1. Contribution from the Leiden Institute of Chemistry, Leiden University, Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Laboratory of Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Federal Institute of Technology, ETH-ITO, Grabenstrasse 3, 8952 Schlieren, Switzerland