1. Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, U.K., Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, U.K., Cambridge Display Technology Ltd., Cambourne Business Park, Cambridgeshire CB23 6DW, U.K., and Laboratory for Chemistry of Novel Materials, University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium