Xiang, L. Kevan, J. Phys. Chem. 1992, 96, 10 258. [26] M. M. Khaled, R. T. Carlin, P. C. Trulove, G. R. Eaton, S. S. Eaton, J. Am. Chem. Soc. 1994, 116, 3465. Improving the Performance of Polyfluorene-Based Organic Light-Emitting Diodes via End-capping** By Tzenka Miteva, Andreas Meisel, Wolfgang Knoll, Heinz G. Nothofer, Ullrich Scherf ,* David C. Müller, Klaus Meerholz, Akio Yasuda, and Dieter Neher* Polyfluorenes (PF) have evolved as an important class of materials for polymer light-emitting diodes (LED). Several reports have demonstrated bright blue emission from PF homo- polymers. [1±4] A second important property of PF homopoly- mers is their thermotropic liquid crystallinity, which allows the orientation of these polymers on rubbed polyimide layers. [5,6] By using hole-transporting alignment layers, LEDs emitting Adv. Mater. 2001, 13, No. 8, April 18 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0935-9648/01/0804-0565 $ 17.50+.50/0 565 COMMUNICATIONS ± [*] Prof. U. Scherf Institut für Chemie, Universität Potsdam Karl-Liebknecht-Str. 24±25, Haus 25 D-14476 Golm (Germany) E-mail: scherf@rz.uni-potsdam.de Prof. D. Neher Institut für Physik, Universität Potsdam Am Neuen Palais 10, D-14469 Potsdam (Germany) E-mail: neher@rz.uni-potsdam.de Dr. T. Miteva, [+] A. Meisel, Prof. W. Knoll, H. G. Nothofer [+] Max-Planck-Institut für Polymerforschung Ackermannweg 10, D-55021 Mainz (Germany) D. C. Müller, Dr. K. Meerholz Department Chemie, Lehrbereich Physikalische Chemie Ludwig-Maximilians-Universität München Butenandtstr. 11, D-81377 München (Germany) Dr. A. Yasuda Sony International (Europe) GmbH, Materials Science Laboratories Heinrich-Hertz-Str. 1, D-70327 Stuttgart (Germany) [+] Present address: Sony International (Europe) GmbH, Materials Science Laboratories, Heinrich-Hertz-Str. 1, D-70327 Stuttgart, Germany. [**] We acknowledge Dr. G. Nelles (Sony International Europe), Prof. Dr. G. Wegner, and Prof. Dr. K. Müllen (MPI-P in Mainz) for generous sup- port and fruitful discussions and Sony International (Europe) GmbH for financial support.