1. Contribution from The Department of Chemistry and Materials Science & Engineering, The University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801, The Beckman Institute for Advanced Science and Technology, 405 North Mathews Avenue, Urbana, Illinois 61801, and Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands