1. Contribution from the Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Department of Polymer Technology, Faculty of Chemistry and Chemical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands, and Philips Research Laboratories, Department of Biomolecular Engineering, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands