1. Contribution from the Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, the Department of Chemistry and Institute of Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium, and the Service de Chimie des Matériaux Nouveaux, Université de Mons-Hainaut, B-7000 Mons, Belgium