1. Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200-F, 3001, Leuven, Belgium, Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, and Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P. O. Box 513, 5600 MB, The Netherlands