1. Contribution from the Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands, Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, and Laboratory for Chemistry of Novel Materials, University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium