1. Contribution from the Department of Chemistry, Temple University, Beury Hall 201, Philadelphia, Pennsylvania 19122, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Chemistry of Novel Materials, University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium